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Subtype tunicates general characteristics. Amazing sea creatures - tunicates (TUNICATA). Subtype Tunicates. Main features and structure of ascidian

Chordata animals are inferior in number to Arthropods, but their role in the life of our planet is paramount. In terms of the level of their organization, Chordates are noticeably superior to all other groups of animals, and therefore it is no coincidence that it was within this type that a thinking creature arose - Homo sapiens. However, this was preceded by a long and difficult path, as evidenced by the amazing diversity of chordates. Among them there are also very primitive creatures. For example, the lancelet has not yet gone very far from some ancient ancestors unknown to us. We will limit ourselves to considering only living chordates, represented by two groups very unequal in volume. A representative of a very ancient group of Skullless that has come down to us is the lancelet. Much later, Vertebrates, or Cranial, animals arose, which include all the main groups of modern vertebrates, represented by six classes: Cartilaginous fish, Bony fish, Amphibians, or Amphibians, Reptiles, or Reptiles, Birds and Mammals. The main feature that unites all Chordates - the presence of an internal axial skeleton, initially represented by a notochord, which was later replaced by the vertebral column (spine). This axial skeleton always lies closer to the dorsal surface, above the intestines and all other internal organs except the nervous system. The latter has shifted to the dorsal side of the body and lies directly above the axial skeleton. The nervous system is laid down in the form of a tube. Other characteristics common to all chordates are as follows. They have a longitudinally differentiated through digestive system, which includes the liver. The most anterior, pharyngeal section of the digestive system is penetrated by gill slits. In the most primitive chordates they remain throughout life and serve for respiration. In terrestrial vertebrates they can be detected only in the early stages of development. The circulatory system is always closed. We will consider the structure of the lancelet separately, since the example of this very ancient and primitive animal most clearly reveals the main features of the organization (structural plan) of chordates. Subtypes Cephalochordates Cephalochordates Branchiostoma lanceolatum Cephalochordata (lat. Cephalochordata) or skullless (lat. Acrania) are small marine fish-like animals with all the features characteristic of chordates. Cranials are a subtype of lower chordates, unlike other chordates (tunicates and vertebrates), which retain the basic characteristics of the type (notochord, neural tube and gill slits) throughout life. There is no brain, the sense organs are primitive. They lead a bottom-dwelling lifestyle and are burrowing filter feeders by the nature of their diet. They may be the ancestors of vertebrates, or the last living members of the group from which vertebrates descended. In total, about 30 species belong to the skullless species, making up one class - lancelets. [edit] Tunicates Tunicates: ascidians Tunicates (lat. Tunicata, Urochordata) are a subtype of chordates. Includes 5 classes - ascidians, appendicularians, salps, fire beetles and barrel beetles. According to another classification, the last 3 classes are considered units of the Thaliace class. More than 1,000 species are known. They are distributed throughout the world and inhabit the seabed. Three large classes of tunicates: Ascidians - lower soft-bodied filter-feeding chordates, leading a sedentary lifestyle as adults; The appendiculars retain larval features such as a tail throughout life. For this reason, for a long time they were considered as larvae of ascidians and salps. Due to the presence of long tails, tunicate larvae are called lat. urochordata; The third group of tunicates - free-swimming salps - feed on plankton. In their life cycle, two generations are known - solitary hermaphroditic and budding colonial asexual. The larvae of these animals have all the main characteristics of chordates, including a notochord and a tail. They are also equipped with a rudimentary brain and light and position (roll) sensors. [edit]Vertebrates Vertebrates (lat. Vertebrata) are the highest subtype of chordates. The dominant group of animals (along with insects) on earth and in the air. They differ from other chordates in the presence of a separate skull and the development of the brain and sensory organs. The notochord in most representatives of higher chordates is replaced by a spine that protects the spinal cord and consists, as a rule, of cartilage and bone tissue. The endostyle, as such, is present only in lamprey larvae. Compared to lower chordates - skullless and tunicates - they are characterized by a significantly higher level of organization, which is clearly expressed both in their structure and in their physiological functions. Among vertebrates, there are no species that lead a sessile (attached) lifestyle. They move over a wide range, actively searching for and capturing food, finding individuals of the opposite sex for reproduction, and escaping from pursuit by enemies. The position of lampreys is ambiguous. They have an underdeveloped skull and rudimentary vertebrae - therefore, they can be considered as vertebrates and true fish. However, molecular phylogeneticists, who used biochemical reactions to classify organisms, eventually assigned this group of vertebrates to the Myxinidae family of the class Cyclostomes]. Hagfish, which have a gill skeleton consisting of a small number of cartilaginous plates and vestigial vertebrae, are not considered true vertebrates - they are considered a group from which vertebrates evolved.

The phylum Chordata unites animals that differ in appearance, living conditions, and lifestyle. Representatives of this type are found in all major environments of life: in water, on land, in the soil, in the air. They are distributed throughout the Earth. Number of species modern representatives There are about 40 thousand chordates.

The phylum Chordata includes skullless, cyclostomes, fish, reptiles, amphibians, mammals and birds. Tunicates can also be classified as this type - this is a unique group of organisms that lives on the ocean floor and leads an attached lifestyle. Sometimes gastrobreathers, which have some characteristics of this type, are included in the phylum Chordata.

Characteristics of the type Chordata

Despite the great diversity of organisms, they all have a number of common structural and developmental features.

The structure of chordates is as follows: all these animals have an axial skeleton, which first appears in the form of a notochord or dorsal string. The notochord is a special non-segmented and elastic cord that embryonically develops from the dorsal wall of the embryonic intestine. The origin of the chord is endothermal.

Further, this cord can develop differently, depending on the organism. It remains throughout life only in lower chordates. In most higher animals, the notochord is reduced, and in its place a vertebral column is formed. That is, in higher organisms, the notochord is an embryonic organ that is replaced by vertebrae.

Above the axial skeleton is the central nervous system, which is represented by a hollow tube. The cavity of this tube is called a neurocoel. Almost all chordates are characterized by a tubular structure of the central nervous system.

In most chordate organisms, the anterior section of the tube grows to form the brain.

The pharyngeal section (anterior) of the digestive tube comes out at two opposite ends. The openings that emerge are called visceral fissures. Lower organisms of the type have gills on them.

In addition to the three above-mentioned features of chordates, it can also be noted that these organisms have a secondary mouth, like echinoderms. The body cavity in animals of this type is secondary. Chordata are also characterized by bilateral body symmetry.

The phylum Chordata is divided into subtypes:

Skullless;
Tunicates;
Vertebrates.

Subtype Skullless

This subphylum includes only one class - Cephalochordates, and one order - Lancelets.

The main difference between this subtype is that these are the most primitive organisms, and all of them are exclusively marine animals. They are widespread in the warm waters of oceans and seas of temperate and subtropical latitudes. Lancelets and epigonychites live in shallow water, mainly burying the back part of their body in the bottom substrate. They prefer sandy soil.

This type of organism feeds on detritus, diatoms or zooplankton. They always breed in the warm season. Fertilization is external.

The lancelet is a favorite object of study, because all the characteristics of chordates are preserved for life, which allows us to understand the principles of the formation of chordates and vertebrates.

Subtype Tunicates

The subtype includes 3 classes:

Salps;
Ascidians;
Appendiculars.

All animals of the subtype are exclusively marine.

The main difference between these chordates is that almost all organisms lack a notochord and a neural tube as adults. In the larval state, all the characteristics of the type in tunicates are clearly expressed.

Tunicates live in colonies or solitarily, attached to the bottom. There are significantly fewer free-swimming species. This subtype of animals lives in the warm waters of the tropics or subtropics. They can live both on the surface of the sea and deep in the ocean.

The body shape of adult tunicates is round, barrel-shaped. The organisms got their name due to the fact that their body is covered with a rough and thick shell - a tunic. The consistency of the tunic is cartilaginous or gelatinous; its main purpose is to protect the animal from predators.

Tunicates are hermaphrodites and can reproduce both sexually and asexually.

It is known that the ancestors of these organisms were free-swimming, but at present only tunicate larvae can move freely in the water.

Subphylum Vertebrates

Cranial animals are the highest subphylum. Compared to other subtypes, they have a higher level of organization, which is evident from their structure, both external and internal. Among vertebrates, there are no species that lead a completely attached lifestyle - they actively move in space, looking for food and shelter, and a pair for reproduction.

By moving, vertebrate organisms provide themselves with the opportunity to change their habitat depending on changing external conditions.

The above general biological features are directly related to the morphological and physiological organization of vertebrates.

The nervous system of cranial animals is more differentiated than that of lower animals of the same type. Vertebrates have a well-developed brain, which contributes to the functioning of higher nervous activity. It is higher nervous activity that is the basis of adaptive behavior. These animals have well-developed sensory organs, which are necessary for communication with the environment.

As a result of the emergence of sensory organs and the brain, a protective organ such as the skull developed. And instead of a chord, this subtype of animals has a spinal column, which serves as a support for the entire body and a case for the spinal cord.

All animals of the subtype have a movable jaw apparatus and an oral fissure, which develop from the anterior part of the intestinal tube.

The metabolism of this subtype is much more complex than that of all the animals discussed above. Cranial animals have a heart that provides rapid blood flow. Kidneys are necessary for removing waste products from the body.

The subtype Vertebrates appeared only in the Ordovician-Silurian, but in the Jurassic period all currently known types and classes already existed.

Total modern species a little over 40 thousand.

Classification of vertebrates

The phylum Chordata is very diverse. The classes existing in our time are not so numerous, but the number of species is enormous.

The cranial subtype can be divided into two groups, these are:

Primary water organisms.
Terrestrial organisms.

Primary water organisms

Proto-aquatic eggs are distinguished by the fact that they either have gills throughout their entire life or only in the larval stage, and during the development of the egg, embryonic membranes are not formed. This includes representatives of the following groups.

Section Agnathans

Class Cyclostomes.

These are the most primitive cranial animals. They actively developed in the Silurian and Devonian; at present, their species diversity is not great.

Section Gastrostomata

Pisces superclass:

Class Bony fish.
Class Cartilaginous fish.

Superclass Quadrupeds:

Class Amphibians.

These are the first animals to develop a jaw apparatus. This includes all known fish and amphibians. All of them actively move in water and on land, hunt and capture food with their mouths.

Terrestrial organisms

The group of terrestrial animals includes 3 classes:

Birds.
Reptiles.
Mammals.

This group is characterized by the fact that in animals, during the development of the egg, embryonic membranes are formed. If the species lays eggs on the ground, the embryonic membranes protect the embryo from external influences.

All chordates of this group live mainly on land and have internal fertilization, which suggests that these organisms are more evolutionarily developed.

They lack gills at all stages of development.

Origin of chordates

There are several hypotheses for the origin of chordates. One of them suggests that this type of organisms originated from the larvae of intestinal-breathers. Most representatives of this class lead an attached lifestyle, but their larvae are mobile. Examining the structure of the larvae, one can see the rudiments of the notochord, the neural tube and other features of chordates.

Another theory states that the phylum Chordates evolved from the crawling, worm-like ancestors of the gastrobreathers. They had the rudiments of a chord, and in the pharynx, next to the gill slits, there was an endostyle - an organ that contributed to the secretion of mucus and the capture of food from the water column.

The article discussed the general characteristics of the type. Chordates are united by many similar features of all organisms, but still each class and each species has individual characteristics.

Type Tunicata (Tunicata) (N. G. Vinogradova)

Tunicates, or tunicats, which include ascidians, pyrosomes, salps And appendiculars, - one of the most amazing groups of marine animals. They got their name because their body is covered on the outside with a special gelatinous membrane, or tunic. The tunica consists of a substance extremely similar in composition to cellulose, which is found only in the plant kingdom and is unknown in any other group of animals. Tunicates are exclusively marine animals, leading a partly attached, partly free-swimming pelagic lifestyle. They can be either solitary or form amazing colonies that arise during alternation of generations as a result of the budding of asexual single individuals. We will specifically talk below about the methods of reproduction of these animals - the most extraordinary among all living creatures on Earth.

Currently, it is believed that tunicates, through secondary simplification, or degradation, evolved from some forms very close to vertebrates.

Together with other chordates and echinoderms, they form the trunk of deuterostomes - one of the two main trunks of the evolutionary tree.

Tunicates are considered either as a separate subtype type chordates- Chordata, which includes three more subtypes of animals, including vertebrates(Vertebrata), or as an independent type -Tunicata, or Urochordata. This type includes three class: Appendiculars(Appendiculariae, or Copelata), Ascidia(Ascidiae) and Salps(Salpae).

Previously, ascidians were divided into three squad: simple, or solitary, ascidian(Monascidiae); complex, or colonial, ascidian(Synascidiae) and pyrosomes, or firebugs(Ascidiae Salpaeformes, or Pyrosomata). However, at present, the division into simple and complex ascidians has lost its systematic meaning. Ascidians are divided into subclasses based on other characteristics.

Salps divisible by two squad - kegmakers(Cyclomyaria) and actually salp(Desmomyaria). Sometimes these units are given the meaning of subclasses. Salps also apparently include a very peculiar family of deep-sea bottom tunicates - Octacnemidae, although until now most authors considered it a strongly deviated subclass of ascidians.

Very often, salps and pyrosomes, leading a free-swimming lifestyle, are combined into the group of pelagic tunicates Thaliacea, which is given class significance. The class Thaliacea is then divided into three subclasses: Pyrosomida or Luciae, Desmomyaria or Salpae, and Cyclomyaria or Doliolida. As can be seen, views on the taxonomy of the higher groups of Tunicata are very different.

Currently, more than a thousand species of tunicates are known. The vast majority of them fall to the share of ascidians; there are about 60 species of appendicularia, about 25 species of salps and about 10 species of pyrosomes (Tables 28-29).

As already mentioned, tunicates live only in the sea. Appendicularips, salps and pyrosomes swim in the ocean waters, while ascidians lead an attached lifestyle on the bottom. Appendicularia never form colonies, while salps and ascidians can occur both as single organisms and as colonies. Pyrosomes are always colonial. All tunicates are active filter feeders, feeding either on microscopic pelagic algae and animals, or on particles of organic matter suspended in water - detritus. By pushing water through the throat and gills outward, they filter out the smallest plankton, sometimes using very complex devices.

Pelagic tunicates live mainly in the upper 200 m water, but sometimes they can go deeper. Pyrosomes and salps are rarely found deeper than 1000 m, appendiculars are known up to 3000 m. However, there are apparently no special deep-sea species among them. Ascidians for the most part are also distributed in the tidal littoral and subtidal zones of oceans and seas - up to 200-500 m, however, a significant number of their species are found deeper. Maximum depth their location - 7230 m.

Tunicates are found in the ocean either in single specimens or in the form of colossal clusters. The latter is especially characteristic of pelagic forms. In general, tunicates are quite common in marine fauna and, as a rule, are caught in plankton nets and bottom trawls of zoologists everywhere. Appendiculars and ascidians are common in the World Ocean at all latitudes. They are as characteristic of the seas of the Arctic Ocean and Antarctica as of the tropics. Salps and pyrosomes, on the contrary, are mainly confined in their distribution to warm waters and are only rarely found in waters of high latitudes, mainly being brought there by warm currents.

Body structure almost all tunicats are unrecognizable and very different from general plan body structure in the chordate phylum. The appendiculars are closest to the original forms, and in the tunicate system they occupy first place. However, despite this, the structure of their body is the least characteristic of tunicates. It is probably best to start getting acquainted with tunicates with ascidians.

The structure of ascidians. Ascidians are bottom-dwelling animals that lead an attached lifestyle. Many of them are single forms. Their body sizes are on average several centimeters in diameter and the same in height. However, among them some species are known that reach 40-50 cm, such as the widespread Cione intestinalis or the deep-sea Ascopera gigantea. On the other hand, there are very small sea squirts, measuring less than 1 mm. In addition to single ascidians, there are a large number of colonial forms in which individual small individuals, several millimeters in size, are immersed in a common tunic. Such colonies, very diverse in shape, grow on the surfaces of stones and underwater objects.

Most of all, single ascidians resemble an elongated, inflated sac of irregular shape, which adheres with its lower part, called the sole, to various solid objects (Fig. 173, A). On the upper part of the animal, two holes are clearly visible, located either on small tubercles, or on rather long outgrowths of the body, reminiscent of the neck of a bottle. These are siphons. One of them - oral, through which the ascidian sucks in water, the second - cloacal. The latter is usually slightly shifted to the dorsal side. Siphons can open and close using muscles called sphincters. Body The ascidian is covered with a single-layer cell cover - epithelium, which secretes a special thick shell on its surface - tunic. The external color of the tunic varies. Typically, ascidians are colored orange, reddish, brown or purple. However, deep-sea ascidians, like many other deep-sea animals, lose their coloration and become dirty white. Sometimes the tunic is translucent and the insides of the animal are visible through it. Often the tunic forms wrinkles and folds on the surface and is overgrown with algae, hydroids, bryozoans and other sessile animals. In many species, its surface is covered with grains of sand and small pebbles, so that the animal can be difficult to distinguish from surrounding objects.

The tunic can have a gelatinous, cartilaginous or jelly-like consistency. Its remarkable feature is that it consists of more than 60% cellulose. The thickness of the tunic walls can reach 2-3 cm, but usually it is much thinner.

Some epidermal cells can penetrate into the thickness of the tunic and populate it. This is possible only due to its gelatinous consistency. In no other group of animals do cells inhabit formations of a similar type (for example, the cuticle of nematodes). In addition, blood vessels can grow into the thickness of the tunic.

Under the tunic lies the body wall itself, or mantle, which includes a single-layer ectodermic epithelium covering the body and a connective tissue layer with muscle fibers. The external muscles consist of longitudinal, and the internal muscles of circular fibers. Such muscles allow ascidians to make contractile movements and, if necessary, throw water out of the body. The mantle covers the body under the tunic, so that it lies freely inside the tunic and grows together with it only in the area of the siphons. In these places there are sphincters - muscles that close the openings of the siphons.

There is no hard skeleton in the body of ascidians. Only some of them have small calcareous spicules of various shapes scattered in different parts of the body.

Alimentary canal The ascidian begins with a mouth located at the free end of the body on the introductory, or oral, siphon (Fig. 173, B). Around the mouth there is a corolla of tentacles, sometimes simple, sometimes quite highly branched. The number and shape of tentacles vary among different species, but there are never less than 6 of them. A huge pharynx hangs inward from the mouth, occupying almost the entire space inside the mantle. The pharynx of ascidians forms a complex respiratory apparatus. Along its walls, gill slits are located in strict order in several vertical and horizontal rows, sometimes straight, sometimes curved (Fig. 173, B). Often the walls of the pharynx form 8-12 rather large folds hanging inward, located symmetrically on its two sides and greatly increasing its internal surface. The folds are also pierced by gill slits, and the slits themselves can take on very complex shapes, twisting in spirals on cone-shaped projections on the walls of the pharynx and folds. The gill slits are covered with cells bearing long cilia. In the spaces between the rows of gill slits, blood vessels pass, also correctly positioned. Their number can reach 50 on each side of the pharynx. Here the blood is enriched with oxygen. Sometimes the thin walls of the pharynx contain small spicules to support them.

The gill slits, or stigmas, of ascidians are invisible if you examine the animal from the outside, removing only the tunic. From the pharynx they lead into a special cavity lined with endoderm and consisting of two halves fused on the ventral side with the mantle. This cavity is called peribranchial, atrial or peribranchial(Fig. 173, B). It lies on each side between the pharynx and the outer wall of the body. Part of it forms a cloaca. This cavity is not an animal body cavity. It develops from special invaginations of the outer surface into the body. The peribranchial cavity communicates with the external environment using the cloacal siphon.

A thin dorsal plate, sometimes dissected into thin tongues, hangs from the dorsal side of the pharynx, and a special subbranchial groove, or endostyle, runs along the ventral side. By beating the cilia on the stigmas, the ascidian drives water so that a constant current is established through the mouth opening. Next, the water is driven through the gill slits into the circumbranchial cavity and from there through the cloaca to the outside. Passing through the cracks, water releases oxygen into the blood, and various small organic remains, unicellular algae, etc. are captured by the endostyle and driven along the bottom of the pharynx to its posterior end. There is an opening leading into the short and narrow esophagus. Curving to the ventral side, the esophagus passes into the swollen stomach, from which the intestine emerges. The intestine, bending, forms a double loop and opens with the anus into the cloaca. Excreta is expelled from the body through the cloacal siphon. Thus, the digestive system of ascidians is very simple, but noteworthy is the presence of an endostyle, which is part of their hunting apparatus. Endostyle cells are of two genera - glandular and ciliated. The ciliated cells of the endostyle capture food particles and drive them to the pharynx, gluing them together with secretions of glandular cells. It turns out that the endostyle is a homologue of the vertebrate thyroid gland and secretes an organic substance containing iodine. Apparently, this substance is close in composition to the thyroid hormone. Some ascidians have special folded processes and lobular masses at the base of the stomach walls. This is the so-called liver. It is connected to the stomach by a special duct.

Circulatory system ascidian is not closed. The heart is located on the ventral side of the animal's body. It looks like a small elongated tube surrounded by a thin pericardial sac, or pericardium. A large blood vessel runs from the two opposite ends of the heart. The branchial artery begins from the anterior end, which stretches in the middle of the ventral side and sends out numerous branches to the gill slits, giving off small side branches between them and surrounding the gill sac with a whole network of longitudinal and transverse blood vessels. The intestinal artery departs from the posterior dorsal side of the heart, giving branches to the internal organs. Here the blood vessels form wide lacunae-spaces between organs that do not have their own walls, very similar in structure to lacunae bivalves. Blood vessels also extend into the body wall and even into the tunic.

The entire system of blood vessels and lacunae opens into the branchial-intestinal sinus, sometimes called the dorsal vessel, with which the dorsal ends of the transverse branchial vessels are connected. This sinus is significant in size and stretches in the middle of the dorsal part of the pharynx. All tunicates, including ascidians, are characterized by a periodic change in the direction of blood flow, since their heart alternately contracts for some time, from back to front, then from front to back. When the heart contracts from the dorsal to the abdominal region, the blood moves through the branchial artery to the pharynx, or gill sac, where it is oxidized and from where it enters the enterobranchial sinus. The blood is then pushed into the intestinal vessels and back to the heart, just as is the case in all vertebrates. With the subsequent contraction of the heart, the direction of blood flow is reversed, and it flows like in most invertebrates. Thus, the type of blood circulation in Tuni Khat is transitional between the blood circulation of invertebrate and vertebrate animals. The blood of ascidians is colorless and acidic. Its remarkable feature is the presence of vanadium, which takes part in the transfer of oxygen in the blood and replaces iron.

Nervous system in adult ascidians it is extremely simple and much less developed than in the larva. Simplification of the nervous system occurs due to the sedentary lifestyle of adult forms. The nervous system consists of the suprapharyngeal, or cerebral, ganglion, located on the dorsal side of the body between the siphons. From the ganglion, 2-5 pairs of nerves originate, going to the edges of the mouth, pharynx and to the insides - the intestines, genitals and to the heart, where there is a nerve plexus. Between the ganglion and the dorsal wall of the pharynx there is a small paranervous gland, the duct of which flows into the pharynx at the bottom of the fossa in a special ciliated organ. This gland is sometimes considered a homologue of the lower appendage of the brain of vertebrates - the pituitary gland. There are no sensory organs, but the oral tentacles probably have a tactile function. Nevertheless, the nervous system of tunicates is not essentially primitive. Ascidian larvae have a spinal tube lying under the notochord and forming a swelling at its anterior end. This swelling apparently corresponds to the brain of vertebrates and contains the larval sensory organs - pigmented ocelli and an organ of equilibrium, or statocyst. When the larva develops into an adult animal, the entire posterior part of the neural tube disappears, and the brain vesicle, along with the larval sensory organs, disintegrates; Due to its dorsal wall, the dorsal ganglion of the adult ascidian is formed, and the abdominal wall of the bladder forms the perinnervous gland. As V.N. Beklemishev notes, the structure of the nervous system of tunicates is one of the best evidence of their origin from highly organized mobile animals. The nervous system of ascidian larvae is higher in development than the nervous system of the lancelet, which lacks a brain bladder.

Special excretory organs Ascidians do not. It is likely that the walls of the digestive canal take part in the excretion to some extent. However, many ascidians have special so-called scattered storage buds, consisting of special cells - nephrocytes, in which excretory products accumulate. These cells are arranged in a characteristic pattern, often clustered around the intestinal loop or gonads. The reddish-brown color of many ascidians depends precisely on the excreta accumulated in the cells. Only after the death of the animal and the disintegration of the body are the excretory products released and released into the water. Sometimes in the second knee of the intestine there is a cluster of transparent vesicles that do not have excretory ducts, in which nodules containing uric acid accumulate. Representatives families Molgulidae, the accumulation bud becomes even more complex and the accumulation of vesicles turns into one large isolated sac, the cavity of which contains nodules. The great originality of this organ lies in the fact that the molgulide kidney sac of some other ascidians always contains symbiotic fungi that do not even have distant relatives among other groups of lower fungi. Fungi form the finest micelle threads that entwine the nodules. Among them there are thicker formations of irregular shape, sometimes sporangia with spores are formed. These lower fungi feed on urates, the excretion products of ascidians, and their development frees the latter from accumulated excreta. Apparently, these fungi are necessary for ascidians, since even the rhythm of reproduction in some forms of ascidians is associated with the accumulation of excreta in the kidneys and with the development of symbiotic fungi. How fungi are transferred from one individual to another is unknown. Ascidian eggs are sterile in this regard, and young larvae do not contain fungi in their buds, even when excreta has already accumulated in them. Apparently, young animals are again “infected” with fungi from sea water. Ascidians are hermaphrodites, i.e. the same individual has both male and female gonads at the same time. The ovaries and testes lie one or several pairs on each side of the body, usually in a loop of intestine. Their ducts open into the cloaca, so that the cloacal opening serves not only for the release of water and excrement, but also for the removal of reproductive products. Self-fertilization does not occur in ascidians, since eggs and sperm mature in different time. Fertilization most often occurs in the peribranchial cavity, where the sperm of another individual penetrate with a current of water. Less often it happens outside. Fertilized eggs exit through the cloacal siphon, but sometimes the eggs develop in the peribranchial cavity and already formed swimming larvae emerge. Such viviparity is typical especially for colonial ascidians.

In addition to sexual reproduction, ascidians also have an asexual method of reproduction through budding. In this case, various ascidian colonies are formed.

Structure ascidiozooid- a member of a colony of complex ascidians - in principle does not differ from the structure of a single form. But their sizes are much smaller and usually do not exceed a few millimeters. The body of the ascidiozooid is elongated and divided into two or three sections (Fig. 174, A): the pharynx is located in the first, thoracic, section, the intestines are in the second, and the gonads and heart are in the third. Sometimes different organs are located slightly differently.

The degree of communication between individuals in an ascidiozoan colony can vary. Sometimes they are completely independent and are connected only by a thin stolon that spreads along the ground. In other cases, the ascidiozooids are enclosed in a common tunic. They can either be scattered in it, and then both the oral and cloacal openings of the ascidiozooids come out, or are located correct figures in the form of rings or ellipses (Fig. 174, B). In the latter case, the colony consists of groups of individuals that have independent mouths, but have a common cloacal cavity with one common cloacal opening into which the cloacas of individual individuals open. As already indicated, the size of such ascidiozooids is only a few millimeters. In the case when the connection between them is carried out only with the help of a stolon, ascidiozooids reach larger sizes, but usually smaller than solitary ascidians.

The development of ascidians, their asexual and sexual reproduction will be described below.

The structure of pyros. Pyrosomas, or firefishes, are free-swimming colonial pelagic tunicates. They got their name because of their ability to glow with bright phosphorescent light.

Of all the planktonic forms of tunicates, they are closest to ascidians. They are essentially colonial sea squirts floating in the water. Each colony consists of many hundreds of individual individuals - ascidiozooids, enclosed in a common, often very dense tunic (Fig. 175, A). Pyros have everything zooids equal and independent in terms of nutrition and reproduction. A colony is formed by the budding of individual individuals, and the buds get to their place, moving through the thickness of the tunic with the help of special wandering cells - forocytes. The colony has the shape of a long elongated cylinder with a pointed end, having a cavity inside and open at its wide rear end (Fig. 175, B). The outside of the pyrosome is covered with small soft subulate-shaped projections. Their most important difference from colonies of sessile ascidians is also the strict geometric regularity of the colony’s shape. Individual zooids stand perpendicular to the wall of the cone. Their mouth openings face outward, while their cloacal openings are located on the opposite side of the body and open into the cavity of the cone. Individual small ascidiozooids capture water with their mouths, which, passing through their body, enters the cavity of the cone. The movements of individual individuals are coordinated with each other, and this coordination of movements occurs mechanically in the absence of muscle, vascular or nervous connections. In the tunic, mechanical fibers are stretched from one individual to another, connecting their motor muscles. The contraction of the muscle of one individual pulls another individual with the help of the fibers of the tunic and transmits irritation to it. Contracting simultaneously, the small zooids push water through the colony cavity. At the same time, the entire colony, similar in shape to a rocket, having received a reverse push, moves forward. Thus, pyrosomes chose the principle of jet propulsion for themselves. This method of movement is used not only by pyrosomes, but also by other pelagic tunicates.

Tunic The pyrosome contains such a large amount of water (in some tunicates water makes up 99% of their body weight) that the entire colony becomes transparent, as if made of glass, and is almost invisible in the water. However, there are also colored pink color colonies. Such pyrosomes are gigantic in size - their length reaches 2.5 and even 4 m, and the diameter of the colony is 20-30 cm- repeatedly caught in Indian Ocean. Their name is Pyrosoma spinosum. The tunic of these pyrosomes has such a delicate consistency that, when caught in plankton nets, the colonies usually break up into separate pieces. Usually the sizes of pyrosomes are much smaller - from 3 to 10 cm long with a diameter from one to several centimeters. A new species of pyrosome, P. vitjasi, has recently been described. The colony of this species also has a cylindrical shape and sizes up to 47 cm. According to the author’s description, the insides of individual ascidiozooids are visible through the pinkish mantle as dark brown (or rather, dark pink in living specimens) inclusions. The mantle has a semi-liquid consistency, and if the surface layer is damaged, its substance spreads in the water in the form of viscous mucus, and individual zooids disintegrate freely.

Structure ascidiozooid the pyrosome is not much different from the structure of a solitary ascidian, except that its siphons are located on opposite sides of the body, and not brought together on the dorsal side (Fig. 175, B). The sizes of ascidiozooids are usually 3-4 mm, and for giant pyrosomes - up to 18 mm length. Their body can be laterally flattened or oval. The mouth opening is surrounded by a corolla of tentacles, or there may be only one tentacle present on the ventral side of the body. Often the mantle in front of the mouth opening, also on the ventral side, forms a small tubercle or a rather significant outgrowth. The mouth is followed by a large pharynx, cut through by gill slits, the number of which can reach 50. These slits are located either along or across the pharynx. Blood vessels run approximately perpendicular to the gill slits, the number of which also varies from one to three to four dozen. The pharynx has an endostyle and dorsal tongues hanging into its cavity. In addition, in the front part of the pharynx on the sides there are luminous organs, which are clusters of cellular masses. In some species, the cloacal siphon also has luminous organs. The luminescent organs of pyrosomes are populated by symbiotic luminous bacteria. Under the pharynx lies the nerve ganglion, and there is also a paranervous gland, the canal of which opens into the pharynx. The muscular system of pyrosome ascidiozooids is poorly developed. There are fairly well-defined circular muscles located around the oral siphon, and an open ring of muscles at the cloacal siphon. Small bundles of muscles - dorsal and abdominal - are located in the corresponding places of the pharynx and radiate along the sides of the body. In addition, there are a couple of cloacal muscles. Between the dorsal part of the pharynx and the body wall there are two hematopoietic organs, which are oblong clusters of cells. Reproducing by division, these cells turn into various elements of the blood - lymphocytes, amoebocytes, etc.

The digestive section of the intestine consists of the esophagus, which extends from the back of the throat, stomach and intestines. The intestine forms a loop and opens with the anus into the cloaca. On the ventral side of the body lies the heart, which is a thin-walled sac. There are testes and ovaries, the ducts of which also open into the cloaca, which can be more or less elongated and opens with a cloacal siphon into the general cavity of the colony. In the region of the heart, pyrosome ascidiozooids have a small finger-like appendage - the stolon. He plays important role during colony formation. As a result of the division of the stolon in the process of asexual reproduction, new individuals bud from it.

The structure of the salps. Like pyrosomes, salps are free-swimming animals and lead a pelagic lifestyle. They are divided into two groups: kegmakers, or doliolide(Gyclomyaria), and salp itself(Desmomyaria). These are completely transparent animals in the shape of a barrel or cucumber, at the opposite ends of which there are oral and anal openings - siphons. Only in some species of salps are certain parts of the body, such as the stolon and intestines, colored bluish-blue in living specimens. Their body is dressed in a delicate transparent tunic, sometimes equipped with outgrowths of different lengths. The small, usually greenish-brown intestine is clearly visible through the body walls. The size of the salps ranges from a few millimeters to several centimeters in length. The largest salp - "Thetys vagina - was caught in the Pacific Ocean. The length of its body (including appendages) was 33.3 cm.

The same types of salps are found either in solitary forms or in the form of long chain-like colonies. Such chains of salps are individual individuals connected to each other in a row. Connection between zooids in the colony, the salp, both anatomical and physiological, is extremely weak. The members of the chain seem to stick together with each other by attachment papillae, and essentially their coloniality and dependence on each other is barely expressed. Such chains can reach a length of more than one meter, but they are easily torn into pieces, sometimes simply when hit by a wave. Individuals and individuals that are members of a chain differ so greatly from each other in size and appearance that they were even described by older authors under different species names.

Representatives of another order - barrel worms, or doliolids - on the contrary, build extremely complex colonies. One of the leading modern zoologists, V.N. Beklemishev, called barrel lizards one of the most fantastic creatures in the sea. Unlike ascidians, in which the formation of colonies occurs due to budding, the formation of colonies in all salps is strictly related to the alternation of generations. Single salps are nothing more than emerging from eggs asexual individuals, which, budding, give rise to the colonial generation.

As already mentioned, the body of an individual, whether it is solitary or a member of a colony, is dressed in a thin transparent tunic. Under the tunic, like the hoops of a barrel, whitish ribbons of annular muscles are visible. They have 8 such rings. They encircle the body of the animal at a certain distance from each other. In barrel snails, the muscle bands form closed hoops, but in salps proper they do not close on the ventral side. Consistently contracting, the muscles push the water entering through the mouth through the animal’s body and push it out through the excretory siphon. Like pyrosomes, all salps move using a jet mode of movement.

IN squad doliolide the barrels are wide open at both ends (Fig. 176). At one end there is a mouth opening, at the opposite end there is an anus. Both openings are surrounded by sensitive tubercles. The inside of the barrel is divided by an oblique septum or dorsal process into two cavities. The anterior cavity is the pharynx, the posterior cavity is the cloaca. The mouth leads directly into a huge pharynx, which occupies almost the entire volume of the body. Unlike ascidians, the lateral walls of the pharynx of barrel snails are solid, and only the posterior wall, which separates the pharynx cavity from the cloaca, is pierced by two converging rows of gill slits. The slits connect the pharynx directly to the cloaca, and the special circumbranchial cavities that ascidians have are absent here. From them only one cloacal cavity remains. At the bottom of the pharynx there is an endostyle, and along the dorsal side, as in other tunicates we have examined, there is a longitudinal outgrowth - the dorsal plate. The endostyle leads from the pharynx to the intestine, very shortened, located on the abdominal part of the septum between the two cavities. The intestine consists of a short esophagus, which passes into a flask-shaped stomach, the back of which is adjacent to the digestive gland, and intestines. The intestine opens from the anus into the cloaca.

Nervous system consists of a cerebral ganglion located above the pharynx, from which nerves arise. The heart sac lies next to the stomach. Blood vessels depart from the heart, which, like all tunicates, form open lacunae arranged in an irregular network.

Like all tunicates, barrel tunicates are hermaphrodites. They have one ovary and one testis. The gonads lie on one side of the stomach and also open through ducts into the cloacal cavity. Only one large egg develops in the ovary at a time.

Excretory organs are missing. Probably, their function is performed by some blood cells in which yellowish-brown nodules are found. These nodules are transported by the bloodstream to the stomach area, where they are concentrated, then penetrate the intestine and are expelled from the body. In some salps, for example in Gyclosalpa, accumulations of ampoules of OFFENSIVE cells are found, very similar to those in ascidians. They are also located in the intestinal area and apparently play the role of storage buds. However, this has not yet been definitively established.

The body structure just described refers to the sexual generation of barrel worms. Asexual individuals do not have sexual gonads. They are characterized by the presence of two stolons. One of them, kidney-shaped, like in pyrosomes, is located on the ventral side of the body and is called the abdominal stolon; second stolon-dorsal.

Actually salps in their structure they are very similar to kegs and differ from them only in details (Fig. 177, A, B). In appearance, they are also transparent, cylindrical animals, through the walls of whose body the compact, usually olive-colored, stomach is clearly visible. The salp tunic can produce a variety of outgrowths, sometimes quite long in colonial forms. As has already been indicated, their muscle hoops are not closed, and their number may be greater than that of keg snails. In addition, the cloacal opening is somewhat shifted to the dorsal side, and does not lie directly at the posterior end of the body, as in barrel snails. The partition between the pharynx and the cloaca is pierced by only two gill slits, but these slits are enormous in size. And finally, the cerebral ganglion in salps is somewhat more developed than in barrel snails. In salps it has a spherical shape with a horseshoe-shaped cutout on the dorsal side. A rather complex pigmented eye is placed here.

Salps and kegworts have the ability to glow. Their luminescent organs are very similar to the luminescent organs of pyrosomes and are clusters of cells located on the ventral side in the intestinal area and containing symbiotic luminous bacteria. The organs of luminescence are especially strongly developed in species of the genus Cyclosalpa, which glow more intensely than other species. They develop so-called “lateral organs” located on the sides on each side of the body.

As has been repeatedly stated, salps are typical planktonic organisms. However, there is one very small group of peculiar benthic tunicates - Octacnemidae, numbering only four species. These are colorless animals up to 7 cm in diameter, living on seabed. Their body is covered with a thin translucent tunic, forming eight rather long tentacles around the mouth siphon. It is flattened and resembles an ascidian in appearance. But in terms of their internal structure, octacnemids are close to salps. In the zone of attachment to the substrate, the tunic produces thin hair-like outgrowths, but, apparently, these animals are weakly anchored in the ground and can swim above the bottom for short distances. Some scientists consider them to be a special, strongly deviated subclass of ascidians, while others are inclined to consider them as salps that have settled to the bottom for the second time. Octacnemidae are deep-sea animals found in the tropical Pacific Ocean and off the coast of Patagonia, as well as in the Atlantic Ocean south of Greenland, mainly at a depth of 2000-4000 thousand. m.

The structure of the appendiculars. Appendicuids are very small, transparent, free-swimming animals. Unlike other tunicates, they never form colonies. Their body sizes range from 0.3 to 2.5 cm. Appendicular larvae do not undergo regressive metamorphosis in their development, i.e., a simplification of the body structure and the loss of a number of important organs, such as the notochord and sensory organs, caused by the transformation of a free-swimming larva into a stationary adult form, as is the case in ascidians. The structure of the adult appendicular is very similar to that of the ascidian larva. As already mentioned, such an important feature of the structure of their body as the presence of a notochord, which puts all tunicates in the same group with chordates, is retained in appendiculars throughout their lives, and this is precisely what distinguishes them from all other tunicates, which in appearance are completely different from their closest relatives.

Body the appendicular divides into the body and tail (Fig. 178, A). The general appearance of the animal resembles a tadpole of frogs. The tail, the length of which is several times greater than the length of the animal’s rounded body, is attached to the ventral side in the form of a long thin plate. The appendicularia keeps it rotated 90° around its long axis and tucked on its ventral side. Along the middle of the tail along its entire length runs a notochord - an elastic cord consisting of a number of large cells. On the sides of the notochord there are 2 muscle bands, each of which is formed by only a dozen giant cells.

At the front end of the body lies a mouth leading into a voluminous pharynx (Fig. 178, B). The pharynx communicates directly with the external environment by two oblong-oval gill openings, or stigmas. There is no circumbranchial cavity with a cloaca, like in ascidians. An endostyle runs along the ventral side of the pharynx; on the opposite, dorsal side, a longitudinal dorsal process is noticeable. The endostyle drives lumps of food towards the digestive section of the intestine, which resembles a horseshoe-shaped curved tube and consists of the esophagus, a short stomach and a short hindgut, which opens outward through the anus on the ventral side of the body.

On the ventral side of the body, under the stomach, lies the heart. It has the shape of an oblong oval balloon, its dorsal side tightly fitting to the stomach. Blood vessels - abdominal and dorsal - go to the anterior part of the body from the heart. In the anterior part of the pharynx they are connected using a ring vessel. There is a system of lacunae through which, like blood vessels, blood circulates. In addition, a blood vessel also runs along the dorsal and ventral sides of the tail. The appendicular heart, like that of other tunicates, periodically changes the direction of blood flow, contracting for several minutes in one direction or the other. At the same time, it works very quickly, making up to 250 contractions per minute.

Nervous system consists of a large suprapharyngeal medullary ganglion, from which the dorsal nerve trunk extends backward, reaching the end of the tail and passing over the notochord. At the very base of the tail, the nerve trunk forms a swelling - a small nerve nodule. Several of the same nerve nodes, or ganglia, are present throughout the entire tail. A small organ of balance, the statocyst, is closely adjacent to the dorsal side of the cerebral ganglion, and there is a small fossa on the dorsal side of the pharynx. It is usually mistaken for the olfactory organ. The appendiculars have no other sensory organs. Special excretory organs are missing.

Appendiculars are hermaphrodites, they have both female and male genital organs. In the back of the body is the ovary, closely compressed on both sides by the testes. Sperm are released from the testes through holes on the dorsal side of the body, and the eggs enter the water only after the body walls have ruptured. Thus, after laying eggs, the appendiculars die.

All appendiculars build extremely characteristic houses, resulting from the isolation of their skin epithelium (Fig. 178, B). This house, somewhat pointed at the front - thick-walled, gelatinous and completely transparent - first fits closely to the body, and then lags behind it so that the animal can move freely inside the house. The house is tunic, but in appendiculars it does not contain cellulose, but consists of chitin, a substance similar in structure to horny. The house is equipped with several holes at the front and rear ends. Being inside, the appendicular makes wave-like movements with its tail, due to which a current of water is formed inside the house, and the water leaving the house forces it to move in the opposite direction. On the same side of the house into which he moves, there are two openings at the top, covered with a very fine lattice with long narrow slits. The width of these slots is 9-46 mk, and length 65-127 mk. The grate is a filter for food particles entering the house with water. The appendicular fish feed only on the smallest plankton that passes through the lattice openings. These are usually organisms ranging in size from 3-20 mk. Larger particles, crustaceans, radiolaria and diatoms, cannot penetrate inside the house.

The flow of water, having entered the house, enters a new lattice, shaped like a top and ending at the end with a bag-like channel, which the appendicular holds with its mouth. Bacteria, small flagellates, rhizomes and other organisms that have passed through the first filter collect at the bottom of the canal, and the appendix feeds on them, making occasional swallowing movements. But the thin front filter gets clogged quickly. In some species, such as Oikopleura rufescens, it stops working after just 4 hours. Then the appendicularity leaves the damaged house and produces a new one in its place. It only takes about 1 hour to build a new house, and again it begins to filter out the smallest nannoplankton. During its operation, the house manages to let through approximately 100 cm 3 waters. In order to leave the house, the appendicular uses the so-called “escape gate”. The wall of the house in one place is very thin and turned into a thin film. Having pierced it with a blow of its tail, the animal quickly leaves the house in order to immediately build a new one. The appendicular house is very easily destroyed by fixation or mechanical action, and it can only be seen in living organisms.

A characteristic feature of the appendicularis is constancy of cellular composition, i.e., the constancy of the number of cells from which the entire body of the animal is built. Moreover, different organs are also built from a certain number of cells. The same phenomenon is known for rotifers and nematodes. In rotifers, for example, the number of cell nuclei and especially their location are always constant for a particular species. One type consists of 900 cells, the other - of 959. This occurs as a result of the fact that each organ is formed from a small number of cells, after which the reproduction of cells in it stops for life. In nematodes, not all organs have a constant cellular composition, but only the muscles, nervous system, hindgut and some others. The number of cells in them is small, but the size of the cells can be huge.

Reproduction and development of tunicates. The reproduction of tunicates provides an amazing example of how incredibly complex and fantastic life cycles may exist in nature. All tunicates, except the appendicular, are characterized by both sexual, so asexual method of reproduction. In the first case, a new organism is formed from a fertilized egg. But in tunicates, development to an adult occurs with profound transformations in the structure of the larva towards its significant simplification. With asexual reproduction, new organisms seem to bud off from the mother, receiving from her the rudiments of all the main organs.

All sexual specimens of tunicates are hermaphrodites, i.e. they have both male and female gonads. The maturation of male and female reproductive products always occurs at different times, and therefore self-fertilization is impossible. We already know that in ascidians, salps and pyrosomes, the ducts of the gonads open into the cloacal cavity, and in the appendicular, sperm enter the water through ducts that open on the dorsal side of the body, while eggs can come out only after the walls have ruptured, which leads to death animal. Fertilization in tunicates, except for salps and pyrosomes, is external. This means that the sperm meets the egg in the water and fertilizes it there. In salpas and pyrosomes, only one egg is formed, which is fertilized and develops in the mother’s body. In some ascidians, fertilization of eggs also occurs in the mother's cloacal cavity, where the sperm of other individuals penetrate with the flow of water through siphons, and the fertilized eggs are excreted through the anal siphon. Sometimes the embryos develop in the cloaca and only then come out, i.e., a kind of viviparity takes place.

Reproduction and development of the appendicular. In appendicularis, viviparity is unknown. Layed egg (about 0.1 mm in diameter) begins to crush entirely, and at first the crushing proceeds evenly. All stages of its embryonic development - blastula, gastrula etc. - the appendiculars pass very quickly, and as a result a massive embryo develops. It already has a body with a pharyngeal cavity and a brain vesicle and a caudal appendage, in which 20 notochord cells are arranged in a row one after another. Muscle cells are adjacent to them. Then from four cells it is formed and neural tube, lying along the entire tail above chord.

At this stage, the larva leaves the egg shell. It is still very little developed, but at the same time it has the rudiments of all organs. The digestive cavity is rudimentary. There is no mouth or anus, but the brain vesicle with the statocyst - the organ of balance - is already developed. The tail of the larva is located in continuation of the anterior-posterior axis of its body, and its right and left sides face right and left, respectively.

This is followed by the transformation of the larva into an adult appendicular. An intestinal loop forms and grows toward the abdominal wall of the body, where it opens outward at the anus. At the same time, the pharynx grows forward, reaches the outer surface and breaks through the oral opening. Bronchial tubes are formed, which open on both sides of the body with gill openings outward and also connect the pharyngeal cavity with the external environment. The development of the digestive loop is accompanied by the pushing of the tail from the very end of the body to its ventral side. At the same time, the tail rotates around its axis 90° to the left, so that its dorsal ridge is on the left side, and the right and left sides of the tail are now facing up and down. The neural tube extends into a neural cord, nerve nodules form, and the larva develops into an adult appendix.

All development and metamorphosis of appendicular larvae is characterized by high speed all processes occurring during this development. The larva hatches from the egg before its formation is complete. This rate of development is not caused each time by the influence of some external causes. It is determined by the internal nature of these animals and is hereditary.

As we will see later, adult appendiculars are very similar in structure to ascidian larvae. Only some structural details distinguish them from each other. There is a point of view that the appendiculars remain at the larval stage of development all their lives, but their larva has acquired the ability of sexual reproduction. This phenomenon is known in science as neoteny. A widely known example is the amphibian Ambystoma, the larvae of which, called axolotls, are capable of sexual reproduction. Living in captivity, axolotls never turn into ambists. They have gills and a caudal fin and live in water, reproducing well and producing offspring similar to themselves. But if they are fed a thyroid drug, axolotls complete the metamorphosis, lose their gills and, when they come onto land, turn into adult ambistos. Neoteny has also been observed in other amphibians - newts, frogs, and toads. Among invertebrate animals, it is found in some worms, crustaceans, spiders and insects.

Sexual reproduction in the larval stages is sometimes beneficial for animals. Neoteny may not occur in all individuals of a given species, but only in those that live in special, perhaps unfavorable conditions for them, for example, at low temperatures. The result is the possibility of reproduction in an unusual environment. In this case, the animal does not expend much energy to complete the complete transformation of the larva into an adult and the rate of maturation increases.

Neoteny probably played a large role in the evolution of animals. One of the most serious theories about the origin of the entire trunk of deuterostome animals - Deuterostomia, which includes all chordates, including vertebrates, derives them from free-swimming coelenterate ctenophores or ctenophores. Some scientists believe that the ancestors of the coelenterates were sessile forms, and ctenophores originated from the floating larvae of the oldest coelenterates, which acquired the ability of sexual reproduction as a result of progressive neoteny.

Reproduction and development of ascidians. The development of ascidians occurs in a more complex way. When the larva emerges from the egg shell, it is quite similar to the adult appendicular (Fig. 179, A). It burned like the appendicular, and resembles in appearance a tadpole, the elongated oval body of which is somewhat laterally compressed. The tail is elongated and surrounded by a thin fin. A chord runs along the axis of the tail. The nervous system of the larva is formed by a neural tube that lies above the notochord in the tail and forms a brain vesicle with a statocyst at the anterior end of the body. Unlike appendiculars, ascidians also have a pigmented ocellus that can react to light. On the front part of the dorsal side there is a mouth leading into the pharynx, the walls of which are penetrated by several rows of gill slits. But, unlike the appendiculars, the gill slits even in ascidian larvae do not open directly outward, but into a special peribranchial cavity, the rudiments of which in the form of two sacs invaginating from the surface of the body are clearly visible on each side of the body. They are called nonribranchial invaginations. At the anterior end of the larval body three sticky attachment papillae are visible.

At first, the larvae swim freely in the water, moving with the help of their tail. Their body sizes reach one or several millimeters. Special observations have shown that the larvae float in water for a short time - 6-8 hours. During this time they can cover distances of up to 1 km, although most of them settle to the bottom relatively close to their parents. However, even in this case, the presence of a free-swimming larva promotes the dispersal of immobile ascidians over considerable distances and helps them spread throughout all seas and oceans.

Settled to the bottom, the larva attaches itself to various solid objects using its sticky papillae. Thus, the larva sits down with the anterior end of the body, and from this moment it begins to lead a motionless, attached lifestyle. In this regard, a radical restructuring and significant simplification of the body structure occurs (Fig. 179, B-G). The tail, together with the chord, gradually disappears. The body takes on a bag-like shape. The statocyst and eye disappear, and instead of the brain vesicle, only the nerve ganglion and perinnervous gland remain. Both peribranchial invaginations begin to grow strongly on the sides of the pharynx and surround it. The two openings of these cavities gradually approach each other and finally merge on the dorsal side into one cloacal opening. The newly formed gill slits open into this cavity. The intestine also opens into the cloaca.

By sitting on the bottom with its front part, on which the mouth is located, the ascidian larva finds itself in a very disadvantageous position in terms of capturing food. Therefore, the settled larva undergoes another important change in the general plan of the body structure: its mouth begins to slowly move from bottom to top and is eventually located at the very upper end of the body (Fig. 179, D-G). The movement occurs along the dorsal side of the animal and entails the displacement of all internal organs. The moving pharynx pushes the cerebral ganglion in front of it, which eventually lies on the dorsal side of the body between the mouth and the cloaca. This ends the transformation, as a result of which the animal turns out to be completely different in appearance from its own larva.

The ascidian formed in this way can also reproduce in another, asexual way, through budding. In the simplest case, a sausage-shaped protrusion grows from the ventral side of the body at its base, or kidney stolon(Fig. 180). This stolon is surrounded by the outer covering of the body of ascidians (ectoderm), the body cavity of the animal continues into it and, in addition, the blind protrusion of the posterior part of the pharynx. The heart also gives off a long process into the stolon. Thus, the rudiments of the most important organ systems enter the kidney stolon. On the surface of the stolon, small tubercles, or buds, are formed, into which all the organ rudiments listed above also give off their processes. Through complex rearrangements, these rudiments form new kidney organs. A new intestine develops from the outgrowth of the pharynx, and a new heart sac develops from the cardiac outgrowth. A mouth opening breaks through the integument of the body of the kidney. By invaginating the ectoderm from the outside inwards, a cloaca and peribranchial cavities are formed. In solitary forms, such a bud, growing, breaks away from the stolon and gives rise to a new solitary ascidian, while in colonial forms, the bud remains sitting on the stolon, grows, begins to bud again, and eventually a new colony of ascidians is formed. It is interesting that the buds of colonial forms with a common gelatinous tunica are always separated within it, but do not remain in the place where they were formed, but move through the thickness of the tunica to their final place. Their kidney always makes its way to the surface of the tunic, where its oral and anal openings open. In some species, these openings open independently of the openings of other kidneys; in others, only one mouth opens outward, while the cloacal opening opens into a cloaca common to several zooids (Fig. 174, B). Sometimes this can create long channels. In many species, zooids form a tight circle around a common cloaca, and those that do not fit into it are pushed away and give rise to a new circle of zooids and a new cloaca. Such a cluster of zooids forms the so-called cormidium.

Sometimes such cormidia are very complex and even have a common colonial vascular system. The cormidium is surrounded by a ring blood vessel into which two vessels flow from each zooid. In addition, such vascular systems of individual cormidia communicate with each other, and a complex colony-wide circulatory vascular system arises, so that all ascidiozooids are interconnected. As we see, the connection between individual members of colonies in different complex ascidians can be either very simple, when individual individuals are completely independent and are only immersed in a common tunic, and the kidneys also have the ability to move in it, or complex, with a single circulatory system. system.

Except budding through the stolon, other types of budding are also possible - the so-called mantle, pyloric, post-abdominal, - depending on those parts of the body that gave rise to the kidney. With mantle budding, the bud appears as a lateral protrusion of the body wall in the pharynx area. It consists of only two layers: the outer one - the ectoderm and the inner one - the outgrowth of the periobranchial cavity, from which all the organs of the new organism are subsequently formed. As on the stolon, the bud gradually becomes rounded and is separated from the mother by a thin constriction, which then turns into a stalk. Such budding begins at the larval stage and is especially accelerated after the larva settles on the bottom. The larva that gives rise to the bud (in this case called an oozooid) dies, and the developing bud (or blastozoid) gives rise to a new colony. In other ascidians, a bud is formed on the ventral surface of the intestinal part of the body, also very early, when the larva has not yet hatched. In this case, the kidney, covered with epidermis, includes branches of the lower end of the epicardium, i.e., the outer wall of the heart. The primary bud elongates, is divided into 4-5 parts, each of which turns into an independent organism, and the larva - oozooid - which gave rise to these buds, disintegrates and serves as a nutritional mass for them. Sometimes the kidney may include parts of the digestive system of the stomach and hindgut. This method of budding is called pyloric. Interestingly, in this very complex case of budding, the entire organism arises from the fusion of two buds into one. For example, in Trididemnum the first kidney includes outgrowths of the esophagus, and the second - outgrowths of the epicardium. After both kidneys merge, the esophagus, stomach and intestines of the daughter organism, as well as the heart, are formed from the first, and the pharynx, penetrated by gills, and the nervous system are formed from the second. After this, the daughter organism, which already has full set organs, is detached from the maternal one. However, other parts of the body can give rise to a kidney. In some cases, even the outgrowths of the larval notochord can enter the kidney and from them the nervous system and gonads of the daughter individual are formed. Sometimes the processes of budding are so similar to the simple division of an organism into parts that it is difficult to say what method of reproduction is present in this case. At the same time, the intestinal section of the body is greatly lengthened, nutrients accumulate in it, which are obtained as a result of the breakdown of the thoracic section. The abdominal section then divides into several fragments, usually called buds, from which new individuals arise. In Amaroucium, soon after the larva attaches, a long outgrowth forms at the posterior end of its body. It increases in size, and ascidians as a result of this strongly develop the posterior part of the body - the post-abdomen, into which the heart is displaced. When the length of the post-abdomen begins to greatly exceed the length of the larval body, it is separated from the maternal individual and divided into 3-4 parts, from which young buds - blastozoids - are formed. They move forward from the post-abdomen and are located next to the maternal body, in which the heart is formed anew. The development of blastozooids occurs unevenly, and when some of them have already completed it, others are just beginning to develop.

The processes of budding in ascidians are extremely diverse. Sometimes even closely related species of the same genus have different budding methods. Some ascidians are able to form dormant buds that have stopped developing, which allows them to survive unfavorable conditions.

During budding in ascidians, the following interesting phenomenon is observed. As is known, in the process of embryonic development, various organs of the animal’s body arise from different, but completely defined parts of the embryo (germ layers) or layers of the body of the embryo that make up its wall at the very first stages of development.

Most organisms have three germ layers: the outer or ectoderm, the inner or endoderm, and the middle or mesoderm. In the embryo, the ectoderm covers the body, and the endoderm lines the internal intestinal cavity and provides its nutrition. The mesoderm mediates the connection between them. In the process of development, as a general rule, the nervous system, skin, and in ascidians the peribranch sacs are formed from the ectoderm; the digestive system and respiratory organs are formed from the endoderm; muscles, skeleton and genital organs are formed from the mesoderm. With different methods of budding in ascidians, this rule is violated. For example, during mantle budding, all internal organs (including the stomach and intestines, arising from the endoderm of the embryo) give rise to the outgrowth of the peribranchial cavity, which is an ectoderm formation in origin. And vice versa, in the case when the kidney includes an outgrowth of the epicardium (and the ascidian heart is formed as an outgrowth of the endoderm pharynx during embryonic development), most of the internal organs, including the nervous system and peribranchial sacs, are formed as a derivative of the endoderm.

Reproduction and development of pyrosomes. Pyrosomes also have an asexual mode of reproduction by budding. But in them, budding occurs with the participation of a special permanent outgrowth of the body - the bud stolon. It is also characterized by the fact that it occurs at very early stages of development. Pyrosom eggs are very large, up to 0.7 mm and even up to 2.5 mm, and rich in yolk. In the process of their development, the first individual is formed - the so-called cyathozoid. The cyathozoid corresponds to the oozooid of ascidians, i.e. it is an asexual maternal individual that developed from an egg. It stops developing very early and collapses. The entire main part of the egg is occupied by the nutritious yolk, on which the cyathozoid develops.

In the recently described species Pyrosoma vitjazi, a cyathozoid is located on the yolk mass, which is a fully developed ascidian with an average size of about 1 mm(Fig. 181, A). There is even a small mouth opening that opens outwards under the egg membrane. The pharynx contains 10-13 pairs of gill slits and 4-5 pairs of blood vessels. The intestine is fully formed and opens into the cloaca, a siphon that has the shape of a wide funnel. There is also a nerve ganglion with a paranervous gland and a heart that pulsates vigorously. By the way, all this speaks about the origin of pyrosomes from ascidians. In other species, during the period of maximum development of the cyathozoid, one can distinguish only the rudiments of the pharynx with two gill slits, the rudiments of two peribranchial cavities, the cloacal siphon, the nervous ganglion with the perinnervous gland, and the heart. The mouth and digestive section of the intestine are absent, although an endostyle is visible. A cloaca with a wide opening is also developed, opening into the space under the egg membranes. At this stage, even in the egg shell of the pyrosomes, the processes of asexual development already begin. At the posterior end of the cyathozoid, a stolon is formed - the ectoderm gives rise to an outgrowth into which continuations of the endostyle, pericardial sac and peribranchial cavities enter. From the ectoderm of the stolon in the future kidney, a nerve cord arises, independent of the nervous system of the cyathozoid itself. At this time, the stolon is divided by transverse constrictions into four sections, from which the first blastozooid buds develop, which are already members of the new colony, i.e. ascidiozooids. The stolon gradually becomes transverse to the axis of the body of the cyathozoid and the yolk and twists around them (Fig. 181, B-E). Moreover, each kidney becomes perpendicular to the axis of the body of the cyathozoid. As the buds develop, the maternal individual - the cyathozoid - is destroyed, and the yolk mass is gradually used as food for the first four ascidiozooid buds - the ancestors of the new colony. The four primary ascidiozooids assume a geometrically regular cruciform position and form a common cloacal cavity. This is a real small colony (Fig. 181, E-G). In this form, the colony leaves the mother’s body and is freed from the egg shell. The primary ascidiozooids, in turn, form stolons at their posterior ends, which, laced together, give rise to secondary ascidiozooids, etc. As soon as the ascidiozooid is isolated, a new stolon is formed at its end and each stolon forms a chain of four new buds. The colony is growing progressively. Each ascidiozooid becomes sexually mature and has male and female gonads.

In one group of pyrosomes, the ascidiozooids retain contact with the mother and remain in the place where they originated. During the formation of buds, the stolon lengthens and the buds are connected to each other by cords. Ascidiozooids are located one after another towards the closed, anterior end of the colony, while the primary ascidiozooids move towards its rear, open part.

In another group of pyrosomes, which includes most of their species, the buds do not remain in place. Once they reach a certain stage of development, they separate from the stolon, which never elongates. At the same time, they are picked up by special cells - phorocytes. Forocytes are large, amoeba-like cells. They have the ability to move through the thickness of the tunic with the help of their pseudopods, or pseudopodia, just as amoebas do. Picking up a bud, the phorocytes carry it through the tunica covering the colony to a strictly defined place under the primary ascidiozooids, and, as soon as the final ascidiozooid breaks away from the stolon, the phorocytes carry it along the left side to the dorsal part of its producer, where it is finally installed in such a way that old ascidiozooids move further and further to the top of the colony, and young ones find themselves at its rear end.

Each new generation of ascidiozooids is transferred with geometric regularity to a strictly defined place in relation to the previous generation and is arranged in floors (Fig. 181, 3). After the formation of the first three floors, secondary, then tertiary, etc. floors begin to appear between them. The primary floors have 8 ascidiozooids, the secondary floors have 16, the tertiary floors have 32, etc. in geometric progression. The diameter of the colony increases. However, as the colony grows, the clarity of these processes is disrupted; some ascidiozooids get confused and end up on other floors. The same individuals in a pyrosome colony that reproduced by budding subsequently develop gonads and begin sexual reproduction. As we already know, each of the many ascidiozooid pyrosomes formed by budding develops only one large egg.

According to the method of formation of colonies, namely, whether the ascidiozooids maintain a connection with the maternal organism for a long time or not, pyrosomes are divided into two groups - Pyrosoma fixata and Pyrosoma ambulata. The former are considered more primitive, since the transfer of buds by forocytes is more complex and a later acquisition of pyrosomes.

The formation of a primary colony of four members was considered so constant for pyrosomes that this feature was even included in the characteristics of all squad Pyrosomida. However, recently new data on the development of pyrosomes have been obtained. It turned out, for example, that in Pyrosoma vitjazi the bud stolon can reach a very large length, and the number of buds simultaneously formed on it is about 100. Such a stolon forms irregular loops under the egg shell (Fig. 181, A). Unfortunately, it is still unknown how their colony is formed.

Reproduction and development of barrel snails and salps. In barrel breams, the reproduction processes are even more complex and interesting. From the egg they develop a tailed larva, like ascidians, which has a notochord in the caudal section (Fig. 182, A). However, the tail soon disappears, and the body of the larva grows greatly and turns into an adult barrel worm, which in its structure is noticeably different from the sexual individual that we described above. Instead of eight muscular hoops, it has nine, there is a small sac-like organ of balance - a statocyst, and gill slits are half as large as in the sexual individual. It completely lacks gonads and, finally, in the middle of the ventral side of the body and on the dorsal side of its posterior end, two special outgrowths develop - stolons (Fig. 182, B). This asexual individual has a special name - feeder. The filamentous abdominal stolon of the nurse, which is the kidney-bearing stolon, contains outgrowths of many organs of the animal - a continuation of the body cavity, pharynx, heart, etc. - a total of eight different primordia. This stolon very early begins to bud tiny primary buds, or so-called preferences. At this time, at its base there is a crowd of large forocytes already familiar to us. Forocytes, in twos or threes, pick up the buds and carry them first along the right side of the feeder, and then along its dorsal side to the dorsal stolon (Fig. 182, C, D). If the kidneys go astray, they die. While the kidneys move and move to the dorsal stolon, they continue to divide all the time. It turns out that buds formed on the abdominal stolon cannot develop and live on it.

The first portions of the buds are seated as phorocytes on the dorsal stolon in two lateral rows on its dorsal side. These lateral buds very quickly develop here into small spoon-shaped barrels with a huge mouth, well-developed gills and intestines (Fig. 182, E). Their other organs atrophy. They are attached to the dorsal stolon of the feeder by their own dorsal stolon, which has the shape of a process. The dorsal stolon of the nurse grows strongly at this time - it lengthens and expands. Eventually it can reach 20-40 cm length. It is a long outgrowth of the body into which two large blood feeding lacunae enter.

Meanwhile, more and more phorocytes with buds are creeping up, but now these buds are no longer seated on the sides, but in the middle of the stolon, between the two rows of the individuals described above. These buds are called median or phorozoids. They are smaller than the lateral ones, and from them barrels develop, similar to sexually mature individuals, but without sexual gonads. These barrel plants grow to the stolon of the feeder with a special thin stalk.

All this time, the feeder supplies the entire colony with nutrients. They enter here through the blood lacunae of the dorsal stolon and through the stalks of the kidneys. But gradually the feeder is depleted. It turns into an empty muscular barrel, which serves only to move the already significant colony formed on the dorsal stolon.

More and more new buds continue to move along the surface of this barrel, which the abdominal stolon continues to form. From the moment the feeder turns into an empty bag, its role in feeding the colony is taken on by large-mouthed lateral individuals, which are called gastrozoids(feeding zooids). They capture and digest food. The nutrients they absorb are not only used by themselves, but also transferred to the middle kidneys. And forocytes still bring new generations of buds to the dorsal stolon. Now these buds are no longer seated on the stolon itself, but on those stalks that attach the middle buds (Fig. 182, E). It is these buds that turn into real sex barrels. After the sexual preference has strengthened on the stalk of the median bud, or phorozoid, it breaks off together with its stalk from the common stolon and becomes a free-swimming small independent colony (Fig. 182, G). The task of the phorozoid is to ensure the development of the sexual preference. It is sometimes called a second-order feeder. During the free period of life of the forozoids, the reproductive bud, settled on its stalk, is divided into several gonozoid reproductive buds. Each such bud grows into a typical sexual barrel, which has already been described in the previous part. Having reached maturity, the gonozoids, in turn, separate from their phorozoid and begin to lead the life of independent solitary barrel-makers, capable of sexual reproduction. It must be said that in both gastrozoids and phorozoids, gonads are also formed during their development, but then they disappear. These individuals only help the development of the third true sexual generation.

After all the middle buds are torn off from the dorsal stolon of the nurse, the nurse, along with the lateral buds, dies. The number of individuals produced on one feeder is extremely large. It is equal to several tens of thousands.

As we can see, the development cycle of barrel clams is extremely complex and is characterized by an alternation of sexual and asexual generations. Its brief diagram is as follows: 1. The sexual individual develops on the abdominal stalk of the phorozoid. 2. The sexual individual lays eggs, and as a result of their development, an asexual tailed larva is obtained. 3. An asexual nurse directly develops from the larva. 4. A generation of asexual lateral gastrozoids develops on the dorsal stolon of the nurse. 5. New generation of asexual median phorozoids. 6. The appearance and development of sexual gonozoids on the abdominal stolon of the phorozoid separated from the nurse. 7. Formation of a sexual individual from a gonozoid. 8. Laying eggs.

In development salp there is also a change of generations, but it is not as amazingly complex as with the barrel-makers. Salp larvae do not have a tail containing a notochord. Developing from a single egg in the mother’s body, in her cloacal cavity, the salp embryo comes into close contact with the walls of the mother’s ovary, through which nutrients are supplied to it. This junction of the fetal body with the tissues of the mother is called the baby's place or placenta. There is no free-living larval stage in salps, and their embryo has only a rudiment (a remnant that has not received full development) of the tail and notochord. This is the so-called eleoblast, consisting of large fat-rich cells (Fig. 183, A). A newly developed embryo, essentially still an embryo, released through the cloacal siphon into the water, has a small kidney stolon on the ventral side near the heart and between the rest of the placenta and the eleoblast. In adult forms, the stolon reaches a considerable length and is usually spirally twisted. This single salp is also the same nurse as the barrelworm formed from the larva (Fig. 183, B). On the stolon, numerous buds are formed from the lateral thickenings, located in two parallel rows. Usually, a certain section of the stolon is first captured by budding, giving rise to a certain number of buds of the same age. Their number varies - in different species from several units to several hundred. Then the second section begins to bud, the third, etc. All buds - blastozoids - of each individual section or link develop simultaneously and are equal in size. While in the first section they already achieve significant development, the blastozoids of the second section are much less developed, etc., and in the last section of the stolon the buds are just emerging (Fig. 183, B).

During their development, blastozoids undergo rearrangement while remaining tied friend with my friend Stolon. Each pair of zooids acquires a certain position in relation to the other pair. It turns its free ends in opposite directions. In addition, in each individual, as in ascidians, a displacement of organs occurs, leading to a change in their original relative position. All the substance of the stolon is used for the formation of kidneys. In salps, all kidney development occurs on the ventral stolon, and they do not require a special dorsal stolon. The buds come off from it not individually, but in whole chains, according to how they arose, and form temporary colonies (Fig. 183, D). All individuals in them are absolutely equal, and each develops into a sexually mature animal.

It is interesting that, while the neural tube, sex cord, peribranchial cavities, etc. have already divided in different individuals, the pharynx remains common within the same chain. Thus, the members of the chain are first organically connected to each other by the stolon. But the detached mature sections of the chain consist of individuals connected to each other only by adherent attachment papillae. Each individual has eight such suckers, which determines the connection of the entire colony. This connection is both anatomically and physiologically extremely weak. -

The coloniality of such chains is essentially barely expressed. Linearly elongated chains - colonies of salps - can consist of hundreds of individual individuals. However, in some species the colonies may be ring-shaped. In this case, individuals are connected to each other by tunic outgrowths directed, like spokes in a wheel, towards the center of the ring along which the members of the colony are located. Such colonies consist of only a few members: in Cyclosalpa pinnata, for example, eight to nine individuals (Table 29).

If we now compare the methods of asexual reproduction of different tunicates, then, despite the great complexity and heterogeneity of this process in different groups, we cannot help but notice common features. Namely: in all of them, the most common method of reproduction is the division of the bud stolon into a larger or smaller number of sections, giving rise to individual individuals. Such stolons are found in ascidians, pyrosomes, and salps.

Colonies of all tunicates arise as a consequence of the asexual method of reproduction. But if in ascidians they appear simply as a result of budding and each zooid in the colony can develop both asexually and sexually, then in pyrosomes and especially in salps their appearance is associated with a complex alternation of sexual and asexual generations.

Tunicate lifestyle. Let's now see how different tunicates live and what practical significance they have. We have already said above that some of them live at the bottom of the sea, and some in the water column. Ascidians are bottom-dwelling animals. Adult forms spend their entire lives motionless, attached to some solid object at the bottom and driving water through their gill-pierced throat to filter out the smallest cells of phytoplankton or small animals and particles of organic matter on which ascidians feed. They cannot move, and only by being frightened by something or swallowing something too large can the ascidian shrink into a ball. In this case, water is forcefully thrown out of the siphon.

As a rule, ascidians simply adhere to stones or other hard objects with the lower part of their tunic. But sometimes their body can rise above the ground surface on a thin stalk. This device allows animals to “catch” a larger volume of water and not drown in soft ground. It is especially characteristic of deep-sea ascidians, which live on the thin mud that covers the ocean floor at great depths. In order not to sink in the ground, they may also have another device. The tunic processes, which normally attach ascidians to stones, grow and form a kind of “parachute” that holds the animal on the surface of the bottom. Such “parachutes” can also appear in typical inhabitants of hard soils, usually settling on rocks, during their transition to life on soft muddy soils. Root-like outgrowths of the body allow individuals of the same species to enter a new and unusual habitat and expand the boundaries of their range if other conditions are favorable for their development.

Recently, ascidians have been discovered among a very specific fauna that inhabits the thinnest passages between grains of sand. Such fauna is called interstitial. Now seven species of ascidians are already known that have chosen such an unusual biotope as their habitat. These are extremely small animals - their body size is only 0.8-2 mm in diameter. Some of them are movable.

Single ascidians sometimes form large aggregations, which grow into entire druses and settle in large clusters. As already mentioned, many species of ascidians are colonial. More often than others, massive gelatinous colonies are found, the individual members of which are immersed in a common rather thick tunic. Such colonies form crusty growths on stones or are found in the form of peculiar balls, cakes and outgrowths on legs, sometimes resembling mushrooms in shape. In other cases, individual colonies can be almost independent.

Some ascidians, such as Claveiina, have the ability to easily restore, or regenerate, their body from different parts. Each of the three body sections of the colonial clavelin - the thoracic region with the gill basket, the body section containing the entrails, and the stolon - when cut out, can recreate an entire ascidian. It is amazing that even from a stolon a whole organism grows with siphons, all the internal organs and a nerve ganglion. If you isolate a piece of the gill basket from a clavelin by simultaneously making two transverse cuts, then at the anterior end of the animal fragment, which has turned into a rounded lump, a new pharynx with gill slits and siphons is formed, and at the rear - a stolon. If you first make a cut in the back and then in the front, then amazingly a pharynx with siphons is formed at the posterior end, and a siphon at the anterior end and the anteroposterior axis of the animal’s body rotates 180°. Some ascidians are capable, in certain cases, of discarding parts of their body themselves, i.e. they are capable of autotomy. And just as a lizard’s severed tail grows back, a new ascidian grows from the remaining piece of its body. The ability of ascidians to restore lost body parts is especially pronounced in adulthood in those species that can reproduce by budding. Species that reproduce only sexually, for example the solitary Ciona intestinalis, have a much less regenerative ability.

The processes of regeneration and asexual reproduction have many similarities, and, for example, Charles Darwin argued that these processes have a single basis. The ability to restore lost body parts is especially highly developed in protozoa, coelenterates, worms and tunicates, i.e. in those groups of animals that are especially characterized by asexual reproduction. And in a sense, asexual reproduction itself can be considered as the ability to regenerate from a fragment of the body, manifested in natural conditions of existence and localized in certain parts of the animal’s body.

Ascidians are widespread in both cold and warm seas. They are found in both the Arctic Ocean and Antarctica. They were even found directly on the coast of Antarctica during the examination by Soviet scientists of one of the fiords of the Banger “oasis”. The fiord was fenced off from the sea by piles of multi-year ice, and its surface water was highly desalinated. On the rocky and lifeless bottom of this fiord, only lumps of diatoms and threads of green algae were found. However, in the very depths of the bay, the remains of a starfish and a large number of large ones, up to 14, were found. cm long, pinkish-transparent gelatinous ascidians. The animals were torn from the bottom, probably by a storm, and washed there by the current, but their stomachs and intestines were completely filled with green mass of somewhat digested phytoplankton. They were probably feeding shortly before they were caught from the water close to the shore.

Ascidians are especially diverse in the tropical zone. There is evidence that the number of tunicate species in the tropics is approximately 10 times greater than in temperate and polar regions. Interestingly, ascidians in cold seas are much larger than in warm seas, and their settlements are more numerous. They, like other marine animals, obey the general rule according to which fewer species live in temperate and cold seas, but they form significantly larger settlements and their biomass is 1 m 2 the bottom surface is many times larger than in the tropics.

Most ascidians live in the most superficial littoral or tidal zone of the ocean and in the upper horizons of the continental shallows or subtidal zone to a depth of 200 m. With increasing depth, the total number of species decreases. Currently deeper than 2000 m 56 species of ascidians are known. The maximum depth of their habitat at which these animals were found is 7230 m. At this depth, ascidians were discovered during the work of the Soviet oceanographic expedition on the ship "Vityaz" in the Pacific Ocean. These were representatives of the characteristic deep-sea genus Culeolus. The round body of this ascidian with very wide open siphons that do not protrude at all above the surface of the tunic, sits at the end of a long and thin stalk, with which the culeolus can be attached to small pebbles, spicules of glass sponges and other objects at the bottom. The stalk cannot support the weight of a rather large body, and it probably floats, oscillating above the bottom, carried away by a weak current. Its color is whitish-gray, colorless, like most deep-sea animals (Fig. 184).

Ascidians avoid desalinated areas of seas and oceans. The vast majority of them live at normal ocean salinity of about 35 0/00.

As already mentioned, the largest number of ascidian species live in the ocean at shallow depths. Here they form the most massive settlements, especially where there are enough suspended particles in the water column - plankton and detritus - that serve as food for them. Ascidians settle not only on stones and other hard natural objects. Favorite places for their settlement are also the bottoms of ships, the surface of various underwater structures, etc. Sometimes settling in huge quantities along with other fouling organisms, ascidians can cause great harm to the economy. It is known, for example, that, settling on the inner walls of water pipelines, they develop in such numbers that they greatly narrow the diameter of the pipes and clog them. When they die off en masse in certain seasons of the year, they clog filtration devices so much that the water supply can stop completely and industrial enterprises suffer significant damage.

One of the most widespread sea squirts, Ciona intestinalis, grows on the bottoms of ships and can settle in such huge numbers that the speed of the ship is significantly reduced. The losses of transport shipping as a result of fouling are very large and can amount to millions of rubles per year.

However, the ability of ascidians to form mass aggregations due to one of their amazing features may be of some interest to people. The fact is that instead of iron, the blood of ascidians contains vanadium, which performs the same role as iron - it serves to transport oxygen.

Vanadium, a rare element of great practical importance, is dissolved in sea water in extremely small quantities. Ascidians have the ability to concentrate it in their body. The amount of vanadium is 0.04-0.7% by weight of animal ash. It should also be remembered that the tunic of ascidians also contains another valuable substance - cellulose. Its quantity, for example, in one copy of the most widespread species Ciona intestinalis is 2-3 mg. These ascidians sometimes settle in huge numbers. Number of individuals per 1 m 2 surfaces reach 2500-10,000 copies, and their wet weight is 140 kg by 1 m 2 .

There is an opportunity to discuss how ascidians can be practically used as a source of these substances. The wood from which cellulose is extracted is not available everywhere, and vanadium deposits are few and scattered. If you set up underwater “sea gardens,” then ascidians can be grown in huge quantities on special plates. It is estimated that from 1 ha sea area can be obtained from 5 to 30 kg vanadium and from 50 to 300 kg cellulose.

Pelagic tunicates live in the ocean water column - appendicularia, pyrosomes and salps. This is a world of transparent fantastic creatures that live mainly in warm seas and in the tropical zone of the ocean. Most of their species are so strongly confined in their distribution to warm waters that they can serve as indicators of changes in hydrological conditions in different areas of the ocean. For example, the appearance or disappearance of pelagic tunicates, in particular salps, in the North Sea in certain periods is associated with a greater or lesser supply of warm Atlantic waters to these areas. The same phenomenon was repeatedly observed in the area of Iceland, the English Channel, near the Newfoundland Peninsula and was associated with both monthly and seasonal changes in the distribution of warm Atlantic and cold Arctic waters. Only three species of salps enter these areas - Salpa fusiformis, Jhlea asymmetrica and the most widespread in the ocean, Thalia democratica. The appearance of all these species in large numbers off the coasts of the British Isles, Iceland, the Faroe Islands and in the North Sea is rare and is associated with warming waters. Off the coast of Japan, pelagic tunicates are an indicator of pulsations of the Kuroshio Current.

Pyrosomes and salps are especially sensitive to cold waters and prefer not to leave the tropical zone of the ocean, where they are very widespread. The geographic distribution areas of most salp species, for example, cover the warm waters of the entire World Ocean, where more than 20 species are found. True, two species of salps living in Antarctica have been described. This is Salpa thompsoni, distributed in all Antarctic waters and not extending beyond 40° S. sh., i.e., the zone of subtropical subsidence of cold Antarctic waters, and Salpa gerlachei, which lives only in the Ross Sea. Appendicularia are more widespread, there are about ten species living, for example, in the seas of the Arctic Ocean, but they are also more diverse and numerous in tropical areas.

Pelagic tunicates occur at normal ocean salinities of 34-36 0/00. It is known, for example, that in the area where the Congo River confluences, where temperature conditions are very favorable for salps, they are absent due to the fact that the salinity in this place on the African coast is only 30.4 0/00. On the other hand, there are no salps in the eastern part of the Mediterranean Sea near Syria, where the salinity, on the contrary, is too high - 40 0/00.

All planktonic forms of tunicates are inhabitants of the surface layers of water, mainly from 0 to 200 m. Pyrosomes apparently do not go deeper than 1000 m. Salps and appendiculars in the main mass also do not descend deeper than several hundred meters. However, in the literature there are indications that pyrosomes are found at a depth of 3000 m, barrel makers - 3300 m and salps even up to 5000 m. But it is difficult to say whether living salps live at such great depths, or whether these were simply their dead, but well-preserved shells.

On the Vityaz, in catches made with a closing net, pyrosomes were not found deeper than 1000 m, and barrel makers - 2000-4000 m.

All pelagic tunicates are generally widespread in the ocean. Often they are caught in a zoologist's net as single specimens, but large clusters are also typical for them. Appendiculars are found in significant quantities - 600-800 specimens in fishing from a depth of up to 100 m. Off the coast of Newfoundland, their number is much larger, sometimes over 2,500 specimens in such a fishery. This amounts to approximately 50 copies per 1 l 3 waters. But due to the fact that the appendiculars are very small, their biomass is insignificant. Usually it is 20-30 mg by 1 m 2 in cold water areas and up to 50 mg by 1 m 2 in tropical areas.

As for salps, they are sometimes capable of gathering in huge quantities. There are cases when clusters of salps stopped even large ships. Here is how zoologist K.V. Beklemishev, a participant in the Soviet Antarctic expedition, describes one such case: “In the winter of 1956-1957, the motor ship “Kooperatsiya” (with a displacement of more than 5000 T) delivered the second shift of winterers to Antarctica, to the village of Mirny. On a clear, windy morning on December 21, 1956, in the southern Atlantic Ocean, from the deck of a ship, 7-8 reddish stripes were seen on the surface of the water, stretching downwind almost parallel to the course of the ship. When the ship approached, the stripes no longer seemed red, but the water in them was still not blue (as around), but whitish-turbid from the presence of a mass of some creatures. The width of each strip was more than a meter. The distance between them is from several meters to several tens of meters. Strip length - about 3 km. As soon as the Kooperatsia began to cross these stripes at an acute angle, the car suddenly stopped and the ship began to drift. It turned out that plankton had clogged the engine filters and the water supply to the engine had stopped. To avoid an accident, the car had to be stopped to clean the filters.

Having taken a sample of the water, we found in it a mass of elongated transparent creatures about 1-2 in size cm, called Thalia longicaudata and belonging to the salp order. IN 1 m 3 water there were at least 2500 copies. It is clear that the filter grates were completely filled with them. The Kooperatsiya water-drawn seawalls are located at a depth of 5 m and 5.6 m. Consequently, salps were found in large numbers not only on the surface, but also at a depth of at least 6 m".

The massive development of tunicates and their dominance in plankton is apparently a characteristic phenomenon for the edges of the tropical region. Accumulations of salps are observed in the northern part of the Pacific Ocean, their massive development is known in the zone of mixing waters of the Kuroshio and Oyashio currents, off western Algeria, west of the British Isles, off Iceland, in the northwestern Atlantic in coastal areas, near the southern border of the tropical region in Pacific Ocean, off southeastern Australia. Sometimes salps can dominate plankton, which no longer contains other typical tropical representatives.

As for pyrosomes, they apparently do not occur in such huge quantities as described above for salps. However, in some marginal areas of the tropical region, their accumulations have also been found. In the Indian Ocean at 40-45° S. w. During the work of the Soviet Antarctic expedition, a huge number of large pyrosomes were encountered. Pyrosomes were located on the very surface of the water in spots. Each spot contained from 10 to 40 colonies, which glowed brightly with blue light. The distance between the spots was 100 m and more. On average by 1 m 2 surfaces of water accounted for 1-2 colonies. Similar accumulations of pyrosomes were noted off the coast of New Zealand.

Pyrosomes are known to be exclusively pelagic animals. However, relatively recently, in the Cook Strait off New Zealand, it was possible to obtain several photographs from a depth of 160-170 m, on which large accumulations of Pyrosoma atlanticum were clearly visible, the colonies of which simply lay on the surface of the bottom.

Other individuals swam in close proximity to the bottom. It was daytime, and perhaps the animals went to great depths to hide from direct sunlight, as many planktonic organisms do.

Apparently, they felt good, since the environmental conditions were favorable for them. In May, this pyrosome is common in the surface waters of Cook Strait. Interestingly, in the same area in October the bottom is at a depth of 100 m sometimes covered with dead, decaying pyrosomes. This massive die-off of pyrosomes is likely due to seasonal phenomena. It gives some idea of the numbers in which these animals can be found in the sea.

Pyrosomes, which translated into Russian means “fireflies,” got their name from their inherent ability to glow. It was found that the light that appears in the cells of the pyrosome luminescence organs is caused by special symbiotic bacteria. They live inside cells luminous organs and, apparently, they reproduce there, since bacteria with spores inside them have been repeatedly observed. Glowing bacteria are passed on from generation to generation. They are carried by the bloodstream to the eggs of pyrosomes, which are in the last stage of development, and infect them. They then settle between the blastomeres of the cleaving egg and penetrate the embryo. Glowing bacteria penetrate through the bloodstream and into the kidneys. Thus, young pyrosomes receive luminous bacteria as an inheritance from their mothers. However, not all scientists agree that pyrosomes glow thanks to symbiont bacteria. The fact is that the glow of bacteria is characterized by its continuity, and pirosomes emit light only after some kind of irritation. The light of ascidiozooids in a colony can be surprisingly intense and very beautiful. In addition to pyrosomes, salps and appendiculars glow.

At night, in the tropical ocean, a luminous trail remains behind a moving ship. The waves beating against the sides of the ship also flare up with a cold flame - silver, bluish or greenish-white. It's not just pyrosomes that glow in the sea. Many hundreds of species of luminous organisms are known - various jellyfish, crustaceans, mollusks, fish. Often the water in the ocean burns with an even, non-flickering flame from myriads of luminous bacteria. Even bottom organisms glow. Soft gorgonian corals glow and shimmer in the dark, now weakening and now intensifying the glow, with different lights - violet, purple, red and orange, blue and all shades of green. Sometimes their light is like white-hot iron. Among all these animals, fireflies undoubtedly take first place in terms of the brightness of their glow. Sometimes, in the general luminous mass of water, larger organisms flash like separate bright balls. As a rule, these are pyrosomes, jellyfish or salps. The Arabs call them "sea lanterns" and say that their light is like the light of a moon slightly obscured by clouds. Oval spots of light at shallow depths are often mentioned when describing sea glow. For example, in an extract from the log of the motor ship "Alinbek", cited by N.I. Tarasov in his book "Glow of the Sea", in July 1938, spots of light were noted in the southern part of the Pacific Ocean, mostly of a regular rectangular shape, the size of which was approximately 45 x 10 cm. The light from the spots was very bright, greenish-blue. This phenomenon became especially noticeable during the onset of a storm. This light was emitted by pyrosomes. A great expert in the field of sea glow, N.I. Tarasov, writes that a colony of pyrosomes can glow for up to three minutes, after which the glow stops immediately and completely. The light of pyrosomes is usually blue, but in tired, overexcited and dying animals it turns orange and even red. However, not all pyrosomes can glow. The giant pyrosomes from the Indian Ocean described above, as well as the new species Pyrosoma vitjazi, do not have luminescent organs. But it is possible that the ability to glow in pyrosomes is not constant and is associated with certain stages of development of their colonies.

As already mentioned, salps and appendiculars can also glow. The glow of some salps is noticeable even during the day. The famous Russian navigator and scientist F. F. Bellingshausen, passing by the Azores in June 1821 and observing the glow of the sea, wrote that “the sea was dotted with luminous sea animals, they are transparent, cylindrical, two and a half and two inches long , float connected to one another in a parallel position, thus forming a kind of ribbon, the length of which is often a yard." In this description it is easy to recognize salps, which are found in the sea both individually and in colonies. More often, only single forms glow.

If salpas and pyrosomes have special luminous organs, then appendicularia have a glowing entire body and some parts of the gelatinous house in which they live. When the house ruptures, a flash of green light suddenly appears throughout the body. The glow is probably due to the yellow droplets of special secretions present on the surface of the body and inside the house. Appendiculars, as already mentioned, are more widespread than other tunicates and are more common in cold waters. Often they are responsible for the glow of water in the northern part of the Bering Sea, as well as in the Black Sea.

The glow of the sea is an incredibly beautiful sight. You can spend hours admiring the sparkling breakers of water behind the stern of a moving ship. We repeatedly had to work at night during the Vityaz expedition in the Indian Ocean. Large plankton networks that came from the depths of the sea often looked like large cones flickering with a bluish flame, and their mounds, in which sea plankton accumulated, resembled some kind of magic lanterns, giving off such a bright light that it was quite possible to read with it. Water flowed from the nets and from the hands, falling onto the deck in fiery drops.

But the glow of the sea also has a very great practical significance, which is not always favorable for humans. Sometimes it greatly interferes with navigation, blinds and impairs visibility at sea. Its bright flashes can even be mistaken for the light of non-existent beacons, not to mention the fact that the luminous trail unmasks warships and submarines at night and guides the enemy fleet and aircraft to the target. The glow of the sea often interferes with marine fishing, scaring away fish and sea animals from nets drenched in a silvery glow. But, it is true, large concentrations of fish can be easily detected in the dark by the glow of the sea caused by them.

Tunicates can sometimes form interesting relationships with other pelagic animals. For example, the empty shells of salps are often used by planktonic crustaceans hyperiids-phronims as a reliable shelter for breeding. Just like sebaceous fish, phronims are absolutely transparent and invisible in water. Having climbed inside the salp, the female phronima gnaws everything inside the tunic and remains in it. In the ocean you can often find empty shells of salps, each of which contains one crustacean. After the small crustaceans hatch in a kind of maternity hospital, they cling to the inner surface of the tunic and sit on it for quite a long time. The mother, working hard with her swimming legs, drives water through the empty barrel so that her children have enough oxygen. Males apparently never settle inside salps. All tunicates feed on tiny unicellular algae suspended in water, small animals, or simply particles of organic matter. They are active filter feeders. Appendicularia, for example, has developed a special, very complex system of filters and trapping nets for catching plankton. Their structure has already been described above. Some salps have the ability to gather in huge schools.

At the same time, they can eat away phytoplankton so much in those areas of the sea where they accumulate that they seriously compete for food with other zooplankton and cause a sharp decrease in their number. It is known, for example, that large accumulations of Salpa fusiformis can form near the British Isles, covering areas of up to 20 thousand square miles. In the area where they gather, salps filter out phytoplankton in such quantities that they almost completely eat it up. At the same time, zooplankton, mainly consisting of small crustaceans Copepoda, also greatly decreases in quantity, since Copepoda, like salps, feed on floating microscopic algae.

If such aggregations of salps remain in the same body of water for a long time and such waters, highly depleted in phyto- and zooplankton, invade coastal areas, they can have a serious impact on local animal populations. The swept larvae of benthic animals die due to lack of food. Even herring becomes very rare in such places, perhaps due to lack of food or large quantity metabolic products of tunicates dissolved in water. However, such large concentrations of salps are a short-lived phenomenon, especially in colder areas of the ocean. When it gets cold they disappear.

Salps themselves, as well as pyrosomes, can sometimes be used as food by fish, but only by very few species. In addition, their tunic contains a very small amount of digestible organic matter. It is known that during the years of the most widespread development of salps in the Orkney Islands area cod fed on them. Flying fish and yellowfin tuna eat salps, and pyrosomes have been found in the stomachs of swordfish. From the intestines of another fish - munus - size 53 cm on one occasion 28 pyros were recovered. Appendiculars are also sometimes found in the stomachs of fish, even in significant quantities. Fish that eat jellyfish and ctenophores can obviously also feed on salps and pyrosomes. Interestingly, large pelagic Caretta turtles and some Antarctic birds eat solitary salps. But tunicates are not of great importance as a food item.

Zoologists of ancient times classified larval-chordates, or tunicates (Tunicata), as a type of mollusk. But already in 1816 Lamarck came to the conclusion that it would be more correct to consider these peculiar animals as an independent group of invertebrate animals, only vaguely similar to mollusks. The famous works of A. O. Kovalevsky, devoted to the study of the history of the development of tunicates and lancelets, revealed the known closeness of larval chordates to aracrania and vertebrates. This closeness is indicated by: the pattern of development of the embryonic layers of tunicates, breathing associated with the anterior part of the intestine, the formation of the rudimentary notochord and its position relative to the intestine and the neural tube.

The following brief definition may characterize tunicates. These are chordate animals in which the notochord is located exclusively in the caudal region of the body; it usually exists in the larval period of development and disappears at the end of this period. The single-layer epithelium of the skin secretes a gelatinous membrane (tunic), which covers the entire body of the animal. The pharynx has the appearance of a gill box. Reproduction occurs partly sexually, partly by budding; There is a change of generations. Almost all species are hermaphroditic. Currently, there are up to 1,500 species of tunicates, of which the vast majority live on the bottom; part floats in the water column and is part of the plankton. The size of animals belonging to this subtype ranges from 1/2 millimeter to 400 millimeters, rarely more. Colonial forms sometimes form ribbons several meters long. The subtype contains 3 classes: ascidians(Ascidiae), salpas(Salpae) appendiculars(Appendiculariae).

Fig.1. Tunicates

Top row - ascidians, from left to right: ascidia mentula, colony of Schlosser's botryllus, clavelina, gastric zion. Bottom row, from left to right: Appendicularia oicopleura, Barrel dolioletta, Salp piebald colony, Pyrosoma atlantis

A group of primitive chordates that, in the larval stage of development, have all the characteristics of Chordata type structural features, but upon transition to adulthood, they lose the notochord and experience a profound transformation of the central nervous system, which turns from the neural tube into a compact nerve ganglion (only the appendiculars retain the notochord and neural tube all their lives!. Simplification of the body with the age of animals is associated with the transition from mobile the existence of larvae to immobile adults.

Specific structural features: there is a skin-muscular sac (epithelium and layers of longitudinal and circular muscles); the circulatory system is not closed, the heart is tubular, the blood circulation is pendular; the nervous system is represented by a nerve ganglion, which does not have an internal cavity, from which nerve cords extend; the excretory system is absent; hermaphrodites, fertilization in the external environment. Ascidians and salps also reproduce asexually.

Fig.2. Similarities and differences between larval chordates and skullless

The body of tunicates is never segmented, although in some ascidians it is noticeably divided into 2 or 3 sections. Externally, the body is covered with a gelatinous, leathery or cartilaginous shell-tunic. It is based on a substance extremely close to plant fiber (cellulose).

Musculature. Under the outer epithelium lies a layer of connective tissue containing muscles; the muscles of ascidians consist of longitudinal and transverse muscle fibers; in salps, they form a series of rings.

Nervous system. The central nervous system in adult tunicates consists of a single node on the dorsal surface with nerves extending from it.

The sense organs are poorly developed: the eye is found in the form of a pigment spot on the nerve ganglion, sometimes with a light-refracting body (in ascidian larvae, in salps, pyrosis), the auditory organ in the form of an unpaired otocyst (in ascidian larvae, in Doliolum), organs of touch in the form outgrowths at the edges of the inlet and outlet holes. Below the ganglion, the wall of the gill sac protrudes, forming an organ that has been compared to the Hypophysis of the vertebrate brain.

Digestive system. Most characteristic feature The intestinal canal is a strong development of the anterior section, which serves as an organ for breathing and eating. In the appendiculars, the wall of this section (gill sac) is pierced by only two openings, which directly open outward; in ascidians, the wall of the gill sac is equipped with numerous openings (gill slits), which open into the so-called peribranchial or perithoracal cavity surrounding most the wall of the gill sac and the component of the anterior part of the cloacal cavity. Blood supply. The heart lies on the ventral side of the body; The appendiculars have no blood vessels; in the remaining tunicates, the anterior and posterior vessels depart from the heart. A remarkable feature of O.'s blood circulation is that the heart contracts for some time in a certain direction, then the contractions stop and then begin again, but in the opposite direction; The movement of blood, therefore, does not have a specific direction, and in each vessel and in the heart the blood moves first in one direction, then in the other.

Reproductive system and characteristics of reproduction. All sexual specimens of tunicates are hermaphrodites, i.e. they have both male and female gonads. The maturation of male and female reproductive products always occurs at different times, and therefore self-fertilization is impossible. In ascidians, salps and pyrosomes, the ducts of the gonads open into the cloacal cavity, and in the appendicular, sperm enter the water through ducts that open on the dorsal side of the body, while eggs can come out only after the body walls have ruptured, which leads to the death of the animal. Fertilization in most tunicates occurs in the cloaca, but there is also external fertilization, when a sperm meets an egg in water and fertilizes it there. In salpas and pyrosomes, only one egg is formed, which is fertilized and develops in the mother’s body.

It should be emphasized that the acquisition of mobility by pelagic tunicates led to the loss of their developed free-swimming larvae. In complex and in most solitary ascidians, fertilization of eggs occurs in the mother's cloacal cavity, where the sperm of other individuals penetrate with the flow of water through siphons, and the fertilized eggs are excreted through the anal siphon. Sometimes the embryos develop in the cloaca and only then come out, i.e. a kind of live birth takes place.

For sessile organisms to reproduce successfully, it is necessary for the eggs and sperm of neighboring individuals to mature simultaneously. This synchronization is achieved by the fact that the reproductive products released by the first sexually mature individuals pass with a stream of water through the introductory siphon to neighboring animals and in a short time stimulate the beginning of their reproduction over large areas. A special role in this case is played by the paranervous gland, which communicates with the ripeness of the pharynx and receives the corresponding signal from the water. Through nervous system it accelerates gonad maturation.

Tunicates, larval chordates, or tunicates, which include ascidians, pyrosomes. Salps and appendiculars are one of the most amazing groups of marine animals. The central place among them belongs to ascidians. Tunicates received their name due to the fact that their body is covered on the outside with a gelatinous membrane, or tunic. Tunica consists of a special substance - tunicin, which is extremely close in composition to plant fiber - cellulose, which is found only in the plant kingdom and is unknown for any other group of animals. Tunicates are exclusively marine animals. Ascidians lead an attached lifestyle, the rest are free-swimming pelagic. They can be solitary or form colonies that arise during alternation of generations as a result of budding of asexual single individuals. Ascidians have a tailed larva that swims freely in the water.
All tunicates, with the exception of a few unusual predatory species, feed on suspended organic particles (detritus) and phytoplankton and are active filter feeders. In the vast majority of cases, in the adult state they have a sac- or barrel-shaped body with two siphons - inlet and outlet. The siphons are either close together on the upper part of the body or located at its opposite ends.

Representative of the subphylum Tunicata (Tunicata). Photo: Minette Layne

The position of tunicates in the system of the animal kingdom is very interesting. The nature of these animals remained mysterious and incomprehensible for a long time, although they were known to Aristotle more than two and a half thousand years ago under the name Tethya.
Only at the beginning of the 19th century was it established that the solitary and colonial forms of some tunicates - salps - represent only different generations of the same species. Before that, they were classified as different types of animals. Single and colonial forms differ from each other not only in appearance. It turned out that only colonial forms have sexual organs, and solitary forms are asexual. The phenomenon of alternation of generations in salps was discovered by the poet and naturalist Albert Chamisso during his voyage in 1819 on the Russian warship Rurik under the command of Kotzebue. Old authors, including Carl Linnaeus, classified the tunicate as a type of mollusk. Colonial forms were assigned by him to a completely different group - zoophytes, and some considered them a special class of worms. But in fact, these outwardly very simple animals are not as primitive as they seem. Thanks to the work of the remarkable Russian embryologist A. O. Kovalevsky, in the middle of the last century it was established that tunicates are close to chordates. A. O. Kovalevsky established that the development of ascidians follows the same type as the development of the lancelet, which represents, in the apt expression of Academician I. I. Shmalhausen, “a kind of living simplified diagram of a typical chordate animal. The group of chordate animals is characterized by a number of certain important structural features. First of all, this will be the presence of a dorsal string, or notochord, which is the internal axial skeleton of the animal. The tailed larvae of ascidians also have a notochord, which disappears when they transform into an adult. The larvae and other important structural features are much higher than their parents forms. For phylogenetic reasons, that is, for reasons connected with the origin of the group, greater importance is attached to the organization of their larvae in tunicates than to the organization of adult forms. Such an anomaly is unknown for any other type of animal. Apart from the presence of a notochord, at least in larval stage, tunicates are similar to true chordates by a number of other characteristics. It is very important that the nervous system of the tunicates is located on the dorsal side of the body and is a tube with a canal inside. The neural tube of tunicates is formed as a groove-shaped longitudinal invagination of the surface integument of the body of the embryo - the ectoderm, as is the case in all other vertebrates and in humans. In invertebrate animals, the nervous system always lies on the ventral side of the body and is formed in a different way. The main vessels of the circulatory system of tunicates, on the contrary, are located on the ventral side, contrary to what is typical for invertebrate animals. And finally, the anterior section of the intestine, or pharynx, is pierced by numerous openings in tunicates and has turned not only into a digestive organ that filters food, but also into a respiratory organ. As we saw above, invertebrate animals have very diverse respiratory organs, but the intestines never form gill slits. This is a characteristic of chordates and is the only characteristic that is retained in adult forms of tunicates. Tunicates have a secondary body cavity, or coelom, but it is greatly reduced.
According to the ideas of A. O. Kovalevsky, accepted by many, although not all modern zoologists, ascidians descended from free-swimming chordates. The peculiarities of their structure are a secondary simplification, as a result of the second, the notochord, the neural tube, and sensory organs are lost, as well as the presence of a tunic that performs protective and support functions, and greater specialization - this is a consequence of adaptation to the attached way of life in adulthood. The structure of their complex, tailed larvae, swimming in water, to some extent reproduces the organization of their ancestors.
The position of tunicates in the system of the animal kingdom remained unresolved for a long time. They were considered either as an independent type, close to chordates, or as a separate subtype of the chordate type. This was due to poor knowledge, first of all, of the embryonic and ontogenetic development of this group of animals. Recent comparative studies of the embryology of lancelets and ascidians, pyrosomas, salps and appendicularia show many common features. And, as is known, the early stages of animal development are extremely great importance for phylogenetic constructions. It should be considered definitively established that tunicates are a special subphylum - Urochordata, or Tuncata, of the phylum Chordata, where they are included along with the subphyla Acrania and Vertebrata. It must be emphasized, however, that even now some questions remain controversial regarding the family relationships between and within these subphyla, as well as the origin of chordates in general.
A well-known specialist in the field of comparative embryology of lower chordates (animals and tunicates), as well as invertebrates O. M. Ivanova-Kazas, notes that the development of tunicates, despite its extreme originality, allows us to consider them even more highly organized animals than the lancelet, which is the most primitive representative of chordates and the type of development of which in the process of evolution led to the ontogeny of vertebrates. The development of tunicates has evolved in a different direction than that of the lancelet. In connection with the sedentary lifestyle of ascidians, highly developed and specialized forms of asexual reproduction arose in tunicates, completely unusual for other chordates, with complex life cycles, with the emergence of coloniality, polymorphism, etc. From ascidians it was inherited by pyrosomes and salps.
Tunicates reproduce both asexually (budding) and sexually. Among them there are also hermaphrodites. The reproduction of tunicates provides an amazing example of the extraordinarily complex and fantastic life cycles of animals that can occur in nature. All tunicates, except appendiculars, are characterized by both sexual and asexual methods of reproduction. In the first case, a new organism is formed from a fertilized egg. But in tunicates, development to an adult occurs with profound transformations in the structure of the larva towards its significant simplification. With asexual reproduction, new organisms seem to bud off from the mother, receiving from her the rudiments of all the main organs.
All sexual specimens of tunicates are hermaphrodites, i.e. they have both male and female gonads. The maturation of male and female reproductive products always occurs at different times, and therefore self-fertilization is impossible. In ascidians, salps and pyrosomes, the ducts of the gonads open into the cloacal cavity, and in the appendicular, sperm enter the water through ducts that open on the dorsal side of the body, while eggs can come out only after the body walls have ruptured, which leads to the death of the animal. Fertilization in most tunicates occurs in the cloaca, but there is also external fertilization, when a sperm meets an egg in water and fertilizes it there. In salpas and pyrosomes, only one egg is formed, which is fertilized and develops in the mother’s body. It should be emphasized that the acquisition of mobility by pelagic tunicates led to the loss of their developed free-swimming larvae. In complex and in most solitary ascidians, fertilization of eggs occurs in the mother's cloacal cavity, where the sperm of other individuals penetrate with the flow of water through siphons, and the fertilized eggs are excreted through the anal siphon. Sometimes the embryos develop in the cloaca and only then come out, i.e., a kind of viviparity occurs.
For sessile organisms to reproduce successfully, it is necessary for the eggs and sperm of neighboring individuals to mature simultaneously. This synchronization is achieved by the fact that the reproductive products released by the first sexually mature individuals pass with a stream of water through the introductory siphon to neighboring animals and in a short time stimulate the beginning of their reproduction over large areas. A special role in this case is played by the paranervous gland, which communicates with the pharyngeal cavity and receives the corresponding signal from the water. Through the nervous system, it accelerates the maturation of the gonads.
Many features of the embryonic development of lancelets and tunicates are similar to those, for example, in echinoderms or hemichordates, and this allows us to consider lower chordates as a kind of link between invertebrates and vertebrates.
However, neither the skullless nor the tunicates appear to be the direct ancestors of vertebrates. The origin of the tunicata is currently presented as follows. Some primitive skullless creatures switched to a sessile lifestyle on a hard substrate at the bottom of the sea and turned into ascidians. A powerful tunic protected them well from enemies, and the well-developed filtration apparatus of the pharynx provided a sufficient amount of food for these animals, which switched to a passive method of feeding and became filter feeders - stenophages. Some important organs in adult organisms were reduced. They remained only in the active free-swimming larva, which allowed the immobile ascidians to spread widely in the ocean. And the amazing ability for asexual reproduction - budding - ensured the rapid settlement of new areas. Then tunicates repopulated aquatic environment and managed to master the reactive method of movement. All this gave them great advantages, but, although tunicates are widespread in modern seas and oceans and are a characteristic component of the marine fauna, they did not give rise to a progressively developing branch on the evolutionary tree. This is like an evolutionary dead end, a side branch extending from the very base of the phylogenetic trunk of chordates.
Together with other chordates and a small number of invertebrate animals, tunicates belong to deuterostomes - one of the main trunks of the evolutionary tree in the kingdom Animalia.
In representatives of deuterostomes, or Deuterostomia, during embryonic development the mouth is not formed in the place of the primary mouth of the embryo, but breaks out anew. The primary mouth turns into an anus. In contrast, in protostomes, or Protostomia, the mouth is formed in place of the mouth of the embryo - the blastopore. These include most types of invertebrate animals.
The subphylum tunicates includes three classes: ascidiae (Ascidiae), salps (Salpae) and appendiculariae (Appendiculariae). Ascidians gave rise to the remaining classes of tunicata.
The subphylum includes 1,100 species living in the seas. Of these, 1000 species are ascidians. There are about 60 species of appendicularia, about 25 species of salps and about 10 species of pyrosomes. The body structure of almost all tunicates is very different beyond recognition from the general plan of the body structure in the phylum chordates.

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