How many types of polychaete worms are there? Structural features of polychaete worms. Type Annelids, or annelids

Compared to other types of worms, annelids exhibit features of a higher organization and constitute an important link in the evolution of the animal world.

Although they belong to protostomes, like and, but, unlike them, they have a secondary body cavity with its own epithelial lining (the so-called coelom).

These worms got their name from the clearly expressed division of their body into segments, or rings. Hence their short name “rings”. The ringlet type is genetically related to other, more complex types - and arthropods.

Most ringworms have a well-developed circulatory system, which is absent in other types of worms. Often the development of respiratory organs (gills) is observed in ringlets. The excretory organs, built according to the type of metanephridia, also became more complex. Deeper differentiation is typical for ringlets digestive systems s (mouth, pharynx, esophagus, crop, stomach, intestines, anus), as well as more complex nervous system, which includes, in addition to the suprapharyngeal and subpharyngeal ganglion and the peripharyngeal ring, the abdominal nerve cord.

Sense organs of annelids

The sense organs (eyes or their rudiments, tentacles, bristles, etc.; the primary ringlets have statocysts) received further development. Some annelids in ontogenesis go through the stage of a kind of larva - a trochophore, which repeats in its development some features of the distant ancestors of annelids. The emergence of metamerism, the essence of which consists in the systematic repetition in each segment of all internal and external organs of the body, should be considered very significant. An important stage in the evolution of worms was the development of parapodia in the rings - the rudiments of legs.

The genetic connection between ringworms and lower worms is known to be established through nemerteans, the study of which is not provided for in the school zoology course. Therefore, the question of the origin of annelids in high school cannot be dealt with accordingly. The teacher must limit himself general indication on a special type of worm-like animals existing in nature (nemerteans), a number of primitive features of which suggest their origin from ancient ciliated worms, and on the other hand, some structural and developmental features indicate their relationship with annelids. The ancestors of annelids were, in all likelihood, freely mobile predatory image life, which contributed to a significant improvement in their organization. Their initial habitat was the sea, and then, in the process of evolution, some of the ringlets adapted to life in fresh water, as well as in soil.

Nervous system of annelids

Due to the metameric structure of the nervous system, each segment of the body has ganglia from which nerves extend, containing both sensory fibers that perceive irritations coming from receptors and motor fibers that conduct irritations to the muscles and glands of the worm. Consequently, ringlets have an anatomical and morphological basis for reflex activity in a wide range. It should be borne in mind that the head ganglia of the worm (supra- and subpharyngeal) with the help of sensory organs receive from the outside such irritations that are not perceived by other parts of the body. However, despite the leading role of the head nerve centers, unconditioned reflex reactions in the rings can also be carried out locally, in each segment of the body, which has its own ganglia. Moreover, the closure of the reflex arc can occur according to the type receptor - sensory axon - motor axon - muscle cell. In this case, the central nervous system only regulates the level of muscle activity.

The meaning of annelids

Annelids play a significant role in the cycle of substances in nature and occupy a prominent place in many biocenoses of land and sea. No less great is the practical significance of the rings as a power source for commercial fish and as an active factor in the soil-forming process. Some species of sea ringlets (polychaetes) have the ability to selectively absorb and accumulate chemicals dispersed in water in their bodies. For example, they found a concentration of cobalt ranging up to 0.002%, and nickel - from 0.01 to 0.08%, i.e. many thousands of times higher than in water. This ability is also characteristic of other ocean inhabitants, which opens up the prospect for humans to extract rare elements directly from sea water with the help of invertebrates.

The food relationships of ringed beetles are very diverse and affect many groups of invertebrates, excluding insects, with which they do not have direct food contacts.

Types of annelids

Currently, over 7,000 species of ringlets are known, grouped into several classes, of which only two are studied in high school: the class Polychaete annelids, or Polychaetes, and the class Oligochaetes, or Oligochaetes. Polychaetes are important for understanding the origin of annelids and at the same time are of interest as an ancestral group in relation to other classes of annelids, and polychaetes can serve as an example of the adaptation of annelids to existence in fresh water and soil. The study of live ringlets is carried out at school only on representatives of the class of oligochaetes (earthworms). Familiarization with polychaete ringlets is carried out on exhibits of zoological museums, using wet preparations.

The phylum Annelida includes three classes: Polychaetes, Polychaetes and Leeches. Characteristics of the type are given using the example of the most numerous class - Polychaetes.

Class Polychaeta

The scientific name of the class “polychaetes” means “polychaetes” in Greek. These worms are the most numerous of the ringworms; there are over 5,000 species of them. Most live in marine bodies of water, inhabiting all areas and depths of the World Ocean. They are found both in the water column and at the bottom, penetrating into the soil layers or remaining on the surface. Among the polychaetes there are predatory and peaceful views, i.e. carnivores and herbivores. Both of them use sharp, strong jaws when eating food. Pelagic worms chase fish fry; bottom worms eat algae, hydroid polyps, other worms, small crustaceans and mollusks. Those living in the soil pass sand with particles of organic substances through their intestines.

Many polychaetes build themselves tube-houses in which they hide from enemies; others live in burrows and, in case of danger, burrow into the ground (sandworms). The lifespan of polychaetes does not exceed 2-4 years. In some species, care for the offspring is clearly expressed (bearing young - in the brood pouch and special cavities or under the cover of dorsal scales).

Polychaete larvae often settle on the bottom of ships and, together with other fouling organisms, cause harm, reducing the navigability of ships. Since polychaetes do not have a hard skeleton, they serve as complete and easily digestible food for fish, constituting an important element of their food supply.

Polychaetes, with a few exceptions, are marine animals that live in extremely diverse ecological conditions.

Body structure of polychaetes

The body of polychaetes is segmented and consists of a head lobe (prostomium), body segments and anal lobe (pygidium). On the head lobe there are sensory organs: touch (on the palps), vision (simple eyes), chemical sense. The body is elongated, worm-shaped, the number of segments varies greatly. Segments of the body can be identical in structure (homonomous metamerism) or different in both structure and functions (heteronomous metamerism). Metamerism is the division of the body of animals into similar sections - metameres located along the longitudinal axis of the body. Polychaetes are characterized by the process of cephalization - the inclusion of one (or more) segments of the body into the head section.

The body segments are equipped with paired lateral motor appendages - parapodia. In fact, parapodia are the first primitive limbs that originated in invertebrates. Each segment bears a pair of parapodia. The parapodium consists of two branches: dorsal (notopodium) and ventral (neuropodium). Each branch contains a tuft of bristles. In addition to thin identical setae, the branches of the parapodia contain thick supporting setae. The sizes, shapes of parapodia and setae within the class are very diverse. Often in sessile forms the parapodia are reduced.

The body of polychaetes is covered with a thin cuticle formed by a single-layer integumentary epithelium. The epithelium contains single-celled glands that secrete mucus onto the surface of the worms' body. In sessile polychaetes, skin glands secrete substances for the construction of tubes in which worms live. The tubes can be inlaid with grains of sand or impregnated with calcium carbonate.

Under the epithelium there are two layers of muscle - circular and longitudinal. The cuticle, epithelium and layers of muscle form a skin-muscle sac. From the inside, it is lined with single-layer epithelium of mesodermal origin, which limits the secondary body cavity, or coelom. Thus, the coelom is located between the body wall and the intestine. In each segment, the coelom is represented by a pair of sacs filled with coelomic fluid. It is under pressure, and individual cells - coelomocytes - float in it. Contacting above and below the intestines, the walls of the pouches form a two-layer partition - the mesentery (mesentery), on which the intestines are suspended from the body. At the border between the segments, the walls of adjacent coelomic sacs form transverse partitions - dissepiments (septa). Thus, the septa are divided as a whole into a certain number of transverse sections.

Functions of the secondary body cavity: supporting (liquid internal skeleton), distribution (transport of nutrients and gas exchange), excretory (transport of metabolic products to the excretory organs), reproductive (in general, maturation of reproductive products occurs).

The mouth leads into a muscular pharynx, which in predatory species may contain chitinous jaws. The pharynx goes into the esophagus, and then the stomach follows. The above sections make up the foregut. The midgut has the shape of a tube and is equipped with. own muscular lining. The hindgut is short and opens with an anal opening on the anal lobe.

Polychaetes breathe through the entire surface of the body or with the help of gills, into which some parts of the parapodia turn.

The circulatory system is closed. This means that it circulates in the animal’s body only through the vascular system. There are two large longitudinal vessels - dorsal and abdominal, which are connected in segments by ring vessels. A very dense capillary network is formed under the epithelium and around the intestine. The capillaries also intertwine the convoluted tubules of the metanephridia, where the blood is freed from waste products. There is no heart; its functions are performed by a pulsating spinal vessel, and sometimes by annular vessels. Blood flows from front to back through the abdominal vessel, and from back to front through the spinal vessel. Blood may be red due to the presence of iron-containing respiratory pigment, or it may be colorless or have a greenish tint.

The excretory organs in primitive polychaetes are represented by protonephridia, and in higher ones - by metanephridia. The metanephridium is a long tubule that opens into a generally ciliated opening. The genital funnels (genital ducts) fuse with the metanephridium tubules, and a nephromyxium is formed, which serves to remove metabolic products and germ cells. Metanephridia are located metamerically: 2 in each body segment. The excretory function is also performed by chloragogenic tissue - modified coelomic epithelial cells. Chlorogenic tissue functions according to the principle of a storage bud.

Nervous system of polychaetes

The nervous system consists of paired cerebral ganglia, the peripharyngeal nerve ring and the ventral nerve cord. The abdominal nerve cord is formed by two longitudinal nerve trunks, on which two adjacent ganglia are located in each segment. Sense organs: organs of touch, chemical sense and vision. The organs of vision can be quite complex.

Reproduction of polychaetes

Polychaete worms are dioecious, sexual dimorphism is not pronounced. The gonads are formed in almost all segments, have no ducts, and the reproductive products exit as a whole, and out through nephromyxia. In some species, reproductive products are released into the water through breaks in the body wall. Fertilization is external, development proceeds with metamorphosis. The polychaete larva - trochophore - swims in plankton with the help of cilia. In the trochophore, two large mesodermal cells lie on the sides of the intestine - teloblasts, from which the sacs of the secondary body cavity subsequently develop. This method of laying down the coelom is called teloblastic and is characteristic of protostomes.

In addition to sexual reproduction, polychaetes have asexual reproduction, timed to coincide with the period of maturation of reproductive products. At this time, some species rise from the bottom (atokine forms) and lead a planktonic lifestyle (epitoke forms). Epitoke forms are morphologically very different from atoce forms. In these animals, the back of the body can form a head and separate from the front. As a result of regeneration processes, chains of individuals are formed.

Polychaetes serve as food for many species of fish - benthophages, large crustaceans and marine mammals.

At school, students get acquainted with polychaetes using the example of representatives of two families - nereids and sandworms. In addition to the information provided about them in the school textbook, some additional data is given below.

Nereids

Students should be informed that there are over 100 species of Nereids in nature. They belong to the subclass of vagrant polychaetes. The body of Nereids is often painted in green tones, cast in all the colors of the rainbow. Nereids of the White Sea feed on kelp and other algae, as well as small animals; Some species of nereids from the seas penetrate through the mouths of rivers into rice fields, where they gnaw young rice shoots, causing damage to the seedlings. One of the tropical Nereids even moved to land and began to live far from seashore on banana and cocoa plantations, where it lives in a humid environment, feeding on rotting leaves and fruits. These facts show that marine forms of polychaetes can adapt to life in fresh water and on land, which sheds light on the origin of ringlets living in fresh water bodies and in moist soils (oligochaetes, leeches).

Some types of nereids live only in clean water and cannot tolerate the presence of even small amounts of hydrogen sulfide in it, while others can live in polluted water bodies with organic matter rotting in silt. Consequently, nereids, like other aquatic organisms, can serve as indicators of water quality.

As a result of the artificial relocation of Nereids from the Azov Sea to the Caspian Sea, the nutrition of those inhabiting it has significantly improved valuable species fish For example, silt, rich in detritus, previously lay on the bottom of the Caspian Sea as if it were “dead capital”; now it serves as food for Nereids, which, in turn, constitute the main food for fish (sturgeon, stellate sturgeon, bream, etc.). The success of the acclimatization of Nereids, carried out under the leadership of Academician L.A. Zenkevich, opened up broad prospects for the reconstruction of the food supply of not only the Caspian, but also the Aral Sea and entailed a number of other similar measures for the reconstruction of marine fauna.

Nereids are capable of forming temporary connections of a conditioned reflex type. For example, one of the White Sea Nereids was systematically illuminated simultaneously with feeding at the moment it emerged from the tube. After several sessions, the worm began to crawl out on lighting alone, without reinforcing this stimulus with food. Then this reflex was converted to darkening, and even later to changing the degree of illumination.

Nereid trochophores have remarkable maneuverability in swimming, which is facilitated not only by the streamlined shape of the larva, but even more so by the peculiar movements of the cilia in the belts covering the body of the trochophore. This movement creates special currents of water that carry the larva forward, and changing the mode of operation of the cilia allows it to move in a variety of directions. Using the principles of trachophora propulsion, a model of a submarine with rotary engines was proposed in the United States. Thus, knowledge of the characteristics of the trochophore found application in technology after the ringlet larva became an object of bionics.

Sand veins

The silty and sandy soils of the littoral zone are inhabited by greenish-brown polychaetes (20-30 cm long), which lead a burrowing lifestyle. They belong to the subclass of sessile polychaetes and feed on plant dotrite, swallowing and passing soil with organic residues through their intestines.

In the littoral zone of the White Sea at low tide, you can see traces of the activity of sandworms in the form of many trapping funnels and cone-shaped emissions of sand. Sandworms make curved burrows with two exits to the surface in the upper layers of coastal shallows. A funnel is formed at one end of the burrow, and a pyramid is formed at the other. The funnel is a sock that has settled near the worm’s mouth as a result of the sandworm’s absorption of soil along with rotting algae, and the hummock is another portion of sand thrown out that has passed through the intestines of the worm. Calculations have shown that sand extractors are capable of renewing and processing up to 16 tons of soil per 1 hectare of sea coast per day.

Class Oligochaeta

The scientific name for this class, “oligochaetes,” comes from a Greek word that means “oligochaetes.” Oligochaetes evolved from polychaetes by changing some structural features due to their transition to other habitats (fresh water, soil). For example, they completely lost parapodia, tentacles, and in some species, gills; The larval stage, the trochophore, disappeared and a cocoon appeared, protecting the eggs from the effects of soil particles.

The sizes of oligochaetes range from 0.5 mm to 3 mm. About 3,000 species of oligochaetes are known, the vast majority of which are soil inhabitants. Several hundred species live in fresh water and very few (several dozen species) belong to marine forms.

Oligochaetes are inhabitants of soil or fresh water; marine representatives are extremely few in number. The parapodia of oligochaetes are reduced; only a limited number of setae are preserved. Oligochaetes are hermaphrodites.

Body structure of oligochaetes

The body of oligochaetes is elongated and has homonomic segmentation. No cephalization processes are observed; there are no sensory organs on the head lobe. Each body segment bears 4 tufts of setae, the number and shape of which are different. The body ends with an anal lobe.

The body of oligochaetes is covered with a thin cuticle, which is secreted by a single-layer epithelium rich in mucous glands. The secreted mucus is necessary for the worm to ensure respiration processes, and also facilitates the animal’s movement in the ground. There are especially many glands concentrated in the girdle area - a special thickening on the body that takes part in the process of copulation. The muscles are circular and longitudinal, the longitudinal ones are more developed.

In the digestive system of oligochaetes, complications associated with feeding habits are observed. The pharynx is muscular and leads into the esophagus, which expands into the goiter. In the crop, food accumulates, swells and is exposed to enzymes that break down carbohydrates. The ducts of three pairs of calcareous glands flow into the esophagus. Calcareous glands serve to remove carbonates from the blood. Carbonates then enter the esophagus and neutralize humic acids, which are contained in rotting leaves - food for worms. The esophagus flows into the muscular stomach, in which food is ground. On the dorsal side of the midgut, an invagination is formed - typhlosol, which increases the absorptive surface of the intestine.

In the circulatory system, the role of “hearts” is performed by the first five pairs of annular vessels. Breathing occurs through the entire surface of the body. Oxygen dissolved in mucus diffuses into a dense capillary network located under the integumentary epithelium.

Excretory organs are metanephridia and chloragogenous tissue covering the outer surface of the midgut. Dead chloragogenous cells stick together in groups and form special brown bodies, which are brought out through unpaired pores located on the dorsal surface of the body of the worms.

The nervous system has a typical structure, the sensory organs are poorly developed.

Reproduction of oligochaetes

The reproductive system is hermaphrodite. The gonads are located in several genital segments. Fertilization is external, cross. During copulation, the worms stick together with girdle mucus and exchange sperm, which is collected in the seminal receptacles. After this, the worms disperse. A mucous muff forms on the girdle, which slides towards the anterior end of the body. Eggs are deposited in the muff, and then the partner's sperm is squeezed out. Fertilization occurs, the muff slides off the body of the worm, its ends close, and a cocoon is formed, inside which direct development of the worms occurs (without metamorphosis).

Oligochaetes can reproduce asexually - by architomy. The body of the worm is divided into two parts, the front part restores the rear end, and the back part restores the head.

Earthworms play important role in soil-forming processes, loosening the soil and enriching it with humus. Earthworms serve as food for birds and animals. Freshwater oligochaetes - essential component in fish nutrition.

Students can become familiar with oligochaete worms using living objects. Among freshwater oligochaetes, naids and tubifex worms are especially accessible, and among soil inhabitants - various earthworms and enchytraeids (pot worms). In addition to observations, a number of elementary experiments can be carried out in a corner of living nature, in particular, on regeneration, which is quite pronounced in oligochaetes.

Earthworms

The zoology textbook describes the common earthworm, one of the representatives of the Lumbricidae family. However, in fact, when working with students, the teacher will have to deal with that specific species, individuals of which will be extracted from the soil of a school plot or obtained on an excursion to study the soil fauna of a certain biocenosis (fields, meadows, forests, etc.). And although in basic features all these worms are similar, they differ from each other in details depending on the species.

It is important that children learn about the existence of many species of earthworms, adapted to various living conditions in nature, and not be limited to a one-sided idea of ​​them only on the basis of textbook materials. In the Lumbricidae family, for example, there are about 200 species, grouped into several genera. The species identification of worms is based on a number of characteristics: size and color of the body, number of segments, arrangement of setae, shape and position of the girdle and other external and internal features buildings. Students should also be informed that in favorable landscapes the biomass of earthworms can reach 200-300 kg per 1 hectare of land.

When becoming familiar with the external structure of earthworms, students should pay attention to the weak development of bristles, which, however, play a significant role in the movement of worms in the soil. During the excursion, it is easy to verify that the earthworm’s body is firmly fixed in the burrow. You can tell students that at the base of each bristle there are small bristles that replace the old ones as they wear off.

While observing the behavior of a worm in a corner of wildlife as it burrows into the ground, students should become familiar with the “mechanics” of this process and clarify the role of bristles in it. The earthworm acts with the front end of its body like a battering ram. It pushes soil particles to the sides while swelling the front part of the body, where fluid is pumped by muscle contraction. At this moment, the bristles of the head section rest against the walls of the stroke, creating an “anchoring”, i.e., an emphasis for pulling up the rear sections, and the bristles of these latter are pressed against the body, reducing friction on the soil during movement. When the head section begins to move forward again, the bristles of the rest of the body rest against the ground and provide support for the extension of the head.

Due to life in the soil, earthworms, compared to free-living oligochaetes, have underdeveloped bristles, and the receptor apparatus has also become simpler. The outer layer contains various sensitive cells. Some of them perceive light stimulation, others - chemical, others - tactile, etc. The head end is the most sensitive, and the rear end is less sensitive. The weakest sensitivity is observed in the middle part of the body. These differences are due to the unequal distribution density of sensitive cells.

Any harmful or unpleasant external influence; factor causes a defensive reaction in the earthworm: burrowing into the ground, contracting the body, secreting mucus on the surface of the skin. It is necessary to conduct elementary experiments that would show the attitude of worms to various stimuli. For example, tapping on the wall of the cage causes negative vibrotaxis (the worm hides in a hole). Bright light causes the worm to crawl into the shadows or hide in a hole (negative phototaxis). However, the worm reacts positively to weak light (heads towards the light source). Impact of even a very weak solution acetic acid at the head end causes negative chemotaxis (contraction of the anterior part of the body). If you place a worm on filter paper or glass, it tends to crawl to the ground. Negative thigmotaxis (avoidance of a foreign substrate from which unusual irritation emanates) operates here. A strong touch to the rear end entails pulling out the front end - the worm seems to run away. If you touch it from the front, the movement of the head end stops, and the tail end produces a backward movement. These experiments cannot be carried out directly on the surface of the earth, since the worms will burrow into the soil (defensive reaction).

When keeping worms in cages, you can observe them pulling leaves into the burrow. If the leaf is fixed in place, not allowing it to move, then the worm, after 10-12 unsuccessful attempts to bring the prey closer to the hole, leaves it alone and captures another leaf. This indicates the ability of worms to vary stereotypical behavior in accordance with specific circumstances. According to Darwin, the worms each time grab the leaves so that they are dragged into the hole more or less freely, for which they give them the appropriate orientation. However, recent observations have shown that the worms achieve desired results by trial and error.

Some scientists, following Darwin, believed that worms could distinguish the shape of objects and thus find leaves, but in reality it turned out that earthworms (like many other invertebrates) tend to find food using chemoreceptors. Thus, in the experiments of Mangold (1924), worms distinguished the petiole from the top of the blade in the foliage not by shape, but by the unequal smell of these parts of the leaf. It is now recognized that earthworms, while crawling on the ground, can perceive the outlines and placement of objects around them based on tactile and kinesthetic sensations.

In earthworms, activity varies throughout the day. About 1/3 of the day they are more active, and the rest of the time their activity decreases almost three times. In addition to the daily rhythm, earthworms also have a seasonal rhythm of activity. For example, during the winter, worms go deeper into the ground and remain there in burrows in a state of suspended animation. There are known cases of living worms being found inside pieces of ice, which indicates their great endurance and ability to withstand adverse conditions.

Studies conducted in Russia and abroad have shown the positive role of earthworms in improving soil structure and increasing their fertility.

Life in the soil, movement in the ground and contact with coarse particles of earth lead to mechanical damage to the delicate skin of the earthworm, and sometimes to tearing of their body into pieces. However, all these injuries do not lead to their death, since the worms have developed protective devices that ensure their survival in natural environment a habitat. For example, the mucus secreted by the skin glands has properties that protect the body from infection by pathogenic microbes and fungi that penetrate wounds and scratches. In addition, mucus moisturizes the surface of the body, preventing it from drying out, and serves as a lubricant during movement. In addition to mucous secretions, regenerative processes play an important role in preserving the life of worms, which are especially important during mechanical dismemberment of the body into pieces.

In a school wildlife corner, it is not difficult to conduct experiments on the regeneration of earthworms and observe the progress of the restoration of lost parts. The cephalic ganglia play an important role in these processes, so in some species of worms (for example, the dung earthworm), cut in half, the anterior end regenerates better and faster.

The adaptability of worms to existence in the soil is also expressed in the presence of durable cocoons, inside of which a small number of eggs develop. Cocoons can lie in the ground for up to 3 years, preserving the viability of the young. Adult worms also live for several years (from 4 to 10) in cages, where their life expectancy was determined. Under natural conditions, many worms do not live to their natural end, since they are eaten by moles in underground passages, and on the surface of the earth they are attacked and destroyed by ground beetles, large centipedes, frogs, toads, and birds. In particular, many worms die after heavy rains, when water floods their passages and burrows, displacing them and forcing the worms to crawl out to breathe.

Under experimental conditions, earthworms are capable of changing innate behavior based on the development of conditioned reflexes. This was clearly shown in the classic experiments of R. Yerkes (1912). He forced an earthworm to crawl through a T-shaped labyrinth consisting of two tubes connected at right angles. At one end of the transverse tube (right) there was an exit to a box with wet soil and leaves, and at the other (left) there was a strip of glass skin and battery electrodes. The worm crawled in the longitudinal tube until it entered the transverse one and then turned either to the right or to the left. In the first case, he found himself in a favorable environment, and in the second he experienced unpleasant sensations: irritation from the glass skin and an electric injection when his body connected the electrodes. After 120-180 trips, the worm began to prefer the path leading to the box. He developed a conditioned reflex to a biologically useful direction of movement. If the electrodes and the box were swapped, then after about 65 sessions the worm acquired a new conditioned reflex.

Class Leeches (Hirudinea)

Medicinal leech (Hirudo medicinalis) is used in medicine for diseases of blood vessels, blood clots, hypertension, sclerosis, etc.

Nereids. Syllides. Palolo

Polychaetes- marine free-living annelids. They lead a benthic, interstitial, and rarely planktonic lifestyle.

rice. 1.
A - top view, B - side view (pharynx extended
state), B - side view (pharynx retracted
condition): 1 - prostomium, 2 - ocelli, 3 - peristomium,
4 - palps, 5 - jaws, 6 - pharynx, 7 - parapodium,
8 - tentacles, 9 - setae, 10 - mouth.

Like all annelids, the body of polychaete worms consists of a head section, a segmented body and an anal lobe. The head is formed by the head lobe (prostomium) and the oral segment (peristomium). On the head of many polychaetes there are eyes and sensory appendages (tentacles, palps, antennae), and a mouth is located on the peristomium below (Fig. 1).

In most species, each of the trunk segments bears a pair of primitive limbs - parapodia (Fig. 2). Each parapodia consists of a basal part, a dorsal lobe and a ventral lobe. The dorsal lobe has a dorsal "barbel", the ventral lobe has a ventral "barbel". The dorsal "antennae" of some species is transformed into feathery gills. The parapodia are permeated with setae; muscles that cause the parapodia to move are attached to the large acicular setae. The movement of parapodia is synchronous.

rice. 2.
1 - dorsal lobe, 2 - abdominal
lobe, 3 - dorsal antenna, 4 - ventral
antenna, 5 - setae, 6 - acicula.

The skin-muscle sac has a structure typical of annelids, which includes a cuticle, a single-layer epithelium and two layers of muscles (Fig. 3). The circular muscles are located under the epithelium, the longitudinal muscles are located under the circular muscles. The longitudinal muscles are located in four “ribbons”, two of these ribbons are on the dorsal side of the body, two on the abdominal side. On the sides of the body there are bundles of muscles that drive the parapodia.

The inner side of the longitudinal muscles is lined with epithelium of mesodermal origin. The body cavity is not limited by muscles, as in roundworms, but has its own epithelial lining - coelomic epithelium. Due to this coelomic epithelium, two-layer transverse partitions between segments (dissepiments) are formed. The secondary cavity is divided into chambers by dissepiments; each segment contains a pair of coelomic sacs filled with fluid. Coelomic fluid performs transport, excretory, homeostatic and musculoskeletal functions.

Fig.3.
1 - epithelium, 2 - circular muscles, 3 - longitudinal
muscles, 4 - gills, 5 - dorsal lobe of parapodia,
6 - supporting seta (acycula), 7 - funnel
metanephridia, 8 - parapodia muscles, 9 - canal
metanephridia, 10 - oblique muscle, 11 - abdominal
blood vessel, 12 - ovary, 13 - abdominal antenna
parapodia, 14 - ventral lobe of parapodia, 15 -
intestine, 16-coelom, 17-dorsal blood vessel.

The digestive system consists of three sections. The anterior section is of ectodermal origin. It begins with the oral opening located on the peristomium on the ventral side. The oral cavity continues into the muscular pharynx. In carnivorous species, the pharynx consists of several layers of circular and longitudinal muscles, is armed with strong chitinous jaws and can be turned outward (Fig. 1B). The pharynx is followed by the esophagus, into which the ducts of the salivary glands come off. Some species have a small stomach. The middle section is of endodermal origin. Serves for final digestion of food and absorption of nutrients. In the hindgut, which is of ectodermal origin, fecal masses are formed. The anal opening is usually located on the dorsal side of the anal blade.

The closed circulatory system includes dorsal, abdominal, annular and peripheral blood vessels. Through a large and pulsating dorsal blood vessel, blood flows to the head end of the body, through the abdominal end - to reverse direction. In the anterior part of the body, blood is distilled through ring vessels from the dorsal vessel to the abdominal, in the posterior part of the body - from the abdominal to the dorsal. Arteries extend from the annular vessels to the parapodia and gills (Fig. 4B).

rice. 4. Internal structure diagram
polychaete worms:
A - nervous and excretory systems (top view),
B - digestive system and whole (top view),
B - circulatory, digestive and nervous systems
(side view): 1 - suprapharyngeal cephalic ganglion, 2 -
peripharyngeal connective, 3 - ventral nervous ganglia
chains, 4 - nerves, 5 - metanephridia, 6 - mouth, 7 - oral
cavity, 8 - pharynx, 9 - esophagus, 10 - intestine, 11 - muscles
pharynx, 12 - whole, 13 - dissepiment, 14 - ovary, 15 -
dorsal blood vessel, 16 - abdominal
blood vessel, 17 - ring blood vessels.

Gas exchange occurs in the blood capillaries of the integument or gills. In some species, gills are formed from parapodial “antennae”, in others - from head appendages.

The excretory organs are metanephridia, each segment has a pair of metanephridia. The metanephridium consists of a funnel (nephrostomy) and a canal. The funnel is lined with cilia and is located in the coelomic chamber. The canal extending from the funnel penetrates the septum between the segments and in the adjacent segment opens outwards with an excretory opening (nephropore). The job of metanephridia is to remove unnecessary waste products from the coelomic fluid. An additional excretory function is performed by chloragogenic cells of the coelomic epithelium, in which grains of guanine and uric acid salts are deposited.

The nervous system consists of the peripharyngeal nerve ring with ganglia and the ventral nerve cord (Fig. 4A). The suprapharyngeal paired ganglion is more developed than the subpharyngeal ganglion, which is why it is sometimes called the “brain”. The nerve chain originates from the subpharyngeal node and consists of segmentally located pairs of nerve nodes connected to each other by transverse and longitudinal commissures. Nerves extend from the ganglia to various organs. The sense organs are developed to varying degrees. Many species have eyes, and all have olfactory and tactile receptors.

Polychaetes are dioecious animals. Gonads are formed on the wall of the coelom and are of mesodermal origin. In some species, gonads develop in all segments of the body, in others - in some segments. The germ cells from the gonads first enter the secondary cavity. From the coelom, gametes enter the water either through breaks in the body (the parent generation dies) or through special ducts (coelomoducts or nephromyxia). Fertilization is external. Development - with transformation. The polychaete larva is called a trochophore. The trochophore has a rounded shape, a parietal plume of cilia, an equatorial ciliary belt, a radially symmetrical nervous system, protonephridia and a primary body cavity (Fig. 5). At the posterior end of the larval body, two cells, teloblasts, appear on the right and left sides of the intestine. The teloblasts will form the mesoderm and then the mesodermal organs. The trochophore sequentially turns into a metatrochophore, then into a nectochaete. The metatrochophore produces larval segments. In Nektochaete, the cephalic ganglia and ventral nerve cord are formed. Nektochaete transforms into a young worm. The larvae lead a planktonic lifestyle, performing the function of dispersal.

rice. 5.
A - appearance trochophores, B - diagram of the structure of a trochophore,
C - diagram of the structure of the metatrochophore, D - diagram of the structure
nektochaetes: 1 - parietal plume of cilia, 2 - equatorial
ciliary girdle, 3 - mouth, 4 - protonephridia, 5 - intestines,
6 - teloblasts, 7 - anus, 8 - setae, 9 - ocelli.

rice. 6.

Sexual reproduction may be accompanied by the phenomenon of epitoky. Epitoky is a sharp morphophysiological restructuring of the body of a polychaete worm with a change in body shape (expansion of segments, appearance of swimming parapodia and bright coloring) during the period of maturation of reproductive products.

Polychaetes can reproduce not only sexually, but also asexually by budding (Fig. 6) or fragmentation.

Nereids (Nereis sp.)(Fig. 7) lead a benthic lifestyle, can bury themselves in silt, and can swim above the surface of the bottom. Predators. Due to their active lifestyle, they have well-developed muscles and sensory organs. In some Nereids, sexual reproduction is accompanied by epitocy: Nereis virens float to the surface of the water during the breeding season, sweep out reproductive cells, after which they die or are eaten by birds and fish. From fertilized eggs, larvae develop, which, after swimming, settle to the bottom and develop into adults.

rice. 7. Nereid
(left) and
sandstone
(on right)

Nereids have food value. To strengthen the food supply of sturgeon, Nereis diversicolor was brought from the Azov Sea to the Caspian Sea, which took root and successfully reproduced in the new place.

Sand veins (Arenicola sp.)(Fig. 7) settle on flat sandbanks and burrow deep into the sand. The body shape and feeding method of sandworms are similar to those of an earthworm. Parapodia are reduced due to the burrowing lifestyle. Digging uses strong body muscles and a hydraulic method of movement by pushing cavity fluid from one end of the body to the other. Just like Nereids, sandworms are a favorite food of fish.

Fig.8. sedentary
polychaetes:
A - sertularia,
B - Spirobranchus.

Sessile polychaetes (Fig. 8) are a collective group of polychaete worms leading an attached lifestyle. They have glandular cells in their epithelium that secrete a secretion from which a protective convoluted or spirally twisted horny tube is built. As construction progresses, the tube is soaked in lime. Polychaetes of this group never leave their shelters. Only the head ends with fan-shaped gills protrude from the tubes. Gills - often brightly colored, are modified appendages of the head. Many species of sessile polychaetes have numerous eyes on their gills. When a predator approaches, these polychaetes contract their bodies with lightning speed and hide deep into the tube.

Parapodia in most species of sessile polychaetes are reduced due to their attached lifestyle.

rice. 9. Lateral
budding polychaete
Syllis ramosa

In Trypanosyllis, the budding zone is located at the caudal end of the parent organism. Here a “bundle” of sexual individuals of different ages is formed. As they mature, older individuals bud off and swim away.

In Autolytes (Fig. 10) there is an alternation of asexual and sexual generations. The asexual generation leads a benthic lifestyle, reproduces by budding, and in some species - longitudinal multiple budding. The sexual generation is epitocine, with pronounced sexual dimorphism. Females and males perform a “mating dance” at the surface of the water; after the release of sperm, the males die. Females carry the eggs on themselves, and after the larvae hatch, they also die.

rice. 10. Reproduction
Polychaetes autolites:
A - multiple budding, B -
"mating dance": 1 - parent
individual, 2 - daughter individuals (“buds”),
3 - female, 4 - male.

Palolo (Eunice viridis) live in Pacific Ocean. Sexual reproduction of these worms is preceded by asexual reproduction. In this case, the front part of the body remains at the bottom, and the rear budded part of the body is transformed into epitocous individuals filled with reproductive products and floats to the surface of the ocean. Here, germ cells are released into the water and fertilization occurs. In the entire population, the emergence of epitocine individuals occurs simultaneously, as if on a signal. The mass appearance of breeding polychaetes occurs in October or November on the day of the new moon. Knowing the timing of the reproduction of palolo, fishermen en masse catch polychaetes stuffed with “caviar”, which are used as food.

TYPE RINGED WORMS

Annelids are animals that have a long, segmented body. The body segments look like rings. They live in seas, fresh waters, and soil.

Annelids have 3 aromorphoses:

· Metamerism (organ systems are repeated in different segments of the body).

· Overall(secondary body cavity, own epithelial lining).

· Lateral outgrowths of the body ( parapodia) - organs of movement, primitive limbs.

The sizes of ringed fish range from fractions of a millimeter to 3 m. The body is divided into three sections: the head. Trunk and anal lobe. The head was formed by the fusion of several body segments. The head contains the mouth opening, eyes, organs of touch (antennae, palps, etc.). The body consists of homogeneous segments, covered with a skin-muscular sac consisting of a thin cuticle, single-layer epithelium and two layers of muscles - external circular and internal longitudinal. In the anterior and middle sections of the intestine there are differentiated areas (crop, stomach). The circulatory system is closed. Blood moves only through blood vessels. Respiration is carried out either over the entire surface of the body (oligochaetes and leeches), or with the help of gills located on the parapodia (polychaetes). The excretory system is presented metanephridia. The nervous system is represented by a peripharyngeal nerve ring, consisting of the suprapharyngeal and subpharyngeal nodes connected by nerve cords, and two nerve trunks with ganglia, connected to each other by jumpers. Sense organs are more developed in movable rings.

The type of annelids is divided into classes:

1. Polychaetes

2. Oligochaetes

3. Leeches

Class polychaetes (polychaetes)

Representatives lead a free-swimming and attached lifestyle. Movement is carried out by parapodia equipped with tufts of bristles. Parapodia are prototypes of arthropod limbs. In some polychaetes, parapodia have gill apparatus that ensures gas exchange in aquatic environment. Representatives of the class have a well-separated head section, where the sensory organs are located: tentacles, light-sensitive eyes, olfactory fossa. In the structure of the nervous, circulatory, excretory and digestive systems, polychaetes repeat the characteristics of their type. Dioecious, development proceeds with metamorphosis (there is a larval stage).

Polychaetes are a progressive branch of animals from which arthropods descend. Serve as food for marine animals. Nereids are specially acclimatized in the Caspian Sea as food for sturgeon. Palolo, which lives in the tropical waters of the Pacific Ocean, is of commercial importance.

Class oligochaetes

Representatives live in soil or fresh water. The head end is not expressed. Sense organs are poorly developed. There are no parapodia and few setae. Hermaphrodites, direct development.

Class representative - earthworm. Earthworms live deep in burrows, which they dig by swallowing soil. As a result, the head section is very weakly expressed, there are no parapodia, tentacles and ocelli. The skin is permeated with blood capillaries and moistened with mucus, which makes gas exchange easier, movement in the soil easier, and mucus also has small bactericidal properties. Earthworms have a girdle that is lighter than the rest of the body.

The digestive system consists of the mouth opening, oral cavity, pharynx, long esophagus, which has an extension - goiter, muscular stomach, and intestines. It all ends with the anus. Earthworms feed on rotting plant debris, passing a lot of earth through the digestive tract.

There is no respiratory system as such; gas exchange occurs over the entire surface of the body.

Closed circulatory system. Blood moves through blood vessels, of which two are especially developed - the dorsal and abdominal. They communicate with each other through annular vessels located in each segment. There is an extensive network of capillaries. The movement of blood is determined by the rhythmic contractions of blood vessels from the 7th to the 11th segment. Blood plasma contains respiratory pigments similar to hemoglobin.

The excretory system consists of paired convoluted tubes (metanephridia), which begin as a funnel with ciliated cells on the walls in the body cavity, and end with an excretory pore that opens outward.

The nervous system is represented by a peripharyngeal nerve ring, consisting of 2 suprapharyngeal and 2 subpharyngeal nodes, connected by nerve cords. Two nerve trunks depart from the subpharyngeal node, having thickenings in each segment - ganglia, which are connected to each other by jumpers.

The earthworm is a hermaphrodite. Fertilization is cross, development is direct. One worm has both male and female reproductive organs: female oviducts, male testes, vas deferens and spermatheca. The girdle forms a special mucus from which the muff is formed. The coupling begins to move towards the head ring, passing by the ducts of the oviducts, where the eggs enter. The muff then passes through the seminal receptacles, where the sperm is released.

Ringed animals are capable of regeneration. Earthworms influence the properties of the soil; By digging numerous burrows, they improve its structure, loosen it, and enrich it with organic matter.

Class Polychaeta

with all the colors of the rainbow bristles. Serpentine phyllodoces (Phyllodoce) swim and crawl quickly. Tomopteris (Tomopteris) hang in the water column on their long whiskers.

The class of polychaetes differs from other ringlets by a well-separated head section with sensory appendages and the presence of limbs - parapodia with numerous setae. Mostly dioecious. Development with metamorphosis.

General morphofunctional characteristics

External structure. The body of polychaete worms consists of a head section, a segmented body and an anal lobe. The head is formed by the head lobe (prostomium) and the oral segment (peristomium), which is often complex as a result of fusion

with 2-3 body segments (Fig. 172). The mouth is located ventrally on the peristomium. Many polychaetes have eyes and sensory appendages on their heads. Thus, in a Nereid, on the prostomium of the head there are two pairs of ocelli, tentacles - tentacles and two-segmented palps, on the peristomium below there is a mouth, and on the sides there are several pairs of antennae. The trunk segments have paired lateral projections with setae - parapodia (Fig. 173). These are primitive limbs with which polychaetes swim, crawl or burrow into the ground. Each parapodia consists of a basal part and two lobes - dorsal (notopodium) and ventral (neuropodium). At the base of the parapodia, there is a dorsal barbel on the dorsal side, and a ventral barbel on the ventral side. These are the sensory organs of polychaetes. Often the dorsal barbel in some species is transformed into feathery gills. Parapodia are armed with tufts of bristles consisting of an organic substance close to chitin. Among the setae there are several large setae-acicules, to which muscles are attached from the inside, driving the parapodia and tuft of setae. The limbs of polychaetes make synchronous movements like oars. In some species leading a burrowing or attached lifestyle, parapodia are reduced.

Skin-muscle bag(Fig. 174). The body of polychaetes is covered with a single-layer dermal epithelium, which secretes a thin cuticle onto the surface. In some species, certain parts of the body may have ciliated epithelium (a longitudinal ventral stripe or ciliated bands around the segments). Glandular epithelial cells in sessile polychaetes can secrete a protective horny tube, often impregnated with lime.

Under the skin lies circular and longitudinal muscles. The longitudinal muscles form four longitudinal ribbons: two on the dorsal side of the body and two on the abdominal side. There may be more longitudinal strips. On the sides there are bundles of fan-shaped muscles that drive the parapodium blades. The structure of the skin-muscle sac varies greatly depending on lifestyle. The inhabitants of the ground surface have the most complex structure skin-muscle sac, close to that described above. This group of worms crawls along the surface of the substrate using serpentine body bending and parapodia movements. The inhabitants of calcareous or chitinous tubes have limited mobility, as they never leave their shelters. In these polychaetes, strong longitudinal muscle bands provide a sharp lightning-fast contraction of the body and retreat into the depths of the tube, which allows them to escape from attacks by predators, mainly fish. In pelagic polychaetes, the muscles are poorly developed, since they are passively transported by ocean currents.

Rice. 172. External structure of the Nereid Nereis pelagica (according to Ivanov): A - anterior end of the body B - posterior end of the body; 1 - antennae, 2 - palps 3 - peristomal antennae, 4 - eyes, 5 - prostomium, 6 - olfactory fossa, 7 - peristomium, 8 - parapodia, 9 - setae, 10 - dorsal antennae, 11 - pygidium, 12 - caudal appendages , 13 – segment

Rice. 173. Parapodia of Nereis pelagica (according to Ivanov): 1 - dorsal antenna, 2 - notopodium lobes, 3 - setae, 4 - neuropodium lobes, 5 - ventral antenna, 6 - neuropodium, 7 - acicula, 8 - notopodium

Rice. 174. Cross section of a polychaete worm (according to Natalie): 1 - epithelium, 2 - circular muscles, 3 - longitudinal muscles, 4 - dorsal antennae (gill), 5 - notopodium, 6 - supporting seta (acicula), 7 - neuropodium, 8 - funnel of nephridium, 9 - canal of nephridium, 10 - oblique muscle, 11 - abdominal vessel, 12 - ovary, 13 - abdominal antenna, 14 - setae, 15 - intestine, 16 - coelom, 17 - dorsal blood vessel

Secondary body cavity- in general - the structure of polychaetes is very diverse. In the most primitive case, separate groups of mesenchymal cells cover the inside of the muscle bands and the outer surface of the intestine. Some of these cells are capable of contraction, while others are capable of turning into germ cells that mature in the cavity, only conventionally called secondary B In a more complex case, the coelomic epithelium can completely cover the intestines and muscles. The coelom is fully represented in the case of the development of paired metameric coelomic sacs (Fig. 175). When paired coelomic sacs close in each segment above and below the intestine, the dorsal and abdominal mesentery, or mesenteries, are formed. Between the coelomic sacs of two adjacent segments, transverse partitions are formed - dissepiments. The wall of the coelomic sac, lining the inside muscles of the body wall, is called the parietal layer of mesoderm, and the coelomic epithelium , covering the intestine and forming the mesentery, is called the visceral layer of mesoderm. Blood vessels lie in the coelomic septa.

Rice. 175. Internal structure of polychaetes: A - nervous system and nephridia, B - intestine and whole, C - intestine, nervous and circulatory systems, side view (according to Meyer); 1 - brain, 2 - peripharyngeal connective, 3 - ganglia of the abdominal nerve chain, 4 - nerves, 5 - nephridium, 6 - mouth, 7 - coelom, 8 - intestine, 9 - diosepiment, 10 - mesentery, 11 - esophagus, 12 - oral cavity, 13 - pharynx, 14 - muscles of the pharynx, 15 - muscles of the body wall, 16 - olfactory organ, 17 - eye, 18 - ovary, 19, 20 - blood vessels, 21 - network of vessels in the intestine, 22 - annular vessel , 23 - muscles of the pharynx, 24 - palps

The whole performs several functions: musculoskeletal, transport, excretory, sexual and homeostatic. Cavity fluid maintains body turgor. When the circular muscles contract, the pressure of the cavity fluid increases, which provides the elasticity of the worm's body, which is necessary when making passages in the ground. Some worms are characterized by a hydraulic method of movement, in which the cavity fluid, when muscles contract under pressure, is driven to the front end of the body, providing energetic forward movement. In general, nutrients are transported from the intestines and dissimilation products from various organs and tissues. The organs for excreting metanephridia by funnels open as a whole and ensure the removal of metabolic products and excess water. In the whole, there are mechanisms to maintain the constancy of the biochemical composition of the fluid and water balance. In this favorable environment, gonads form on the walls of coelomic sacs, germ cells mature, and in some species even juveniles develop. Derivatives of the coelom - coelomoducts - serve to remove sexual products from the body cavity.

Digestive system consists of three sections (Fig. 175). The entire anterior section consists of derivatives of the ectoderm. The anterior section begins with the oral opening located on the peristomium on the ventral side. The oral cavity passes into the muscular pharynx, which serves to capture food objects. In many species of polychaetes, the pharynx can turn outward, like the finger of a glove. In predators, the pharynx consists of several layers of circular and longitudinal muscles, is armed with strong chitinous jaws and rows of small chitinous plates or spines that can firmly hold, wound and crush captured prey. In herbivorous and detritivorous forms, as well as in sestivorous polychaetes, the pharynx is soft, mobile, adapted for swallowing liquid food. Following the pharynx is the esophagus, into which the ducts of the salivary glands open, also of ectodermal origin. Some species have a small stomach

The middle section of the intestine is a derivative of the endoderm and serves for final digestion and absorption of nutrients. In carnivores, the midgut is relatively shorter, sometimes equipped with paired blind side pouches, while in herbivores, the midgut is long, convoluted, and usually filled with undigested food debris.

The hind intestine is of ectodermal origin and can perform the function of regulating water balance in the body, since there water is partially absorbed back into the coelom cavity. Fecal matter forms in the hindgut. The anal opening usually opens on the dorsal side of the anal blade.

Respiratory system. Polychaetes mainly have cutaneous respiration. But a number of species have dorsal cutaneous gills formed from parapodial antennae or head appendages. They breathe oxygen dissolved in water. Gas exchange occurs in a dense network of capillaries in the skin or gill appendages.

Circulatory system closed and consists of the dorsal and ventral trunks, connected by annular vessels, as well as peripheral vessels (Fig. 175). Blood movement is carried out as follows. Through the dorsal, largest and most pulsating vessel, blood flows to the head end of the body, and through the abdominal - in the opposite direction. Through the ring vessels in the front part of the body, blood is distilled from the dorsal vessel to the abdominal one, and in the back part of the body - vice versa. Arteries extend from the annular vessels to parapodia, gills and other organs, where a capillary network is formed, from which blood collects into venous vessels that flow into the abdominal bloodstream. In polychaetes, the blood is often red due to the presence of the respiratory pigment hemoglobin dissolved in the blood. Longitudinal vessels are suspended on the mesentery (mesentery), annular vessels pass inside the dissepiments. Some primitive polychaetes (Phyllodoce) lack a circulatory system, and hemoglobin is dissolved in nerve cells.

Excretory system polychaetes are most often represented by metanephridia. This type of nephridia appears for the first time in the phylum annelids. Each segment contains a pair of metanephridia (Fig. 176). Each metanephridia consists of a funnel, lined inside with cilia and open as a whole. The movement of the cilia drives solid and liquid metabolic products into the nephridium. A canal extends from the funnel of the nephridium, which penetrates the septum between the segments and in another segment opens outwards with an excretory opening. In the convoluted channels, ammonia is converted into high-molecular compounds, and water is absorbed as a whole. In different species of polychaetes, excretory organs can be of different origins. Thus, some polychaetes have protonephridia of ectodermal origin, similar in

Rice. 176. The excretory system of polychaetes and its relationship with coelomoducts (according to Briand): A - protonephridia and genital funnel (in a hypothetical ancestor), B - nephromyxium with protonephridium, C - metanephridia and genital funnel, D - nephromyxium; 1 - coelom, 2 - genital funnel (coelomoduct), 3 - protonephridia, 4 - metanephridia

structure with those of flatworms and roundworms. Most species are characterized by metanephridia of ectodermal origin. In some representatives, complex organs are formed - nephromyxia - the result of the fusion of protonephridia or metanephridia with the genital funnels - coelomoducts of mesodermal origin. Additionally, the excretory function can be performed by chloragogenic cells of the coelomic epithelium. These are peculiar storage buds in which grains of excreta are deposited: guanine, uric acid salts. Subsequently, chloragogenic cells die and are removed from the coelom through nephridia, and new ones are formed to replace them.

Nervous system. Paired suprapharyngeal ganglia form the brain, in which three sections are distinguished: proto-, meso- and deutocerebrum (Fig. 177). The brain innervates the sense organs on the head. Periopharyngeal nerve cords extend from the brain - connectives to the abdominal nerve cord, which consists of paired ganglia, repeating in segments. Each segment has one pair of ganglia. The longitudinal nerve cords connecting the paired ganglia of two adjacent segments are called connectives. The transverse cords connecting the ganglia of one segment are called commissures. When the paired ganglia merge, a nerve chain is formed (Fig. 177). In some species, the nervous system becomes more complex due to the fusion of ganglia from several segments.

Sense organs most developed in motile polychaetes. On the head they have eyes (2-4) of a non-inverted type, goblet-shaped or in the form of a complex eye bubble with a lens. Many sessile polychaetes living in tubes have numerous eyes on the feathery gills of the head. In addition, they have developed organs of smell and touch in the form of special sensory cells located on the appendages of the head and parapodia. Some species have balance organs - statocysts.

Reproductive system. Most polychaete worms are dioecious. Their gonads develop in all segments of the body or only in some of them. The gonads are of mesodermal origin and form on the wall of the coelom. The germ cells from the gonads enter the whole, where their final maturation occurs. Some polychaetes do not have reproductive ducts and the germ cells enter the water through breaks in the body wall, where fertilization occurs. In this case, the parent generation dies. A number of species have genital funnels with short channels - coelomoducts (of mesodermal origin), through which the reproductive products are excreted out into the water. In some cases, germ cells are removed from the coelom through nephromyxia, which simultaneously perform the function of the reproductive and excretory ducts (Fig. 176).

Rice. 177. Nervous system of polychaetes: 1 - nerves of the antennae, 2 - neopalps, 3 - mushroom body, 4 - eyes with a lens, 5 - nerves of the peristomal antennae, 6 - mouth, 7 - peripharyngeal ring, 8 - abdominal ganglion of the peristomium, 9- 11 - parapodia nerves, 12 - ganglia of the ventral nerve chain, 13 - nerve endings of the nuchal organs

Reproduction Polychaetes can be sexual or asexual. In some cases, alternation of these two types of reproduction (metagenesis) is observed. Asexual reproduction usually occurs by transverse division of the worm's body into parts (strobilation) or by budding (Fig. 178). This process is accompanied by the regeneration of missing body parts. Sexual reproduction is often associated with the phenomenon of epitoky. Epitoky is a sharp morphophysiological restructuring of the worm's body with a change in body shape during the period of maturation of reproductive products: segments become wide, brightly colored, with swimming parapodia (Fig. 179). In worms that develop without epitocy, males and females do not change their shape and reproduce in benthic conditions. Species with epitocy may have several life cycle options. One of them is observed in Nereids, the other in Palolos. Thus, in Nereis virens, males and females become epitocous and float to the surface of the sea to reproduce, after which they die or become prey to birds and fish. From eggs fertilized in water, larvae develop, settling to the bottom, from which adults are formed. In the second case, as in the palolo worm (Eunice viridis) from the Pacific Ocean, sexual reproduction is preceded by asexual reproduction, in which the anterior end of the body remains at the bottom, forming an atokny individual, and the posterior end of the body is transformed into an epitokny tail part filled with sexual products. The back parts of the worms break off and float to the surface of the ocean. Here the reproductive products are released into the water and fertilization occurs. Epitocene individuals of the entire population emerge to reproduce simultaneously, as if on a signal. This is the result of the synchronous biorhythm of puberty and biochemical communication of sexually mature individuals of the population. The massive appearance of reproducing polychaetes in the surface layers of water is usually associated with the phases of the Moon. Thus, the Pacific palolo rises to the surface in October or November on the day of the new moon. The local population of the Pacific Islands knows these periods of reproduction of palolos, and fishermen en masse catch palolos stuffed with “caviar” and use them for food. At the same time, fish, seagulls, and sea ducks feast on worms.

Development. The fertilized egg undergoes uneven, spiral crushing (Fig. 180). This means that as a result of fragmentation, quartets of large and small blastomeres are formed: micromeres and macromeres. In this case, the axes of the cell cleavage spindles are arranged in a spiral. The inclination of the spindles changes to the opposite with each division. Thanks to this, the crushing figure has a strictly symmetrical shape. Egg crushing in polychaetes is determinate. Already at the stage of four blastomeres, determination is expressed. Quartets of micromeres give derivatives of ectoderm, and quartets of macromeres give derivatives

Rice. 178. Development of polychaetes (family Sylhdae) with metagenesis (according to Barnes): A - budding, B - multiple budding, C - alternation of sexual reproduction with asexual

Rice. 179. Reproduction of polychaetes: A - budding of the polychaete Autolytus (no Grasse), B, C - epitocous individuals - female and male Autolytus (according to Sveshnikov)

endoderm and mesoderm. The first mobile stage is the blastula - a single-layer larva with cilia. The blastula macromeres at the vegetative pole plunge into the embryo and the gastrula is formed. At the vegetative pole, the primary mouth of the animal is formed - the blastopore, and at the animal pole, a cluster of nerve cells and a ciliated crest - the parietal plume of cilia - is formed. Next, the larva develops - a trochophore with an equatorial ciliary belt - a troch. The trochophore has a spherical shape, a radially symmetrical nervous system, protonephridia and a primary body cavity (Fig. 180). The blastopore of the trochophore shifts from the vegetative pole closer to the animal along the ventral side, which leads to the formation of bilateral symmetry. The anal opening breaks through later at the vegetative pole, and the intestines become through.

Previously, there was a point of view that in all polychaetes the mouth and anus are formed from the blastopore. But, as was shown by the research of polychaete specialist V.A. Sveshnikov, this situation only represents special case development of polychaetes, and in most cases only the mouth is formed from the blastopore, and the anus forms independently at later phases of development. In the area of ​​the posterior end of the larva, in the immediate vicinity of the anus, on the right and left sides of the intestine, a pair of cells appears - teloblasts, located in the growth zone. This is the rudiment of mesoderm. The trochophore consists of three sections: the head lobe, the anal lobe and the growth zone. -In this area, the zone of future growth of the larva is formed. The structural plan of the trochophore at this stage resembles the organization of lower worms. The trochophore successively turns into a metatrochophore and a nectochaete. In the metatrochophore, larval segments are formed in the growth zone. Larval, or larval, segmentation involves only ectodermal derivatives: ciliary rings, protonephridia, rudiments of the setal sacs of future parapodia. Nektochaete is distinguished by the fact that it develops a brain and an abdominal nerve cord. The setae from the setal sacs are exposed, and the parapodial complex is formed. However, the number of segments remains the same as in the metatrochophore. Different types of polychaetes may have different numbers of them: 3, 7, 13. After a certain time pause, postlarval segments begin to form and the juvenile stage of the worm is formed. In contrast to larval segmentation, postlarval segments in juvenile forms capture derivatives of not only ectoderm, but also mesoderm. At the same time, in the growth zone, teloblasts sequentially separate the rudiments of paired coelomic sacs, in each of which a metanephridia funnel is formed. The secondary body cavity gradually replaces the primary one. At the borders of contact of the coelomic sacs, dissepiments and mesenterium are formed.

Due to the remaining primary body cavity, longitudinal vessels of the circulatory system are formed in the lumen of the mesentery, and circular vessels are formed in the lumens of the septa. Due to the mesoderm, the muscles of the skin-muscular sac and intestines, the lining of the coelom, gonads and coelomoducts are formed. The nervous system, metanephridia channels, foregut and hindgut are formed from the ectoderm. The midgut develops from the endoderm. After metamorphosis is completed, an adult animal develops with a certain number of segments for each species. The body of an adult worm consists of a head lobe, or prostomium, developed from the head lobe of the trochophore, several larval segments with a primary cavity, and many postlarval segments with a coelom and an anal lobe without a coelom.

Thus, the most important features of the development of polychaetes are spiral, determinate fragmentation, teloblastic anlage of mesoderm, metamorphosis with the formation of trochophore larvae, metatrochophore, nectochaete and juvenile form. The phenomenon of the dual origin of metamerism in annelids with the formation of larval and postlarval segments was discovered by the prominent Soviet embryologist P. P. Ivanov. This discovery shed light on the origin of annelids from oligomeric ancestral forms.

The consistent change in the phases of individual development of polychaetes from oligomeric to polymeric reflects a phylogenetic pattern. Comparative morphological data indicate that the ancestors of polychaetes had a small number of segments, i.e. they were oligomeric. Among modern polychaetes, the closest to ancestral forms are some primary ringlets of the class Archiannelida, in which the number of segments usually does not exceed seven. Manifestations of primitive organizational features at the trochophore and metatrochophore stages (primary cavity, protonephridia, orthogon) indicate the relationship of coelomic animals with the group of lower worms.

The biological significance of the development of polychaete worms with metamorphosis lies in the fact that the floating larvae (trochophores, metatrochophores) ensure the dispersal of species that, as adults, lead a predominantly bottom lifestyle. Some polychaete worms show care for their offspring and their larvae are inactive and lose their distribution function. In some cases, live births are observed.

The meaning of polychaete worms. The biological and practical significance of polychaete worms in the ocean is very great. The biological significance of polychaetes lies in the fact that they represent an important link in trophic chains, and are also important as organisms that take part in the purification of sea water and the processing of organic matter.

substances. Polychaetes have food value. To strengthen the food supply of fish in our country, for the first time in the world, the acclimatization of nereids (Nereis diversicolor) in the Caspian Sea, which were brought from the Azov Sea, was carried out. This successful experiment was carried out under the leadership of Academician L.A. Zenkevich in 1939-1940. Some polychaetes are used as food by humans, such as the Pacific palolo worm (Eunice viridis).

General characteristics of polychaete annelids

Most polychaetes are predators, but among them there are also many herbivorous and omnivorous forms. Sessile polychaetes feed on small animals, plants and plant detritus. Most polychaetes are undoubtedly very useful animals, since they constitute a significant part of the food of many commercial fish and other marine animals.

External structure

The body shape of most polychaetes is elongated, worm-shaped, or slightly flattened in the dorsoventral direction. The body is divided into three sections: the head, trunk, often consisting of a very significant number of segments (up to several hundred), and a short anal section, or pygidum.

The head section, in turn, consists of two parts: 1) the prostomium, or the head lobe itself, and 2) the peristomium, or the oral section. The prostomium contains eyes, a pair of tentacles, or tentacles, a pair of more massive palps, and olfactory pits, or olfactory organs. In some polychaetes the cephalic appendages are reduced, in others they develop in large numbers. On the ventral side of the oral section, or peristomium, there is a mouth, and on its sides there are several pairs of antennae. The peristomium of most polychaetes is formed by the fusion of several (2-4) first trunk segments and only in the most primitive forms corresponds to a single first trunk segment.

Thus, in polychaetes we must note: 1) the process of differentiation of the head lobe (prostomium) and the peristomium formed by the first trunk segments, i.e., the process of cephalization, or the formation of the head section by attaching part of the trunk segments to the prostomium; 2) development of various cephalic appendages: tentacles, palps and peristomal antennae.

The trunk section of the most primitive forms consists of a larger or smaller number of identical segments. If in animals that have a segmented body, the segments are identical and repeat each other throughout the entire body, then such segmentation is called homogeneity. If there are differences between body segments (in shape, size, presence or absence of appendages, or in internal structure), then they speak of heteronomy of segmentation. Heteronomy is expressed to a certain extent in all polychaetes already in the attachment of the first body segments to the head.
The degree of heteronomy of other body segments in polychaetes varies. The greatest degree of heteronomy is observed in sessile and partly burrowing forms.

In the vast majority of polychaetes, on the sides of each body segment there are movable outgrowths of the body wall, lined with bristles. These are locomotor organs - parapodia, with the help of which polychaetes crawl and swim.

Parapodia vary greatly in their structure among different species. In the most typical cases, they are bilobed protrusions of the body wall into which the secondary body cavity (coelom) extends. Each parapodia consists of a main, or basal, part and two lobes. The dorsal lobe of the parapodia is called notopodia, the ventral lobe is called neuropodia. Both lobes bear more or less bristles. One of the setae of each lobe is especially strongly developed and begins deep inside the parapodium. This seta is called an acicula. The bristles are extremely varied in shape and size and consist of a substance close to chitin. They sit in special recesses of the skin epithelium - bristle sacs, to which muscle bundles approach. At the base of the ventral lobe of the parapodium there is a ventral barbel, and at the base of the dorsal lobe there is a dorsal barbel and often an adjacent gill. In burrowing forms, parapodia have undergone reduction to varying degrees.

In the primary ringlets, which are sometimes isolated in independent class Archiannelides, no parapodia or setae. Some of them have cilia located in bands at the boundaries of the segments.

Some zoologists are inclined to consider the absence of parapodia in primary ringlets as primary characters.
In sessile forms, the parapodia on the posterior part of the body are reduced; only the setae and hooks are retained, with the help of which the worms are held in the tubes.

The latter - the anal section, or pygidium - has no limbs.

Thus, in polychaetes, for the first time among invertebrate animals, special body appendages appear, which serve as organs of locomotion, or primitive limbs. This fact is of especially great interest, since the complexly structured articulated limbs of arthropods apparently originated from parapodia through their complication. Therefore, parapodia should be considered as the rudiment of a more perfect limb of higher invertebrates. Ring parapodia have not one, but several functions. They, as a rule, perform not only a locomotor function, but also a tactile function (with the help of bristles and antennae), and in many polychaetes the dorsal antennae turns into a respiratory organ - a gill.

Skin-muscle bag

The body of polychaetes is covered with a single-layer epithelium, which secretes a thin cuticle on its surface. The epithelium may be ciliated. It is rich in unicellular glands that secrete mucus and substances from which many sessile polychaetes build their tubes.

The muscles of the skin-muscle sac are located in two layers. Under the epithelium lies a layer of circular muscles, which extend into the parapodia. Under the circular muscles there are highly developed longitudinal muscles. The longitudinal muscles, however, do not form a continuous layer, but are differentiated into four highly developed muscle bands stretching along the entire body. Two of these muscle bands lie in the dorsal and two in the ventral part of the worm's body. In addition, there are individual muscle bundles, already described above, that move the bristles. Polychaetes are also characterized by the presence of dorsoventral muscles. These are muscle bundles that run obliquely from the dorsal part of the skin-muscular sac to the abdominal part.

The movements of polychaetes are much more complex than the movements of lower worms, especially nematodes. Moving along the ground, polychaetes make worm-like or, more precisely, wave-like movements with their whole body, resting on parapodia. In these movements, the main role is played by the most highly developed longitudinal muscles and parapodia muscles. In addition, many polychaetes, as well as many other ringlets, are characterized by peristaltic movements of the body, consisting of a wave of contractions passing along the length of the body - successive compression and lengthening of one section of the body and expansion and shortening of the neighboring section. These movements, which are especially important in burrowing forms, involve the entire musculature of the animal’s body, especially the annular and dorsoventral muscles.

Body cavity

Polychaetes are characterized by a secondary body cavity, or coelom, filled with abdominal fluid, with internal organs located in it (excretory, reproductive, etc.). Morphologically, the coelom differs from the blastocoel (primary body cavity) in that it is lined with a special coelomic, or peritoneal, epithelium that separates the cavity fluid from all surrounding tissues and organs. Longitudinal muscles, intestines and other organs are covered with single-layer peritoneal epithelium.

Another feature of the polychaete coelom is its metameric structure. Primary, each segment essentially has its own cavity, completely separated from the cavity of neighboring segments by special partitions-dissepipements and consisting of two layers of peritoneal epithelium with an intermediate substance between them. Moreover, the coelomic cavity in each segment is completely divided into right and left halves by a longitudinal (also two-layer) septum. The intestine runs inside this longitudinal septum, and the dorsal and abdominal blood vessels are located above and below the intestine. We can say that in each internal segment of polychaetes there are two coelomic sacs. The epithelial walls of these sacs are closely adjacent on one side to the muscles of the skin-muscular sac, forming the somatopleura, and on the other hand, to the intestines and to each other, forming the splanchnopleura, or intestinal layer. The splanchnopleura of the right and left sacs, covering the intestine and blood vessels on both sides, forms the dorsal and abdominal mesentery, or mesentery. The walls of the sacs facing adjacent segments form dissepiments.

In many polychaetes, dissepiments between adjacent segments disappear and in this case several segments have a common body cavity. Most often, such reduction of septa is observed in the anterior segments.

The coelomic fluid receives nutrients digested in the intestines, as well as breakdown products (potassium urates, sodium urates, etc.). The cavity fluid is rich in amoeboid cells - various leukocytes and eleocytes. The former perform mainly a phagocytic function, but are also capable of a trophic function, i.e., storing reserve substances. The latter function is more characteristic of eleocytes, which are usually filled with fatty inclusions and play the role of a kind of adipose tissue. Their reserves are used in the formation of reproductive products. Circulation of the cavity fluid occurs due to the presence of areas of ciliated coelomic epithelium. It helps to carry out the main functions of the coelom: nutritional, excretory and respiratory. In addition, the coelom performs a musculoskeletal function due to the incompressibility of the coelomic fluid and the peristaltic movements of the muscles.

Digestive system

The digestive system consists of three sections: the ectodermic foregut, the endodermic midgut and the ectodermic hindgut. The foregut includes the pharynx and esophagus. A pair of salivary glands open into the pharynx. In many mobile predatory polychaetes, for example in Nereis, the pharynx turns into an attack weapon. It is armed with powerful chitinous jaws and can twist or move far forward. In some sessile polychaetes, the role of the catching apparatus is played by modified palps.

The midgut is a straight tube passing through the dissepiments and continuing into the short hindgut.

Respiratory system

Typical respiratory organs for many polychaetes are gills, usually located on the dorsal lobes (branches) of parapodia. The gills are equipped with a system of capillaries in which gas exchange occurs. The skin, which also has a dense network of capillaries, plays an important role in breathing. Some polychaetes do not have special respiratory organs (nereids and some others) and the respiratory function is performed entirely by the skin. In many sessile forms, the gills are concentrated in the front of the body.

Circulatory system

The presence of a well-developed closed circulatory system is a very important feature of ringlets in general and polychaetes in particular. The most important parts of the circulatory system are two main vessels - dorsal and abdominal, which run along the entire body. These vessels are located in the mesentery above and below the intestine. The dorsal and abdominal vessels are connected by annular vessels covering the intestine. The ring vessels are arranged metamerically. Blood vessels extend from them to the gills, skin, parapodia, excretory organs and intestines. The dorsal vessel has contractile walls. Its pulsation drives blood forward to the head, and through the annular vessels down to the abdominal vessel. Through the abdominal vessel, blood moves in the opposite direction. In addition, blood circulation is promoted by the peristaltic movements of the worm's body.
The blood of polychaetes consists of plasma and cellular elements - hemocytes. Blood plasma usually contains a substance similar to hemoglobin, which gives blood its red color. Some polychaetes have colored blood green color, since plasma contains the green respiratory pigment chlorocruorin.

Excretory system

In most highly organized polychaetes, metanephridia serve as excretory organs.

Metanephridia are open excretory organs, in contrast to closed protonephridia. Metanephridia are arranged metamerically and in pairs, but in such a way that each nephridia begins in one (anterior) segment and ends with an excretory opening in the next (posterior).
The metanephridium begins with a more or less expanded funnel - a nephrostomy, seated with cilia and opening into the cavity of the segment. The nephridial canal begins from the nephrostomy, which passes through the dissepiment to the next segment. Here the nephridial canal forms a more or less complex ball, approaches the body wall and opens with an excretory opening to the outside. Blood vessels approach the metanephridia. The lumen of the nephridial canal usually contains cilia.

At first glance, metanephridia are completely different from protonephridia. However, these two types of excretory organs are genetically related to each other, and metanephridia developed from protonephridia. Polychaetes also have protonephridia. The polychaete larva - trochophore - has true protonephrondia with excretory canals and closed terminal cells. Many polychaetes (Alciopa, Phyllodoce, etc.) have protonephridia of a somewhat special type. In them, the nephridial canal in the terminal part is closed and ends in a group of cells resembling pins with expanded heads. In the expanded part of these cells the nucleus and cytoplasm are located, and in the tubular leg there is a channel in which there is an oscillating cord that reaches the beginning of the nephridial canal. Such peculiar nephridial cells are called solenocytes. Further, some polychaetes (Trypanosyllis) have open nephridia, but without a funnel, but with a bunch of flickering long cilia facing the lumen of the nephridial canal. This is like the next stage in the evolution of the excretory organs. Obviously, during the transformation of protonephridia into metanephridia, first there was a reduction of the closed terminal part and the formation of an open channel, and then the development of the funnel.

Primary to each segment there corresponds a pair of metanephridia, but in many polychaetes metanephridia are not present in all segments. The number of metanephridia is especially reduced in sessile polychaetes. The sandworm (Agenicola), which leads a burrowing lifestyle, has only six pairs of metanephridia; others may have even fewer (up to two pairs).

The excretory function of metanephridia is of two kinds. The wall of the nephridial canal, especially in its middle part, is penetrated by a network of blood capillaries. From the blood, liquid dissimilation products enter the nephridial canal. On the other hand, through the ciliary apparatus of the funnel, together with part of the coelomic fluid, excreta are carried out, which are preliminarily accumulated in the amoeboid cells of the coelom, as well as in special cells that have a phagocytic function and are located in certain places of the coelomic epithelium. As granules of solid excreta accumulate, these cells, along with the excreta they contain, are carried out. Thus, the development of metanephridia is closely related to the participation of the coelom in excretory processes.

Some polychaetes containing gonads in segments have, along with nephridia, coelomoducts, or sex funnels equipped with cilia. Coelomoducts develop from coelomic epithelium, in contrast to proto- and metanephridia, which are of ectodermic origin. In many polychaetes, the majority of coelomoducts do not exist independently, but are attached in various ways to the nephridial canal. In this case, so-called nephromyxia or mixed organs are formed, which simultaneously play the role of both excretory organs and excretory ducts for reproductive products.

Nervous system

In various forms of polychaetes there is a gradual transition from a rather primitive nervous system to a nervous system of a more complex structure.
The most important part of the nervous system of polychaetes is the paired suprapharyngeal ganglion, or brain. The brain of higher polychaetes reaches a high degree of development; in its anterior section, associative centers first appear - stalked, or mushroom-shaped, bodies, which are apparently homologous to the more developed mushroom-shaped bodies of higher insects. The peripharyngeal connectives extend from the brain and form the peripharyngeal nerve ring. Paired abdominal nerve trunks stretch from the nerve ring along the body.
On the nerve trunks, paired nerve nodes - ganglia - are differentiated to one degree or another, connected by transverse commissures. The first nerve ganglia of the abdominal trunks are called subpharyngeal. The trunks can be widely spaced or brought together until they are completely merged. Further complication in the structure of the nervous system lies in the more significant differentiation of the nerve ganglia in which nerve cells. Parts of the nerve trunks connecting successive pairs of ganglia turn into connectives. The ganglia are arranged metamerically according to the number of segments. There are transverse commissures between the ganglia, and the nervous system takes on the appearance of a scalene nervous system. The latter is typical of primary rings related to polychaetes. The scalene nervous system is observed in some highly organized sessile polychaetes, for example in Sabellaria.

Further, the evolution of the nervous system of polychaetes went in the direction of bringing the paired ganglia closer together until the commissures between them disappeared, and then their complete fusion. As a result, a typical ventral nerve cord developed. In some polychaetes, the connectives connecting successive ganglia remain paired; in others they also merge.

The position of the abdominal trunks and nerve chain in relation to the integument may also be different. The nervous system develops from the ectoderm, and therefore the primary position of the nerve trunk will be directly under the epithelium of the skin, as is observed in the primary rings. In other polychaetes, the nervous system is more or less immersed in the skin-muscular sac.

Sense organs

The sense organs of polychaetes are also much more developed than those of lower worms. It is clear that due to active nutrition The organs of vision and touch reach their greatest development in wandering polychaetes. Touch cells are located throughout the body. There are especially many of them on the tentacles, palps and parapodial sensitive antennae.

Polychaetes also have organs of chemical sense (olfaction). These are the palps and ciliated fossae, or nuchal organs, located on the dorsal part of the prostomium.

Some polychaetes also have statocysts - organs of balance. They can be closed organs or open pits communicating through channels with the external environment. Statocysts, consisting of five or more pairs, are located at the anterior end of the body and are more common in sessile polychaetes.

The eyes of these worms are most often located on the prostomium at the base of the antennae and palps. These are the so-called “supracerebral eyes”, usually available in one or two pairs. They can be quite different in structure. In simpler cases, these are eyes shaped like a glass, the bottom of which is formed by the retina. However, in many wandering polychaetes the eyes have a much more complex structure. The eyes are quite complex in Nereids and especially in swimming polychaetes from the family Alciopidae. These polychaetes have bubble-shaped eyes with a cornea, lens, vitreous body and retina. Such eyes have the ability to accommodate.

In addition, many polychaetes have special eyes, much simpler in structure and located on different places of the body: on the palps, antennae, and on the sides of the body. These ocelli can have a very different structure and are sometimes built like the inverted eyes of lower worms.

Reproduction

The vast majority of polychaetes are dioecious. Their genitals, unlike those of flatworms and even roundworms, have a very simple structure. In some polychaetes, gonads develop in all segments of the body, in others - in certain segments, most often only in the posterior part of the body. The gonads develop in the peritoneal epithelium. The gonad primordium is covered with epithelium. As soon as the reproductive products (eggs or sperm) mature, the epithelium ruptures and the reproductive cells fall into the body cavity.

In most polychaetes, fertilization is external and copulation does not occur. Only in very few forms does copulation take place, and in this case the structure of the reproductive apparatus is somewhat complicated by the presence of spermatheca in females and a copulatory organ in males, for example in Saccocirrus.

Reproductive products - eggs and sperm - enter the water in various ways. For many, there is simply a rupture of the wall of a segment overflowing with mature reproductive products. Others have genital funnels - coelomoducts. Very often, as already noted, reproductive products are excreted using nephromyxia.

Of great interest is the phenomenon of epitoky observed in many polychaetes.

Thus, in Nereis the usual benthic form changes greatly with the onset of sexual maturity. The body of the worm is differentiated into two sections: the anterior - atokny, which does not form reproductive products, and the posterior - epitokny, in the segments of which the gonads develop. These two parts of the worm's body differ sharply from one another in appearance. The greatest changes occur in the segments of the epitocine part. By increasing the number of setae and developing membranes, the parapodia of the epitocine part transform into swimming limbs. The intestine in these segments degenerates. The muscles are also reduced. Such forms of Nereids were previously described as a special “species” of polychaetes, called Heteronereis.

The heteronereid form of Nereis, in contrast to the immature form, leads a pelagic lifestyle and floats to the surface of the sea, where reproduction occurs.

In the palolo worm (Eunice viridis), the epitocial part reaches very large sizes (up to 20 cm in length or more). It breaks away from the attack part of the worm and independently floats to the surface of the ocean. This happens at certain times of the year and is widespread. Palolo lives in the Pacific Ocean, especially around the islands of Samoa and Fiji. In October or November, palolos rise to the surface of the ocean in huge masses and at this time are caught in large quantities by local residents, who eat them.

In some polychaetes (Autolytus), the epitocine part, before separating from the atocean part, forms a head, and then epitocene individuals with pronounced sexual dimorphism are separated. In this case, the sexual process is preceded by asexual reproduction and the formation of sexual individuals by dividing the worm.

Finally, among polychaetes there are also forms in which, through sequential budding, a whole chain is formed - strobila of epitocine individuals, successively breaking off and reproducing sexually.

It should be noted that many polychaetes, in accordance with the ability to reproduce by budding, have a very high ability to regenerate.

Development

The crushing of polychaete eggs is complete and has a clearly defined spiral character.

Gastrulation in polychaetes occurs by invagination at the vegetative pole. During the formation of the gastrula, the previously isolated mesoblast cell, representing the rudiment of the future mesoderm, divides into two cells - teloblasts, which are then located on the sides of the blastopore, and the mesoderm rudiment becomes paired.

Subsequently, in most polychaetes, a very characteristic trochophore larva develops from the egg. The trochophore is not at all similar in structure to a worm. The transparent body of the trochophore is pear-shaped. In its upper part there is a thickening of the integument, the so-called parietal plate, usually with a plume of cilia. The anus is located at the narrowed lower (posterior) end. Equatorially, the trochophore is surrounded by two rows of cilia, between which the mouth is located on one side (ventral). The belt of cilia lying in front of the mouth is called preoral or preoral, and behind the mouth is called postoral or postoral.

Trochophore has a characteristic V-shaped intestine, consisting of an expanded foregut, pouch-like midgut and hindgut. Between the intestine and the integument there is a cavity, which is a preserved blastocoel, or the primary body cavity. The trochophore has larval mesenchymal muscles and a pair of excretory organs of the protonephridial type. Finally, on the sides of the hindgut there are two groups of cells, slightly shifted to the ventral side. They are mesodermal primordia, or mesodermal stripes. Each of these stripes was formed by division of the primary teloblast.

How to explain the formation and position of the mouth and anus of the trochophore?

We noted earlier that during gastrulation the blastopore lies almost at the vegetative pole. But then it shifts to one side. This determines the ventral side of the embryo and larva. Then the blastopore extends along the trochophore meridian in the form of a slit-like opening and is partially overgrown on the side of the vegetative pole. From the part of the blastopore lying at the level of the equator of the embryo, the primary mouth is formed. The hindgut and anus are formed secondarily, by ectodermic invagination, merging with the gastric cavity.

Trochophora leads a planktonic lifestyle and, being at the mercy of sea currents, is transported over long distances. Thus, the biological significance of the planktonic larva is to ensure the spread of the species.

However, not all polychaetes produce a larva - a trochophore - from the egg. In many, the stage corresponding to the trochophore takes place in the egg, and the larva emerges from the egg at a later stage of development, the so-called metatrochophore or nektochaete. Nektochaete consists of two sections. The anterior, cephalic section resembles the corresponding part of the trochophore and bears the preoral corolla of cilia. The posterior section is elongated and consists of several (in Nereis - three, in other polychaetes - up to nine) segments with a corresponding number of parapodia.

The process of transformation of a trochophore into a worm in most polychaetes occurs as follows. The trochophore consists of two hemispheres: the preoral (preoral), including the parietal plate and the preoral belt of cilia, and the postoral, which gradually stretches and takes on a worm-like shape, forming the body of the worm. On early stage of this transformation (metatrochophore stage), the larva consists of a head section formed from the preoral hemisphere of the trochophore, and the trunk section consists of several (from 3 to 9, and sometimes more) so-called larval or larval (from larva - larva) segments. The larval section is formed by the growth of the postoral part of the trochophore in length, with the teloblasts remaining all the time at the posterior end, on the sides of the hindgut, and the mesodermal rudiments in the form of two stripes are located on the sides of the intestine along the larval section. In the larval region, external segmentation occurs and parapodia are formed. By moving apart the cells inside the mesodermal primordia, a cavity is formed, divided into metameric sections, depending on the external metamerism of the larval region.

Thus, the larval rudiment of the mesoderm is segmented depending on the external segmentation of this part of the body according to the number of larval segments. At this stage of development, the so-called oligomeric rings from primary annelids, for example Dinophilus, remain.

In most polychaetes, the growth of the worm's body continues at the lower (posterior) end of the nectochaete larva. A growth zone is formed here, as a result of which this part of the body begins to grow in length. The mesoderm strips preserved after the separation of the larval mesoderm, as the worm grows, separate paired mesodermic rudiments, or somites, in front of them. This process is not determined by the development of external metamerism of the body, but, on the contrary, external segmentation develops depending on the number of pairs of somites separated from the mesodermal stripes. At first, each somite is a group of cells, but then the somites grow, and a cavity is formed inside each of them. So the somite turns into a closed bubble that continues to grow. Growing paired somites displace the primary body cavity and separate the rudiments from themselves, due to which the longitudinal muscles of the worm are formed. Thus, simultaneously with the growth of somites, the formation of a skin-muscle sac occurs. Ultimately, as many pairs of somites are detached from the mesodermal stripes as there are segments in an adult worm.

The growth of somites continues until the somite wall, which has become single-layered, comes close from the outside to the muscles of the skin-muscular sac, forming a somatopleura. On the opposite side, the walls of paired somites converge above and below the intestine, covering it on both sides and thus forming the splanchnopleura, or mesentery. At the same time, the anterior and posterior walls of the somites come close to adjacent pairs of somites and thus form two-layer dissepiments, separating the cavity of one segment from the cavity of the other.

Thus, the secondary cavity - the coelom - arises inside the paired mesodermic primordia - somites. The coelomal epithelium, separating the cavity from the walls of the body and internal organs, is the peritoneal epithelium formed from the somite wall. Dissepiments are the walls of two somites adjacent to each other, and the mesentery, on which the intestine is suspended, is formed by the contiguous leaves of paired somites. These sheets are not equally close to each other everywhere. Spaces remain in the dorsal and ventral mesentery. Here, the walls of blood vessels (dorsal and abdominal) are formed from mesodermal elements.

Classification

The class of polychaetes is divided into three subclasses: 1. Wandering polychaetes (Errantia) - freely moving polychaetes; 2. Sessile polychaete ringlets (Sedentaria); 3. Myzostomida.

Subclass Wandering polychaete ringlets (Errantia)

This subclass includes a significant number of species of polychaetes, most of which lead a bottom-dwelling, crawling lifestyle. These include the families of Nereids and Syllides. Some groups of benthic polychaetes have lost their worm-like body shape, which has become wide and flattened in the dorsoventral direction. These are the species of the genus Aphrodite.

Vagrants also include polychaetes that lead a pelagic lifestyle and have modified parapodia that have acquired the character of swimming organs. The palolo (Eunice viridis) described above belongs to this subclass, as well as the largest of the polychaetes - E. gigantea, reaching almost three meters in length, and the swimming polychaete Phyllodoce. Finally, some polychaetes, previously classified as a special class of primary rings, also belong to Errantia. These are the genera Polygordius, Protodrilus, as well as oligomeric Dinophilidae.

Subclass. Sessile polychaete ringlets (Sedentaria)

This subclass includes a large number of mainly sessile forms living in tubes consisting of organic matter secreted by the epithelium of the skin, or in calcareous tubes. However, this subclass includes many burrowing forms, for example sandworms (Arenicola), living in burrows dug in the sand.

Sessile polychaetes are characterized by the greatest heteronomy of body segmentation, weaker development of parapodia, etc.

Subclass. Myzostomida

Practical significance of polychaete rings

Polychaetes inhabit the seas and oceans in large numbers and serve as the main food for many marine animals, primarily various commercial fish. The most valuable sturgeon fish (sturgeon, stellate sturgeon) feed preferably on various polychaetes and only when there is a lack of this food do they eat mollusks, crustaceans and other animals.
The closed Caspian Sea, famous for its most valuable sturgeon, has a depleted marine fauna, in which there are only five species of polychaetes. Therefore, in order to enrich the fauna of the Caspian Sea with animals valuable for food, especially for sturgeon fish, under the leadership of the famous Soviet sea explorer Academician. L.A. Zenkevich carried out work on the acclimatization of the polychaete Nereis (Nereis succinea), living in the Sea of ​​Azov, in the Caspian Sea. Preliminary studies have shown that this worm is most suitable for solving the task. It is very undemanding to fluctuations in sea salinity, which is especially important in connection with the low salinity of the Caspian waters, it is adapted to life in muddy soil, where it feeds on plant detritus, and easily tolerates a lack of oxygen. In 1939-1941. 65 thousand individuals were released in the Caspian Sea. Already in 1944, many of these worms were discovered in the stomachs of sturgeon, and in 1948, the reis inhabited an area of ​​30 thousand km 2 in the Northern Caspian Sea. Thus, the experiment of acclimatization of polychaetes in the Caspian Sea was completed, which significantly improved the food supply of commercial fish and sharply increased the biological productivity of the sea.

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