Echinoderms - deuterostomes - an extensive, about 5000 species, group of marine bottom animals, mostly free-moving, less often attached to the bottom through a special stalk, these are starfish, sea ​​urchins, holothurians (Fig. 514).

Type Echinodermata is characterized by the following features.1. Echinoderms have radial and, moreover, usually five-ray symmetry, but their ancestors were bilaterally symmetrical animals.2. In the subcutaneous connective layer of echinoderms, a skeleton of calcareous plates develops with spikes, needles, etc. sticking out on the surface of the body.

3. Internal organs lie in a vast body cavity (coelom). One of the most original features of the structure of echinoderms should be considered a complex differentiation of a part of the coelom into a number of systems, including the formation of an ambulacral (water-vascular) system of organs of movement due to the coelom. There is a circulatory system; respiratory organs are poorly developed or absent; there are no special excretory organs.5. Nervous system primitive and partly lies directly in the thickness of the skin epithelium or in the epithelium of parts of the body wall that are pushed inwards.6. Echinoderms are dioecious. Eggs experience complete radial crushing. In the development of echinoderms, there is a characteristic dipleurula larva, which undergoes complex metamorphosis.

60. Class Starfish

Sea stars are predators, their main food is molluscs. Stars swallow small mollusks whole, stretching their mouth wide. If the prey is so large that the star cannot swallow it, then it turns its stomach out through the mouth opening and envelops the prey with the walls of the stomach. Digestion takes place already outside the body of the star. Sea stars are very voracious and can cause significant damage to commercial mollusks, such as mussels and oysters, in places of their mass settlement. Circulation. The starfish has a circulatory system consisting of two annular vessels in the central disk and vessels in the rays. Through the blood vessels, nutrients from the walls of the stomach are carried throughout the body of the star. Her blood is colorless. Breath. The starfish breathes oxygen dissolved in water; which enters her body through the skin and through the water-vascular system. Nervous system. Around the esophagus is a nerve ring, from which the nerves depart to the rays. A star can distinguish light from darkness: at the tips of its rays, light-sensitive eyes are located. From bright light the star crawls into the shadows. The sense organs of the star are the legs. The starfish has a developed sense of smell, guided by which it finds prey. Reproduction. Male sea stars do not differ in appearance from females. The reproductive organs are located in the rays. The eggs and sperm are released into the water, where fertilization takes place. After the development of the egg, a larva covered with cilia is obtained. Outwardly, the larva does not look like a star and has a bilateral body symmetry. This indicates the origin of starfish from bilaterally symmetrical animals. For some time, the larva swims in the water column, and then sinks to the bottom and turns into a small starfish. Regeneration in starfish is highly developed. Even from one separated beam, a starfish can grow back

Echinodermata (Echinodermata) is a type of invertebrate deuterostome. Them feature- radial symmetry of the body - is secondary and developed under the influence of a sedentary lifestyle; the earliest echinoderms were bilaterally symmetrical.

Another characteristic feature of echinoderms is ambulacral system, consisting of fluid-filled channels and serving for movement, breathing, touch and excretion. Filling the relaxed canals of the ambulacral system with liquid, echinoderms stretch in the direction of movement, sticking to the ground or some object. A sharp contraction of the lumen of the channels pushes water out of them, as a result of which the animal pulls the rest of the body forward.

The intestines are in the form of a long tube or voluminous bag. The circulatory system consists of annular and radial vessels; the movement of blood is caused by the axial complex of organs. The excretion is carried out by amoebocytes, which are excreted through a gap in the body wall to the outside along with the decay products. The nervous system and sense organs are poorly developed. Some echinoderms, fleeing from enemies, are able to discard individual rays and even most of the body with the entrails, regenerating them subsequently within a couple of weeks.

All echinoderms are crushed sexually; starfish, brittle stars and holothurians are capable of dividing in half, followed by regeneration of the missing half. Fertilization takes place in water. Development proceeds with metaformosis; there is a free-swimming larva (in some species, the larvae remain in the female's brood chambers). Some echinoderms live up to 30 years.

The type is divided into two subtypes; riveted echinoderms are represented by crinoids and several extinct classes, free-moving ones by starfish, sea urchins, holothurians and brittle stars. About 6000 known modern species twice as many extinct species. All echinoderms are marine animals that live only in salt water.

Consider briefly the main classes of echinoderms.

The crinoids (Crinoidea) are the only modern class of attached echinoderms. In the center of the cup-shaped body is the mouth; a corolla of feathery branching rays departs from it. With their help, the sea lily captures the plankton and detritus that it feeds on. A stalk up to 1 m long or numerous movable processes extend down from the calyx, with which the animal is attached to the substrate. Stemless sea lilies are able to slowly crawl and even swim. The total number of species is about 6000; of these, less than 700 currently exist. Crinoids have been known since the Cambrian.

Most sea stars (Asteroidea), in full accordance with the name, have the shape of a flattened five-pointed star, sometimes a pentagon. However, among them there are species with more than five rays. Many of them are brightly colored. Starfish are predators that can slowly crawl along the bottom with the help of numerous ambulacral legs. Some species are able to invert their stomach, wrapping it around a prey such as a mollusk, and digesting it outside the body. About 1500 species; known from the Ordovician. Some starfish are harmful by eating commercial oysters and mussels. The crown of thorns destroys coral reefs, and touching it can cause severe pain.

Sea urchins (Echinoidea) are another class of echinoderms. A disc-shaped or spherical body up to 30 cm in size is covered with skeletal plates bearing long and thin needles. One of the most important purposes of these needles is protection from enemies. Some sea urchins feed on detritus; others, scraping algae from stones, have a mouth with a special chewing apparatus -

Echinodermata, a type of marine invertebrate. Appeared in the early Cambrian, by the end of the Paleozoic reached a great diversity. Sizes from a few millimeters to 1 m (rarely more - in modern species) and up to 20 m in some fossil crinoids. The body shape is varied: star-shaped, disc-shaped, spherical, heart-shaped, cup-shaped, worm-shaped or resembling a flower. About 10,000 fossil species and about 6,300 modern ones are known. Of the 20 known classes, 5 subtypes have survived to the present: crinozoans (sessile forms, oriented with their mouth up, with the only class crinoids), echinoses (combines sea urchins and holothurians) and asterozoans (includes starfish and brittle stars). According to another classification, representatives of the last 2 subtypes are combined into a subtype of Eleutherose.

All modern echinoderms are characterized by the presence of an ambulacral system and five-ray symmetry; the latter extends in many cases to the outlines of the body, the arrangement of individual organs (the nervous and circulatory systems), and the details of the skeleton. Deviations from five-ray symmetry in modern echinoderms (for example, in holothurians) are a secondary phenomenon; at the same time, early Paleozoic homalazoans were initially devoid of radial symmetry.

In most modern species, the mouth is in the center of the body (on the oral side) and the anus is at the opposite pole (on the aboral side). The intestine is poorly differentiated, has the form of a long narrow tube, spirally twisting clockwise, or saccular; in some groups it is secondarily blindly closed. There are no digestive glands. The circulatory system consists of a near-oral annular vessel and radial canals extending from it, devoid of their own walls - a system of lacunae. There is no gas exchange in this system; it serves to deliver nutrients from the intestines to all parts of the body. Weak blood movement occurs due to the pulsation of the heart - a plexus of blood vessels surrounded by epithelial-muscular tissues. The function of the respiratory organs is performed by ambulacral legs, the back of the intestine and other formations. The excretion products are removed by coelomocytes, ambulacral legs and through thin-walled areas of the body.

The nervous system is primitive, without a pronounced brain center. It consists of 3 rings, from each of which 5 radial nerves depart, which do not have direct contacts with each other. Thus, we can talk about the presence of echinoderms, as it were, three nervous systems. In accordance with this, ectoneural (dominant, predominantly sensory, located on the oral side in the integumentary epithelium), hyponeural (controlling the motility of skeletal muscles, connective tissue cells and located in the middle layer) and aboral (controls motor function, prevails in sea lilies, weakly developed in other echinoderms) systems. Echinoderms are dioecious (rarely hermaphrodites). The ducts of the genital glands open outwards. Fertilization is mainly external. A floating larva from bilaterally symmetrical in the course of metamorphosis is transformed into a radially symmetrical adult animal.

Lit .: Beklemishev VN Fundamentals of comparative anatomy of invertebrates. M., 1964. T. 1-2; Invertebrates: a new generalized approach. M., 1992.

S. V. Rozhnov, A. V. Chesunov.

There are more than 6,500 species living in the seas and oceans, both at great depths and in shallow waters.

The body of echinoderms is 5 mm to 5 m long, has ray (radial) symmetry(most often five-ray symmetry).

The body shape of echinoderms is very diverse: with developed rays (sea stars and lilies), spherical (sea urchins), barrel-shaped, elongated in the oral-aboral direction (holothurians). There is no division of the body into sections, but on the body of an echinoderm they distinguish oral pole(on which the mouth is located) and aboral pole(the anus is located). Most echinoderms face down with their oral pole and crawl on their oral side.

The integument of echinoderms consists of two layers: outdoor- single-layered epithelium, and internal, formed by fibrous connective tissue, where various elements develop calcareous skeleton, often with numerous needles, spikes. In starfish, the skeleton is formed by calcareous plates arranged in longitudinal rows and usually bearing spines protruding outwards. The body of sea urchins is enclosed in a calcareous shell of rows of tightly connected plates with long needles sitting on them. The skeleton of holothurians is formed from small calcareous bodies. different shapes scattered all over the skin.

All echinoderms have water vascular (ambulacral) system, with the help of which they can move, and representatives of some species can touch and even breathe. The water-vascular system is represented by an annular canal surrounding the esophagus, and

Five radial channels extending from it into the rays. The latter give branches to the legs - thin, highly elongated and extensible tubes, equipped on one side with a suction cup, on the other - with a bubble. The system is connected to the external environment through a channel through which water enters. Slow movement along the bottom is carried out when the tubule legs are filled with liquid, often with suction cups at the ends.

Nervous system echinoderms has radial structure: from parapharyngeal nerve ring depart radial nerve cords according to the number of body rays. sense organs poorly developed. Primitive eyes located at starfish at the ends of the rays, and at sea urchins - on the upper body. There are also sense organs.

Circulatory system usually consists of two annular vessels, one of which surrounds the mouth, and the other - the anus. There are also radial vessels, the number of which in starfish coincides with the number of rays.

Digestive system starts mouth opening, it leads to short esophagus, Further stomach, intestine and anus(in some species it is absent).

Respiratory organs serve with starfish and hedgehogs skin gills- thin-walled outgrowths on the upper side of the body. In a number of echinoderms, respiration occurs through the body or when participation of channels of the water vascular system.

Special excretory organs echinoderms do not. The excretion of metabolic products takes place through the walls of the channels of the water vascular system.

Echinoderms usually dioecious, but there are also hermaphrodites. Development occurs with a number of complex transformations (metamorphoses). Echinoderm larvae (bilaterally symmetrical) swim in the water column, where, in the process of transformation, they acquire radial symmetry and switch to a crawling lifestyle.

Differ a high degree regeneration. In a starfish, the body can be restored from a beam alone.

Class Sea lilies

Among sea lilies there are sessile and free-floating forms. The mouth and anus of these echinoderms open at the top of the body. All crinoids feed on small planktonic organisms. Breathe on the surface of the body. There are usually 5 tentacles, but they can branch up to 200 or more processes.

Class Starfish

These are sedentary animals with 5 to 50 rays. Their mouth opening is on the underside of the body. Starfish feed mainly on dead animals, as well as silt and sedentary animals. Some predatory starfish destroy commercial mollusks. The stomach of these echinoderms can turn out through the mouth opening and envelop the prey.

Among starfish there are hermaphrodites and dioecious. Reproduction is asexual and sexual.

The fecundity of sea stars can be different: for one individual from several tens to 200 million eggs. in shallow water northern seas sea ​​ice freezes in winter and thaws in spring.

Class Sea urchins

Free-moving animals with a hard shell covered with movable spines. Representatives of some species can use them to move along the bottom. The mouth is equipped with a gnawing apparatus (five teeth) and is located on the underside of the body. They feed on algae, sedentary animals, silt. One female spawns up to 20 million eggs.

In some species of sea urchins, care for offspring is observed: they bear eggs and juveniles on the body.