Subphylum larvalochordates or tunicates. Subtype Tunicates. The main features and structure of ascidians. Reproduction and development of barrel snails and salps

Tunicates (larval chordates; Tunicata or Urochordata), a subphylum of chordates, includes three classes (ascidians , Appendicularia and salps), uniting 1100-2000 species. These are widespread, sedentary marine organisms, the body of which is enclosed in a shell secreted by the outer epithelium - the tunic (hence the name). Body length is from 0.3 cm to 30 m. Only larval forms have a notochord. Some lead an attached lifestyle and are solitary forms or branching colonies. Others swim slowly in the water column. The most prominent organ of tunicates is the front part of the U-shaped digestive tract - the pharynx, which occupies most of the body volume. Nutrition is carried out by filtration. Their prey is small unicellular animals and plants and small organic remains. The circulatory system of the tunicates is open, lacunar type, and consists of a heart sac and a developed network of lacunae. Blood moves through large vessels, and then pours into the cavities that wash the organs. The nervous system is represented by the cerebral ganglion on the dorsal side of the body and the nerve trunk extending from it. Tunicates are hermaphrodites, many of them are capable of asexual reproduction by budding. Ascidian class ( Ascidiae) . The majority of tunicates belong to this class, represented by sessile forms, both solitary and colonial. Colonial forms sometimes lead a free-swimming lifestyle. Ascidia looks like a two-necked jar. With the base of its body (sole) it is attached to the protrusions of the bottom. On the upper part of the body there is a tube-like outgrowth with an opening leading into a huge pouch-like pharynx. This is an oral siphon. Another hole is located lower on the side - this is the cloacal siphon. Throat pierced a large number small openings - gill slits, or stigmas, through which water circulates. At the bottom of the pharynx there is an opening leading into the short esophagus. The esophagus passes into the pouch-shaped stomach. The short intestine opens into the atrial cavity, which communicates with the external environment through an opening - atriopore, located on the cloacal siphon. Passive nutrition. There is an endostyle. Food particles that enter the throat with water are deposited on it. The endostyle begins at the bottom of the pharynx and along its ventral side rises up to the oral opening. Here it bifurcates, forming a peripharyngeal ring, and passes into a dorsal outgrowth stretching along the dorsal side of the pharynx. Food lumps are driven by the ciliated cells of the endostyle upward to the peripharyngeal ring, from where they descend along the dorsal process to the esophagus. There is a stomach, a short intestine opens into the atrial cavity near the cloacal siphon. The circulatory system is open, lacunar. The nervous system consists of a ganglion without an internal cavity, located between the oral and cloacal siphons. There are no sense organs. Reproductive system. Ascidians are hermaphrodites: in the body of one individual there is both an ovary and a testis. During asexual reproduction, a flask-shaped protrusion appears on the ventral side of the mother's body - the kidney stolon. The bud soon separates and turns into a sessile form: in colonial ascidians, the bud remains on the stolon and itself begins to reproduce by budding. All organs of the maternal form are formed in the kidneys. Sexual reproduction of ascidians: a free-swimming larva quickly forms from a fertilized egg. Outwardly, it resembles a tadpole: its “head” contains all the organs, and its tail allows it to move quickly. In addition to the muscles and fin fold, the tail contains a notochord and a neural tube. Soon it is attached to the substrate by two outgrowths of the head and undergoes a regressive metamorphosis. The chord disappears. The neural tube, light-sensitive eye and brain vesicle decrease in size and then disappear. Only the posterior thickened part of the vesicle remains, which forms a ganglion. The pharynx grows, the number of gill openings increases sharply. The oral and anal openings move upward. The body takes on the sac-like appearance typical of an adult. A tunic quickly forms on the surface of the body. Tunicates had common ancestors. The ancestors of tunicates were free-swimming animals that moved through the water using a long caudal fin. They had a developed neural tube with an expanded brain vesicle at the anterior end, sensory organs in the form of an auditory vesicle and a pigmented ocellus, and a well-developed notochord. Later, most species switched to a sedentary lifestyle and their body structure became significantly simpler. Adaptations due to a sedentary lifestyle have progressively developed: a thick tunic - reliable protection for internal organs, complex branchial apparatus, endostyle, reproduction not only sexually, but also by budding.

The phylum Chordata has a number of features:

I. The presence of an internal axial skeleton (notochord). The notochord performs a supporting function. The second function is movement. The notochord is retained throughout life only in the lower representatives of the type. In higher chordates, it is formed in embryogenesis, then replaced by the spine, which is formed in its connective tissue membrane. The notochord is formed from the endoderm.

II. The central nervous system (CNS) is represented by the neural tube. During embryogenesis, a neural plate is formed in the ectoderm (neurula stage), which then folds into a tube. Formed spinal cord with a cavity (neurocoel or spinal canal) inside. The cavity is filled with liquid. In higher chordates, the anterior part of the neural tube differentiates into the brain. Biological significance This type of structure of the central nervous system is that nutrition of the nervous system occurs not only from the surface, but also from the inside, through the cerebrospinal fluid.

III. The anterior section of the digestive system (pharynx) is penetrated by gill slits. Gill slits are openings that connect the pharynx to the external environment. They appear as a filtration apparatus for nutrition, but they also combine a respiratory function. In vertebrates, the respiratory organs - gills - are located on the gill slits. In terrestrial vertebrates, gill slits exist only during the early stages of embryonic development.

IV. Chordates have bilateral (two-sided) symmetry. This type of symmetry is characteristic of most types of multicellular animals.

V. Chordata are secondary-cavity animals.

VI. Chordata are deuterostome animals, together with hemichordates, echinoderms and pogonophora. Unlike protostomes, the mouth breaks through anew, and the blastopore corresponds to the anus.

VII. The structural plan of chordates is determined by the strictly regular arrangement of the main organ systems. The neural tube is located above the notochord, and the intestine is located below the notochord. The mouth opens at the anterior end of the head, and the anus at the posterior end of the body - in front of the base of the tail. In the abdominal part of the body cavity there is a heart, blood from the heart moves forward.

Subphylum Tunicata (Tunicata)

Tunicates are a unique group of marine organisms whose structure does not contain full set morphological features inherent in chordates; they can be solitary or form colonies. There are planktonic forms and forms leading an attached lifestyle. Before the work of A. O. Kovalevsky, who studied the ontogeny of tunicates, they were classified as invertebrate animals. A. O. Kovalevsky proved that these are undoubtedly chordates, and the primitiveness of their structure is due to an immobile or sedentary lifestyle. The subphylum is divided into three classes - Ascidia, Salpa and Appendicularia.

Class Ascidiae

Externally, ascidians have a bag-like shape, motionlessly attached to the substrate. On the dorsal side of the body there are two siphons: the oral siphon, through which water is sucked into the intestines, and the cloacal siphon, from which water is expelled. By type of feeding, ascidians are filter feeders.

The body wall is formed by the mantle, which consists of a single-layer epithelium and layers of transverse and longitudinal muscles. On the outside there is a tunic, which is secreted by epithelial cells. Muscle contractions ensure the flow of water through siphons. The flow of water is facilitated by the ciliated epithelium of the oral siphon. At the bottom of the oral siphon there is a mouth opening surrounded by tentacles.

The mouth leads into a pouch-shaped pharynx, pierced by many gill openings. Under the epithelium of the pharynx there are blood capillaries in which gas exchange occurs. The pharynx performs two functions - breathing and filtering food particles. The food suspension settles on the mucus secreted by a special formation - the endostyle. Then the mucus, together with food, due to the work of the ciliated epithelium, enters the esophagus, and then into the stomach, where it is digested. The stomach passes into the intestine, which opens with the anus near the cloacal siphon.

The nervous system is formed by the dorsal ganglion, from which nerves extend to internal organs.

The circulatory system is not closed. There is a heart. From the heart, blood moves through the vessels and pours into the gaps between the internal organs.

The excretory system is represented by storage buds - peculiar cells that absorb metabolites - uric acid crystals.

Ascidians can reproduce both asexually (budding) and sexually. As a result of budding, ascidian colonies are formed. Ascidians (like other tunicates) are hermaphrodites, external, cross fertilization. Fertilized eggs develop into larvae that actively swim in the water column.

The larva consists of a body and a tail and has all the characteristics of chordates: in the tail there is a notochord, above it is a neural tube, in the anterior extension of which there is an organ of balance and a primitive ocellus. The pharynx is equipped with gill slits. The larva settles to the bottom with its front end. The further transformation of the larva is an example of regressive metamorphosis: the tail disappears, and with it the notochord, the neural tube turns into a dense neural ganglion, the pharynx increases in volume. The larva serves for dispersal.

Class Salpae

In structure and characteristics of life activity they resemble ascidians, but, unlike them, they lead a planktonic lifestyle. Most salps are colonial organisms. These animals are characterized by a natural alternation of sexual and asexual reproduction(metagenesis). Fertilized eggs produce asexual individuals, which reproduce only by budding, and individuals resulting from asexual reproduction begin sexual reproduction. This is the only example of metagenesis in chordates.

Class Appendiculariae

They lead a free planktonic lifestyle. The body is divided into a trunk and a tail. The body contains internal organs. The gill slits open to the external environment. On the dorsal side there is a nerve ganglion, from which a nerve trunk extends back into the tail. The notochord is in the tail. The outer epithelium of the appendicular forms a mucous house. In the front part of the house there is a hole made of thick mucous threads, and in the back part of the house there is a hole of a smaller diameter. With the help of its tail, the animal produces a current of water in the house. Small organisms pass through the lattice of the entrance hole and adhere to the mucous threads that form “ catching net" Then the net with the stuck food is pulled into the mouth opening. The water coming out of the rear hole of the house contributes to the reactive movement of the animal forward. Appendiculars from time to time destroy their house and build a new one.

Appendiculars reproduce only sexually; development proceeds without metamorphosis. Fertilization occurs in the ovaries of the mother, from where young animals emerge through breaks in the wall of the mother’s body. As a result, the mother's body dies. Perhaps appendicularia represent an example of neoteny, that is, reproduction at the larval stage.

Subtype Skullless (Acrania)

Cranials exhibit all the basic characteristics of chordates. By type of nutrition - filter feeders. Among them there are species leading a pelagic lifestyle, others are benthic forms, living buried in the ground and exposing only the anterior end of the body. They move using the lateral bends of the body.

Class Cephalochordata

A representative of the cephalochordates is the lancelet. It has an oval body that tapers to a tail. The epithelium is single-layered; under the epithelium there is a thin layer of connective tissue. There is a fin on the dorsal side and tail; at the end of the tail it has the shape of a lancet, hence the name of the animal. Metapleural folds are formed on the sides of the trunk. Metapleural folds grow downward and then grow together, forming a special space - the atrial cavity. It covers the pharynx and part of the intestine and opens outwards with a special opening - the atriopore. The atrial cavity protects the gill slits from the entry of soil particles.

The skeleton is formed by a chord that stretches along the entire body. The connective tissue surrounding the notochord forms the supporting tissue that supports the fin and penetrates between muscle segments (myomeres). As a result, partitions are formed - myosepta. The muscles are striated. Successive contractions of the myomeres cause lateral bending of the body. The notochord at the anterior end of the body extends forward of the neural tube, which is why the animals are called cephalochordates. The walls of the neural tube contain light-sensitive ocelli. The spinal and abdominal nerves depart from the neural tube, according to the alternation of myomeres. Nerve nodes are not formed. In the anterior part of the neural tube, the neurocoel expands. At this point the olfactory organ is adjacent to the neural tube.

By feeding type, the lancelet is a filter feeder. The oral opening lies deep in the preoral funnel, surrounded by tentacles. There is a sail around the mouth, which is also equipped with tentacles that prevent large particles from entering the mouth. The mouth leads into a long pharynx, pierced by numerous gill openings. They open into the atrial cavity. The interbranchial septa are covered with ciliated epithelium, which creates a flow of water. In the walls of the interbranchial septa there are blood capillaries, where gas exchange occurs. Breathing can also be carried out over the entire surface of the body.

A groove formed by ciliated and mucous cells, the endostyle, runs along the ventral side of the pharynx. With the help of semi-circular grooves located on the interbranchial septa, it connects to the epibranchial groove. The cilia drive mucus with adherent food particles forward along the endostyle, upward along the interbranchial grooves, and backward along the epibranchial groove into the esophagus. A blind hepatic outgrowth extends from the intestine at its very beginning. It performs a number of functions - secretory, absorption and intracellular digestion. The digestive tract ends with the anus in front of the caudal fin.

The circulatory system has a primitive structure. The heart is missing. Paired venous vessels that collect blood from the main veins empty into the venous sinus. The abdominal aorta is located under the pharynx and arises from the confluence of the venous vessels. The abdominal aorta gives off a large number of branchial arteries, which pass through the interbranchial septa. Gas exchange occurs in them. Oxidized blood collects in the dorsal aorta and is distributed to all organs of the body. The lancelet has one circle of blood circulation, the blood is colorless, gases dissolve in the plasma.

The excretory system of the protonephridial type is represented by numerous cells - solenocytes, whose structure resembles protenefridia annelids. The excretory organs are located on the interbranchial septa.

Skullless dioecious sexes. The gonads are located at the walls of the atrial cavity and do not have ducts. The reproductive products exit into the atrial cavity through ruptures in the walls of the gonads. Gametes are released into the external environment through the atriopore. The development of the lancelet proceeds with metamorphosis: there is a larva, the body of which is covered with cilia, with the help of which it moves in the initial stages of development.

Subphylum vertebrates

The vertebrate subphylum (Vertebrata) is generally characterized by the following characters:

1. The notochord is formed in embryonic development; in adult organisms it is partially or completely replaced by the spine.
2. The anterior part of the neural tube extends in front of the notochord and differentiates into the brain, consisting of the brain vesicles. The cavities of the bladders are a continuation of the spinal canal.
3. The brain is located in the cranial cavity.
4. In primary aquatic organisms, respiratory organs - gills - are formed on the interbranchial septa. In terrestrial vertebrates, gill slits are found only in the early stages of embryonic development.
5. The heart appears - a muscular organ located on the ventral side of the body.
6. Excretory organs are the kidneys, which, in addition to the excretory function, perform the function of osmoregulation (maintaining a constant internal environment of the body).

Class cyclostomes (Cyclostomata)

The second name for cyclostomes is jawless (Agnatha). The most primitive and the most ancient representatives vertebrates. Known since the Cambrian, they reached their peak in the Silurian (class Scutellaceae). In the modern fauna they are represented by two orders - Lampreys and Hagfishes. Cyclostomes do not have paired limbs and jaws. The body is elongated, there is no distinct division into the head, body and tail. The skin is bare, there are no scales, and there are many single-celled mucous glands in the skin.

There is a suction funnel on the head, at the bottom of which the mouth opens. Inside the funnel and at the end of the muscular tongue are horny teeth. On the head there is an unpaired nostril leading to the olfactory sac. Spherical gill openings are located on the sides of the head and lead into the gill sacs.

The axial skeleton is formed by the notochord. The notochord, together with the neural tube, is surrounded by a connective tissue membrane. The cranium, that is, that part of the skull that protects the brain and sensory organs, is formed by cartilage that covers the brain from below and from the sides. The olfactory capsule is adjacent to the skull in front, and the auditory capsules are adjacent to the sides. From above, the brain is closed by a connective tissue membrane, that is, the roof of the skull has not yet formed.

Vertebrates have a visceral skull. It includes elements that form in the walls of the anterior part of the digestive system (pharynx). From a functional point of view, this is the skeleton of the gill and oral apparatus. In cyclostomes, the visceral skull is formed by cartilages that support the oral funnel and tongue, as well as the skeleton of the gill sacs and the pericardial cartilage surrounding the heart.

The muscles of the trunk and tail are segmented - formed by distinct myomeres separated by myoseptae.

The digestive system begins with the mouth. In lampreys, the pharynx functions only during the larval stage. In adults, it is divided into two different sections - the respiratory tube and the esophagus. The stomach is undeveloped, and the esophagus immediately passes into the midgut. The intestine is straight and does not form any bends. A fold is formed on the intestinal mucosa - a spiral valve, which increases the absorption surface of the intestine. The liver is large. Using a mouth funnel, lampreys attach themselves to the body of the prey - fish - and use their tongue to make holes in the skin of the fish. The tongue, acting like a piston, forces blood into the oral cavity, from where it enters the esophagus.

Hagfishes have short tentacles in place of the oral sucker. Hagfish feed on carrion. They bite into the body dead fish where the moves are made.

In cyclostomes, gill sacs develop in the gill slits. They are of endodermal origin. The gill sacs contain folds intertwined with blood capillaries in which gas exchange occurs. During respiration, water enters the gill sacs through the gill openings and exits out the same way.

The heart of cyclostomes is two-chambered and consists of an atrium and a ventricle. The venous sinus departs from the atrium, into which all venous vessels flow. The afferent branchial arteries, which carry blood to the gill filaments, are separated from the abdominal aorta. The efferent branchial arteries flow into the azygos root of the aorta. The dorsal aorta extends backward from the root of the aorta, and forward are the carotid arteries, carrying oxidized blood to the head. Venous blood flows from the head through paired jugular veins, which flow into the venous sinus. From the body, blood collects in the posterior cardinal veins. Through the intestinal vein, blood from the intestines passes to the liver, forming the portal system of the liver. There is no renal portal system. Cyclostomes have one circulation.

The excretory organs are represented by ribbon-shaped paired kidneys.

The brain consists of five sections: the forebrain, diencephalon, midbrain, cerebellum and medulla oblongata. The parts of the brain are located in the same plane. That is, they do not form bends characteristic of highly organized vertebrates. Sense organs: organs of vision, hearing, balance, smell, touch and lateral line.

The gonads are unpaired and do not have genital ducts. Gametes enter the body cavity through ruptures in the gonad wall, and then out through special pores on the urogenital sinus. Development with metamorphosis. The lamprey larva is called a sand miner. It lives in fresh water bodies, buried in the ground. Larvae are filter feeders. Development continues for several years. After metamorphosis, the young lamprey migrates to the sea. Hagfishes have direct development. The eggs hatch into young individuals.

Class Cartilaginous fish (Chondrichthyes)

Sharks, rays and chimeras belong to this class. The skeleton is completely cartilaginous. The scales are placoid. Five to seven pairs of gill slits. The arrangement of the paired fins is horizontal. There is no swim bladder. The class is divided into two subclasses: elasmobranchs and whole-headed.

Subclass Elasmobanchii

Unites sharks and rays. The structure will be examined using the example of sharks. The body shape is streamlined, fusiform. On the sides of the head there are five pairs of gill slits. Two openings (sprays) are located behind the eyes and lead into the pharynx. The tail has a cloaca. The axis of the skeleton extends into the upper, large lobe of the caudal fin; this type of structure is called heterocercal. The paired pectoral and ventral fins represent the limbs. In males, parts of the ventral fins are transformed into copulatory organs.

The epidermis contains numerous glands. The scales are placoid and are a plate with a tooth directed backwards. On the jaws the scales are larger and form teeth. The teeth of the scales are covered with enamel on the outside. In front of the mouth on the head are paired nostrils. The body is divided into two sections: the trunk, which starts from the last gill slit and ends with the cloaca opening, and the caudal. The skeleton is cartilaginous.

Consists of the spine, skull, skeleton of paired fins and their belts and the skeleton of unpaired fins.

The spine is formed by cartilaginous vertebrae, within which runs a strongly reduced notochord. The upper arches of the vertebrae form the canal in which the spinal cord is located. The brain section of the skull consists of the braincase, rostrum and paired capsules of the sensory organs. A cartilaginous roof appears in the braincase. The visceral skeleton consists of the jaw arch, hyoid arch and branchial arches. The skeleton of the forelimb belt is formed by a cartilaginous arch lying deep in the muscles. The girdle of the hind limbs is formed by unpaired cartilage located across the body in front of the cloaca. Paired limbs, pectoral and ventral fins are attached to the belts. Unpaired fins represented by dorsal, caudal and anal.

The jaws have large teeth. The oral cavity leads to the pharynx. The pharynx is perforated by gill slits, and squirts open into it. The esophagus is short, passes into an arched stomach. The small intestine begins from the stomach, into the anterior section of which the bile duct of a large bilobed liver flows. The pancreas lies in the mesentery of the small intestine. The large intestine contains a spiral valve that increases the absorption surface. The spleen is located next to the stomach.

The gill openings are delimited from each other by interbranchial septa, in the thickness of which cartilaginous gill arches are located. Gill filaments sit on the anterior and posterior walls of the gill slits.

The heart of cartilaginous fish is two-chambered and consists of an atrium and a ventricle. The venous sinus flows into the atrium, into which venous blood flows. The conus arteriosus arises from the ventricle. The abdominal aorta originates from the conus arteriosus. It gives off five pairs of branchial arterial arches. Oxidized blood collects in the efferent gill arteries, which flow into paired longitudinal vessels - the roots of the aorta, which, when fused, form the dorsal aorta. It runs under the spine and supplies blood to the internal organs. The carotid arteries extend to the head from the roots of the aorta. From the head, venous blood collects in paired jugular veins, and from the body - in paired cardinal veins, which at the level of the heart merge with the jugular veins, forming paired ducts of Cuvier, which flow into the sinus venosus. There is a renal portal system. From the intestine, blood flows through the intestinal vein into the liver, where the portal system of the liver is formed, and then through the hepatic vein flows into the venous sinus. Cartilaginous fish have one circulation.

The brain consists of five sections. The large forebrain passes into the diencephalon. The midbrain forms the optic lobes. The cerebellum is well developed and lies behind the medulla oblongata. 10 pairs of cranial nerves arise from the brain.

1. Olfactory nerve - originates from the olfactory lobes of the forebrain.
2. Optic nerve - departs from the bottom of the diencephalon.
3. Oculomotor nerve - originates from the floor of the midbrain.
4. Trochlear nerve - arises from the posterosuperior part of the midbrain.
5. The remaining nerves arise from the medulla oblongata.
6. Abducens nerve.
7. Trigeminal nerve.
8. Facial nerve.
9. Auditory nerve.
10. Glossopharyngeal nerve.
11. Nervus vagus.

In terrestrial vertebrates, in addition, the hypoglossal and accessory nerves arise.

The sense organs of cartilaginous fish are very well developed. Large eyes have a flat cornea, a spherical lens, and no eyelids. The hearing organs are formed by the inner ear. The lateral line organ is a canal located in the skin and communicates with the external environment through openings. The channel contains receptors that perceive water vibrations.

Excretory organs are paired kidneys. Gonads are paired. In the male, seminiferous tubules extend from the ribbon-shaped testes, flowing into top part kidneys The seminiferous tubules merge into the vas deferens, which, together with the ureters, open into the cloaca on the urogenital papilla.

In females, paired oviducts merge, forming a common funnel; the expansion of the oviducts forms shell glands, the secretion of which forms the shell of the egg. The oviduct ends with the “uterus”. It opens with separate holes into the cloaca. Paired ovaries. Mature eggs from the ovary exit into the body cavity and are captured by the oviduct funnel. Fertilization is internal and occurs in the oviduct. Eggs develop in the uterus: in viviparous sharks until the embryo fully matures, and in oviparous sharks, eggs covered with a dense shell are released from the uterus.

Class Bony fishes (Osteichthues)

They are characterized by a more or less developed bone skeleton. A bony operculum is formed, covering the outside of the gill apparatus. Gill filaments are located on the gill arches. In most species, the swim bladder arises as an outgrowth of the dorsal intestine. Fertilization is external, development with metamorphosis.

Subclass Cartilaginous ganoids (Chondrostei)

This subclass includes ancient fish that have retained a number of primitive features in which they resemble cartilaginous fish. Representatives: sturgeon - sturgeon, beluga, stellate sturgeon, etc. - and paddlefish.

The head end ends in an elongated rostrum, the mouth in the form of a slit is located on the underside of the head. The paired fins are arranged horizontally, the caudal fin is of the heterocercal type. The body is covered with bone scales, the largest scales are called bugs.

The notochord persists throughout life. The vertebral bodies are not formed, but there are upper and lower vertebral arches. The gill covers are bony. Like sharks, the intestines contain a spiral valve. The swim bladder maintains contact with the intestines. Heart with conus arteriosus. The eggs are small, fertilization is external. They have commercial significance.

Subclass Lungfish (Dipnoi)

They live in tropical, fresh, oxygen-poor waters. They arose in the Devonian and reached their peak at the beginning of the Mesozoic. Modern representatives: monopulmonates - neoceratodes, bipulmonates - protopterus, lepidosiren.

The skeleton is mainly cartilaginous. The notochord is well developed and remains throughout life. The intestine has a spiral valve. The heart has a conus arteriosus. The paired fins are fleshy, the scales are bony, the caudal fin is diphycercal. Breathing is gill and pulmonary. One or two bladders serve as peculiar lungs, which open on the abdominal side of the esophagus. Pulmonary breathing is carried out through through nostrils. The circulatory system acquires a unique structure in connection with pulmonary respiration. They can breathe simultaneously with gills and lungs, or with each of them separately. When water is depleted of oxygen or during hibernation, breathing is only pulmonary. Commercial significance Dont Have.

Subclass lobe-finned fish (Crossopterygii)

Peculiar ancient fish in the modern fauna are represented by one species - coelacanth (Latimeria halumnae). They live in the Comoros area at depths of up to 1000 meters. The group flourished in the Devonian and Carboniferous, and became extinct in the Cretaceous.

The notochord is well developed, the vertebrae are rudimentary. The fish have a degenerated lung. Like lungfishes, ancient lobe-finned birds had double breathing. Paired fins in the form of fleshy blades containing the fin skeleton and motor muscles. This is the fundamental difference between the structure of the limbs of lobe-finned fish and the limbs of other fish. The body is covered with rounded thick bone scales.

Lobefin ​​and lungfishes probably have common origin. They lived in fresh water bodies with a deficiency of oxygen, so they developed double breathing. With the help of fleshy fins, lobe-finned fish moved along the bottom of the reservoir, and also crawled from reservoir to reservoir, which was the prerequisite for the transformation of their fleshy fins into a five-fingered limb of the terrestrial type. Lobe-finned fish gave rise to amphibians - stegocephals, the first primitive terrestrial vertebrates. The possible ancestor of amphibians is considered to be extinct lobe-finned fish - Ripidistia.

Subclass ray-finned (Actinopterygii)

The most numerous subclass of modern fish. The skeleton is bony, the presence of cartilage in the skeleton is insignificant. The paired fins are located vertically in relation to the body, and not horizontally, as in cartilaginous fish. The mouth is located at the front end of the head. Rostrum is missing. There is no cloaca. The caudal fin is of the homocercal type - the fin blades are identical, the spine does not extend into the blades. The scales are bone, in the form of thin plates, tiled-like overlapping each other.

Superorder bony fishes (Teleostei)

Fish have a streamlined body covered with bony scales. The scales can be cycloid - with a smooth leading edge, and ctenoid - with a serrated leading edge. Scales form in the skin. On the outside, the scales are covered with a multilayered epidermis, which contains a large number of single-celled mucous glands. The glands secrete mucus, which reduces the friction of the fish on the water when moving. Scales grow throughout the life of the fish. A lateral line runs along the sides of the body. The holes that pierce the scales lead into the canals where the lateral line organs are located. Nerve endings perceive water vibrations.

The spine consists of the trunk and caudal sections. The vertebrae are bony and have upper and lower arches. The superior arches close and form the spinal canal, which contains the spinal cord. In the trunk region, the ribs are attached to the lower arches of the vertebrae. In the caudal region, the lower arches have spinous processes, the fusion of which gives rise to the hemal canal. The hemal canal contains the tail veins and arteries.

The skull consists almost entirely of bone tissue and is formed by many individual bones. The skull has a foramen magnum through which the spinal cord and brain connect. The visceral skull is formed by a series of visceral arches: the maxillary, hyoid and five branchial arches. The gill apparatus is covered by gill covers.

The girdle of the forelimbs is attached to the brain skull. Skeleton pectoral fins(forelimbs) attaches to the forelimb girdle. The belt of the hind limbs is paired and lies in the thickness of the muscles. The skeleton of the ventral fins (hind limbs) is attached to it. Unpaired limbs are represented by dorsal, caudal and anal fins. The muscles that move the limbs are located on the body. The movement of fish is ensured by the wavy bends of the tail.

In the oral cavity of most fish species, conical teeth are located on the bones. There are no clear boundaries between the oral cavity and pharynx. The pharynx, penetrated by gill slits, continues into a short esophagus, which passes into the stomach. At the border of the stomach and midgut there are pyloric appendages that increase the surface of the intestine. The midgut is poorly differentiated, there is no spiral valve. The anterior section of the small intestine is called the duodenum. Under the stomach there is a large lobed liver with a gall bladder. The bile duct drains into the duodenum. The pancreas is formed by small lobules scattered throughout the mesentery of the midgut. The compact spleen is located under the stomach in the first bend of the intestine.

Most bony fish have a swim bladder. It forms as an outgrowth of the dorsal side of the esophagus. In closed-vesical fish, the connection between the bladder and the esophagus is lost, while in open-vesical fish it is maintained throughout life. The function of the swim bladder is hydrostatic. The volume of gases in the bubble changes, which leads to a change in the density of the fish’s body. In closed-vesical fish, changes in the volume of the swim bladder occur as a result of gas exchange in the network of capillaries entwining the bladder. In open-vesical fish, the volume of the bladder changes due to its compression and expansion.

The gills, which serve as respiratory organs, are of ectodermal origin. There are no interbranchial septa; the gill filaments sit directly on the gill arches. There are four full gills and one half gill on each side of the body. Each gill bears two rows of gill filaments. On the inside of the gill arches there are gill rakers - processes extending in the direction of the adjacent gill arch. The stamens form a filtering apparatus that prevents the release of food out of the pharynx through the gill cavity. The gill filaments contain an extensive network of capillaries in which gas exchange occurs. The presence of an operculum increases the efficiency of respiratory movements. By movements of the mouth, water is forced into the oral cavity, and due to the operation of the lids, water is sucked into the gill cavity and passes through the gills.

Sharks use a different type of ventilation: the fish swims with open mouth, water is pushed through the gills. The higher the speed of movement, the more intense the gas exchange.

Fish have a two-chambered heart and one circulation. The heart consists of an atrium and a ventricle. The venous sinus departs from the atrium, into which blood from the veins collects. In the heart of fish, blood is only venous. The abdominal aorta arises from the ventricle. It forms four pairs of afferent gill arteries (according to the number of gills). Oxygen-enriched blood is collected in the efferent branchial arteries, which on the dorsal side of the body flow into the paired roots of the dorsal aorta. The roots of the dorsal aorta fuse to form the dorsal aorta, from which vessels extend to all parts of the body. Venous blood from the caudal region flows through the tail vein. The vein bifurcates and enters the kidneys, forming a portal system only in the left kidney. From the kidneys, blood flows forward through paired veins, and from the head, also through paired veins, blood flows backward; these veins merge and form paired ducts that flow into the venous sinus. Blood from the intestines passes through the portal system of the liver and enters the venous sinus through the hepatic vein.

The brain is more primitive than that of cartilaginous fish. The forebrain is small, the roof does not contain nerve cells. The midbrain and cerebellum are relatively large in size. The eyes are large, the cornea is flat, the lens is round.

The hearing organ consists of the inner ear (membranous labyrinth), which is enclosed in a bone capsule. The capsule is filled with liquid in which auditory pebbles - otoliths - float. Pisces are capable of uttering and receiving. Sounds are produced when bones rub against each other and when the volume of the swim bladder changes.

Olfactory organs: olfactory capsules lined with sensitive olfactory epithelium.

Taste organs are special taste buds located in the mouth and on the skin.

On the sides of the swim bladder are paired gonads. In females, the ovaries have a granular structure, the posterior sections of the ovaries serve as excretory ducts. The genital opening opens on the urogenital papilla. The testes are long, smooth, their posterior sections are transformed into efferent ducts. The male genital opening also opens on the urogenital papilla.

The kidneys are long, ribbon-shaped, stretching along the sides of the spine above the swim bladder. The ureters depart from the kidneys and drain into the unpaired canal. Some fish have bladder, the duct of which opens on the urogenital papilla.

The caviar is small and has a gelatinous shell. Fertilization is external. Development with metamorphosis. The fertilized egg develops into a larva, which feeds from the yolk sac; the larva does not break through its mouth. As a result of metamorphosis, the larva turns into a fry - a self-feeding stage of fish development. Few fish species, such as sea bass, are hermaphrodites.

The bony fishes include the following orders: Herrings, Cyprinids, Eels, Pikefishes, Perchfishes, Sargans, Sticklebackfishes, Codfishes, Flounders, etc. Bony fishes are of great commercial importance.

Superorder Bony ganoids (Holostei)

The heyday of these fish occurred in the middle of the Mesozoic era. The modern fauna is represented by two species - armored pike and amia (mud fish), which live in fresh water bodies.

Superorder Polyteri

They live in fresh water bodies of Tropical Africa. The dorsal fin consists of small individual fins, hence the name.

Tunicates, or tunicates, which include ascidians, pyrosomes, sebaceous 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.

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. Until then, they were classified as different types of animals. These 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 solitary tunicates as a type of mollusk. Colonial forms were attributed by him to a completely different group - zoophytes, and some considered them to be 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 chordates is characterized by a number of certain important structural features. First of all, this is the presence of a dorsal string, or notochord, which is the internal axial skeleton of the animal. Tunicate larvae, freely swimming in water, also have a dorsal string, or notochord, which completely disappears when they transform into an adult. The larvae are much higher than the parental forms in other important structural characteristics. For phylogenetic reasons, that is, for reasons related to the origin of the group, higher value in tunicates, the organization of their larvae is more important than that of the adult forms. Such an anomaly is unknown for any other type of animal. In addition to the presence of a notochord, at least in the larval stage, tunicates are similar to real 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 into a respiratory organ. As we have seen in other chapters, invertebrate animals have very diverse respiratory organs, but the intestines never form gill slits. This is a characteristic of chordates. The embryonic development of the tunica also has many common features with the development of Chordata.

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 subphylum of the chordate phylum- Chordata, which together with them includes three more subtypes of animals, including vertebrates (Vertebrata), or as an independent type - Tunicata, or Urochordata. This type includes three classes: Appendiculars(Appendiculariae, or Copelata), Ascidia(Ascidiae) and Salpy(Salpae).

Before ascidian divided into three groups: simple or single, ascidians (Monascidiae); complex, or colonial, ascidians (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.

Salpas are divided into two groups - kegmakers(Cyclomyaria) and salp itself(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.

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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. Appendicularia, 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 in the form of single organisms and in the form of 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 of water, but can sometimes 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 is 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 just as characteristic of the North Seas Arctic Ocean and Antarctica, as well as for 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.

The body structure of almost all tunicates is very different beyond recognition from the general plan of the body structure in the phylum chordates. 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 the ascidian.

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 there are some species that reach 40-50 cm, for example 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 solitary 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 look like an oblong, inflated sac irregular shape, growing with its lower part, which is 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 is oral, through which the ascidian absorbs water, the second is cloacal. The latter is usually slightly shifted to the dorsal side. Siphons can open and close using muscles called sphincters. The body of the ascidian is covered with a single-layer cellular cover - epithelium, which secretes on its surface a special thick shell - 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 coloring 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 walls of the tunic 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 tunica lies the body wall itself, or the 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.

The digestive canal of ascidians begins with the 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 gland 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 fishing 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.

The circulatory system of the 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 tunicates 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.

The nervous system of adult ascidians is extremely simple and much less developed than that of 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. In representatives of the family Molgulidae, the storage 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. The structure of an ascidiozooid - a member of a colony of complex ascidians - is, in principle, no different 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 single ascidians.

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

The structure of pyrosomes.

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). In pyrosomes, all zooids are 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 spine-like projections. The most important difference Their difference from colonies of sessile ascidians also lies in 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 for themselves the principle jet propulsion. This method of movement is used not only by pyrosomes, but also by other pelagic tunicates.

The tunica of pyrosomes 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 pink-colored colonies. Such gigantic pyrosomes - their length reaches 2, 5 and even 4 m, and the diameter of the colony is 20-30 cm - have been repeatedly caught in the 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. Typically, the sizes of pyrosomes are much smaller - from 3 to 10 cm in length with a diameter of 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 dimensions of 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.

The structure of the ascidiozooid 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 are not brought together on the dorsal side (Fig. 175, B). The dimensions of ascidiozooids are usually 3-4 mm, and for giant pyrosomes they are up to 18 mm in 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 glowing organs, which are accumulations of cell 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: kegworts, or doliolid(Cyclomyaria), 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 her 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. The connection between zooids in a salp colony, 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 asexual individuals emerging from eggs, 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

Our distant relatives are tunicates