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Type of echinoderm: characteristic

The size of invertebrate creatures belonging to this type ranges from 5 cm to 50. But there are species whose dimensions can be several millimeters, or vice versa, two meters.

All the "residents" studied by scientists differ from each other in their structure. But, of course, there are a number of signs by which these vertebrates can be recognized from other marine life. All these signs have not undergone any changes, and have not undergone evolution over time.

Sea stars are echinoderms

Type echinoderms, main features:

1. Only these marine inhabitants have a special ambulacral (water-bearing) system. It consists of a group of channels with thin walls filled with liquid;
2. In their structure, they are radiant (usually this is an internal skeleton, well developed, the rays are a multiple of five).

Holothuria (sea cucumber) - a representative of echinoderms

With the help of the ambulacral system, representatives of the echinoderm type are able to move around, touch objects. And in some varieties of sea lilies and hedgehogs, it can perform the function of breathing. This whole system consists of the same type: an annular canal, which surrounds the oral opening from the inside, and radial canals, multiples of 5, extending from the annular canal. They end blindly or with a sensitive process.

If we talk about the characteristics of echinoderms, then another distinguishing feature of these vertebrates is that their skeleton is laid in the connective layer of the skin and it is calcareous. The outer epithelium of most representatives of echinoderms is covered with an uneven layer of cilia. They are designed to cleanse the body of impurities, perform a respiratory function and feed food into the mouth opening.

Sea lily

The outer layer of the epidermis may contain a large number of specific glandular cells that secrete a poisonous or sticky enzyme. There are some species such as echinoderms that secrete enzymes that can glow.

Skeletal plates in echinoderms are arranged like rays. But this manifests itself not only with outside, but also displayed on the internal organs of these sea creatures. For example, the nervous and circulatory systems also have a radiant arrangement.

Representative of echinoderms - ophiura

Zoologists believe that the senses of smell and taste are well developed. They are represented large quantity sensitive cells located on ambulacral legs, which is an important aspect in the characterization of echinoderms. Creatures of this class are able to perceive taste stimuli at great distances.

Their reproductive system also has a radiant structure. It consists of a cord and gonads. Much depends on the variety of the echinoderm type, the gonads are located in grape-shaped sacs, which are placed either in the radial base or go deep into it.

The digestive organs do not have a similar structure. At different types echinoderms they are arranged differently. This may be due to the difference in nutrition. The presence of skeletal plates or its absence may also depend on nutrition. In some species of these representatives of vertebrates, the mouth may be surrounded by tentacles that help in catching prey.

Sea urchin "Slate pencil"

In spite of various characteristics echinoderms, scientists have not yet fully understood the complete process of digestion in these creatures. What is clear is that the walls of their intestines contain a large number of cells that secrete digestive juices and enzymes. Also, no answer was found to the question of the excretion of the decay products of these invertebrates. Representatives of the phylum echinoderms do not have special bodies to bring them out.

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 protruding spines. Body sea ​​urchins 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 of various shapes, scattered throughout 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. WITH external environment the system is connected 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.

They have a high degree regeneration. At starfish the body can recover from the ray 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 the shallow waters of the northern seas, marine ones freeze in winter and thaw 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.

Echinoderms are an independent and very peculiar type of animal world. According to the plan of their structure, they are completely incomparable with any other animals and, thanks to the peculiarities of their external organization and the original shape of the body, resembling a star, a flower, a ball, a cucumber, etc., have attracted attention for a very long time. The name "echinoderms" was given by the ancient Greeks. In ancient times, sea stars, holothurians and sea urchins were already distinguished among the animal world, and the history of the study of these animals is associated with the names of Aristotle and Pliny. In the 18th and early 19th centuries such prominent scientists as Klein (Kiein, J.), Linnaeus (Linnaeus, C), Lamarck (Lamarck, J. B.) and Cuvier (Cuvier, G.) were engaged in them. However, most of the old zoologists connected echinoderms with either worms or coelenterates.

Name radiant, or zoophytes, remains for echinoderms until 1847, when X. Frey and R. Leuckart (N. Frey und R. Leuckart) identified them as a separate independent type, pointing out significant differences between echinoderms and radiant animals, in particular from intestinal animals .

Subsequent works of such scientists as Ludwig (Ludwig, N.), Agassiz (Agassiz, A), A. Kovalevsky, I. Mechnikov, Til (Theel, N.), Mack Bride (Mac Bride, E. W.), Lieberkind (Lieberkind, I.), Fischer, (Ficher, W. K.), Mortensen, (Mogtensen T. N.) and others, significantly expanded their understanding of the systematics, anatomy and embryology of echinoderms, of the relationship between individual representatives of this type . The Russian scientist Mechnikov, with his work on the development of echinoderms, established their relationship with the enterobranchs, showing that the branch of echinoderms is related to the branch of chordates. The Danish scientist Mortensen not only developed a system for modern and fossil classes of echinoderms, but also gave a classification of larvae parallel to the adult system. This is especially important, since echinoderm larvae are very peculiar, very different from adult forms, lead a pelagic lifestyle and for a long time considered to be independent organisms.

Among the Soviet researchers, who did a lot for the development of knowledge about echinoderms, first of all, it is necessary to note A. M. D'yakonov and D. M. Fedotov, who contributed to this group with their work on anatomy, morphology, development, regeneration, phylogeny and paleontology. elucidate the position of echinoderms among deuterostomes.

At present, there is no doubt that type of echinoderm belongs to the group of the most highly organized invertebrates - to group of deuterostomes.

Echinoderms appeared on Earth a long time ago, over 520 million years ago. The remains of these animals are already found in the sediments of the early Cambrian. The possession of a calcareous skeleton contributed to a rather good preservation of echinoderms in the fossil state, therefore their history is known to us better than the history of other invertebrates, especially since the features of the echinoderm skeleton largely allow us to judge the entire structure of the animal.

Ancient echinoderms reached in the Paleozoic era a huge number and a wide variety of forms. These were predominantly bilaterally symmetrical, mostly attached animals with a sac-like, spherical, or cup-shaped body covered with shell-like plates.

Most of the animals were subphylum of attached echinoderms - pelmatozoa(Pelmatozoa), to its five classes: heterostele(Heterostelea) cystideum, or cystoidea(Cystidea) blastoid(Blastoidea), edrioasteroid(Edrioasteroidea) and sea ​​lilies(Crinoidea). And from subtype of free-moving echinoderms-Eleuterozoa - only one small fossil class Ophiocistia, numbering only 5 genera and less than 10 species, was limited to the Paleozoic, while the remaining classes of this subtype are flourishing at the present time. Representatives of the class Ophiocistia (Fig. 122) had a disc-shaped or spherical body, devoid of protruding rays and completely covered with plates. The mouth was placed in the center of the side facing the substrate, and was equipped with strong movable jaws. These animals appeared in the Middle Silurian and disappeared already in the Middle Devonian.

The most ancient and primitive forms, closest to the ancestors of echinoderms, belonged to heterostele class(Fig. 122). They had a sac-like compressed body, devoid of radial symmetry, and possessed a number of features that bring them closer to lower chordates and to the lancelet. Characteristic features of the echinoderm type are significantly developed in Cystidae, which dominated the seas 500-400 million years ago. In connection with attachment to the substrate, they developed and consolidated radial symmetry, and it was already possible to observe the appearance of five-ray symmetry. Cystidae were extremely diverse forms (Fig. 122), flourished in the Silurian and died out in the Devonian. It is likely that blastoids and sea ​​lilies. Blastoid(Fig. 122) were very isolated forms that died out in the Permian without leaving any offspring. On the contrary, some representatives sea ​​lily class have survived to this day.

Fossil sea ​​lilies(Fig. 122), represented in the Silurian, Devonian and Lower Carboniferous, according to Fischer, 5 thousand species belonging to 5 orders, almost completely died out by the end of the Paleozoic. Only a few of their representatives went to mesozoic era and gradually disappeared. However, at the beginning of the Mesozoic, approximately 180 million years ago, a new group sea ​​lilies - articulate detachment(Articulata), whose representatives not only survived to this day, but also reach their greatest prosperity at the present time.

Edrioasteroidei(Fig. 122), known from the early Cambrian to the Carboniferous, probably also descended from the early Cystidae, since their most primitive representatives were close to the Cystidae in terms of their level of organization. However, they already possessed a number of features that bring them closer to starfish, which is why they are often called sessile starfish. The edryoasteroids were located on the substrate with their mouths up. Most of them adhered to the substrate with a wide surface of the body, and only very few had a stalk, and some forms, obviously, did not adhere to the substrate at all, although they lay motionless on the bottom. Thus, the evolution of the ancient classes of echinoderms subphylum palmatozoa almost completely passed in the Paleozoic, and only one order of the sea lily class.

The living echinoderms, of which there are about 6000 species, with the exception of the class of sea lilies, belong to subtype of free-moving echinoderms - Eleutherozoa(Eleutherozoa).

Among modern echinoderms (Table 17) there are Starfish class(Asteroidea) with a star-shaped or multi-beam body and a mouth located in the center of the lower side facing the substrate; Ophiura class, or Serpenttails(Ophiuriidea), very similar to starfish, but with rays strongly laced from the disc; class Sea urchins(Echinoidea) - almost spherical animals, without elongated rays; class Sea capsules, or Holothurians(Holothurioidea), - sac-like or worm-like animals, whose mouth and anus are almost always located at different ends of the body; class Sea lilies(Crinoidea) - resembling flowers, with strongly branching rays, in which the mouth and anus are brought together and lie on the same side, facing upwards.

The body dimensions of echinoderms usually range from 5 to 50 cm, but there are species whose length does not exceed a few millimeters, while in others, on the contrary, it can even reach 2 m.

The classes of echinoderms known to us are very peculiar and very different from each other, but, despite the great variety of forms, they have a number of features characteristic only of these animals, which indicate both the common origin of this entire group and the peculiar way it evolution. Indeed, only echinoderms have a special aquifer (ambulacral) system, consisting of a series of thin-walled canals filled with liquid, have a radiant body plan, usually a multiple of 5, and a highly developed skeleton, which, unlike other invertebrate animals, is internal in origin.

The ambulacral system in most forms serves for movement, touch, and in sea lilies and some urchins it performs a respiratory function. In the most typical case, it consists of an annular canal a surrounding the oral opening from the inside, and 5 radial canals (Fig. 123) extending from the annular and ending blindly or with a small sensitive process.

Radial canals, passing along the inner side of the beam, give in pairs lateral branches to the legs, which are called ambulacral legs. The legs are thin cylindrical very extensible tubes, usually equipped with a suction cup at the end, less often without it. At the base of each leg is a thin-walled vesicle, or ampulla. Between the radial canals (and inter-radially) from the annular canal of the ambulacral system, thin-walled, highly extensible, often very large sacs - polyeus bubbles, which serve as reservoirs of the fluid of the ambulacral system, depart. The number of polyovial vesicles varies greatly, sometimes there are several of them even in one interradial space, in other forms they may be absent altogether. An unpaired stony canal connects interradially with the annular canal, so named because of the hardness of its walls, which contain deposits of calcium carbonate. At its opposite end, the stony channel is connected to a plate lying on the surface of the animal in most forms, which is called a madrepor plate or madrep orite. It is permeated with tiny holes - pores through which sea water enters the ambulacral system. Details of the structure and function of the ambulacral system may undergo a number of changes both in different classes and within the same class, order, and even family of echinoderms. Thus, in some holothurians, the radial canals are greatly shortened to the extent of short stumps or disappear altogether. Some stars, brittle stars, and holothurians have several stony channels and several madreporous plates, and in most holothurians, the stony channel loses communication with the external environment and, together with madreporite, hangs down into the body cavity. In starfish, urchins and representatives of the same order of brittle stars, the annular canal has special grape-like appendages, consisting of simple or branched tubules. These are Tiedemann's bodies, the purpose of which is not exactly established. Ambulacral legs undergo especially large changes in structure and function. For example, in sea lilies, they lack ampoules, suction discs, are pointed, never serve as organs of movement, but perform a sensitive and respiratory function. Two pairs of legs located near the mouth turn into mouth tentacles that help in capturing food. In many holothurians, the same specimen has both motor legs equipped with suckers and legs devoid of suckers, turned into papillae and already performing other functions, and in representatives of two orders of holothurians, the legs on the body are generally reduced and disappear. The oral tentacles of holothurians are partially modified legs.

Among sea urchins, there are also species with ambulacral legs very diverse in form and function. Along with motor legs, they have conical, rosette-shaped, racemose legs, which already have other functions. Despite such a change in the legs, their main significance in starfish, most holothurians and hedgehogs is the movement of the animal along the bottom of the seas. In this case, the fluid of the ambulacral system is driven from the annulus into the radial canals, from where it enters the ampullae of the legs along the side branches. Due to muscle contraction, the liquid from the ampoules is distilled into the legs, which are greatly elongated from this, and the soles of the legs are in contact with the substrate. From this contact, the muscles of the sole (sucker) contract, the latter is pressed in, and a rarefied space is formed between the sole and the substrate, which determines the attachment of the leg to the substrate. The attachment of the legs to the substrate due to the pressure of air and water and the action of the sticky mucus secreted by the glandular cells of the sucker is so strong that when trying to get the attached animal, the legs break off rather than lag behind the substrate. The subsequent contraction of the attached legs tightens the entire body of the animal, and the liquid from the contracted leg again enters the ampoule, which is relaxed by this time. The valves located in the ampoules regulate their filling with liquid.

Another characteristic feature of echinoderms is the radiant structure of the body, usually a multiple of 5, which is reflected in the fact that their body is divided into 10 parts (sectors). The sectors in which the ambulacral legs and the radial channels of the ambulacral system are located are called ambulacral or radii. They alternate with interradial sectors, which are called interradii or interambulacras. In different classes of echinoderms, radii and interradii are expressed differently. In some, they are fused with the common body of the animal (holothurians and sea urchins), in others, the radii can be strongly extended, forming moving or fixed rays, sometimes called arms. In such cases, animals acquire a star-like shape (stars, brittle stars). Sometimes the rays can branch out in a tree-like manner, which is observed in branching brittle stars, or divide pinnately, as in sea lilies. In star-shaped forms, the number of rays can also vary from 5 to 16 and even up to 50. Occasionally, among the usual five-ray forms, four and six-ray specimens can come across.

An equally characteristic feature of the echinoderm type, along with the ambulacral system and the radiant body plan, is the presence of a calcareous skeleton embedded in the connective tissue layer of the skin. The skin of echinoderms consists of a single layer of integumentary epithelium, a thick layer of cutis, or skin proper, and again a layer of ciliated epithelium, which limits the secondary body cavity, or coelom. The outer integumentary epithelium in most forms (with the exception of holothurians) is provided with cilia, which are unevenly spaced. The action of these cilia causes the appearance of water currents on the surface of the animal's body, going in certain directions and serving to supply food, breathe and cleanse the body of dirt. In the outer epithelium of the skin there are a large number of glandular cells that secrete a mucous, sticky or poisonous secret, and in some species there are glandular cells, the secret of which causes the organisms to glow. There are also clusters of pigment cells here, giving a surprisingly beautiful and bright color echinoderm animals. The cutis, which underlies the outer epithelium, consists of a connective tissue in which three layers can be distinguished. In its lower, inner layer, there are muscles that are highly developed in holothurians, in which they form a skin-muscular sac, and significantly reduced in sea lilies, hedgehogs, and most brittle stars due to the development of a strong skeleton in them. In the outer layer of the cutis, under the epithelium, skeletal elements are formed, therefore, in their origin from the mesodermal layer, they rather correspond to the integumentary bones of vertebrates, and not to the shell of snails.

To build their skeleton, echinoderms use calcium carbonate dissolved in sea water. By its origin, the skeleton of all echinoderms is internal and appears in the form of a small grain of calcium carbonate, barely visible under a microscope, inside the cells of the connective tissue. This grain grows, acquires a three-beam shape, falls out of the cell that gave rise to it and becomes an intercellular calcareous body. With further development, the rays of the calcareous body begin to branch, neighboring branches are connected to each other, closing the holes and forming small perforated plates. Larger skeletal lamellae are obtained by fusing many small lamellae at their edges, so they always have a spongy texture. The shape of the resulting skeletal elements is specific and inherited. It depends on the role played by skeletal elements in the body. Although the shape of the skeletal elements of echinoderms is very diverse, they all share the optical and crystallographic properties of a single calcareous spar crystal. Therefore, like the latter, each skeletal element of the echinoderm has birefringence. However, the skeletal elements of echinoderms are not just calcareous crystals, since their specific gravity and refractive index are less than those of calcareous crystals. These are biocrystals that arise in the plasma of cells mainly from calcite with a slight addition of organic matter. Therefore, they are not limited by the planes of the crystal, but their shape is determined by the activity of the intracellular educational plasma. The calcareous skeleton of echinoderms can be poorly developed and consist of individual very small plates of extremely diverse shapes, which we observe in almost all holothurians and in the ambulacral legs of sea urchins, or reach a powerful development (all other classes of echinoderms). In the latter case, it consists of well-marked plates, which can be fastened motionless to each other in the form of a dense shell, or form a more or less loose or dense network, or connect to each other like vertebrae. The plates of the skeleton quite often carry on their surface a variety of needles, tubercles, small granules or special grasping organs - pedicellaria.

The possession of such a skeleton with all kinds of outgrowths served as the basis for calling these animals "echinoderms", although not all representatives of this type have needles.

Among the skeletal elements, perhaps the most remarkable are the grasping organs - pedicellaria. They are found in starfish and urchins, sometimes in very large numbers and are modified needles. Often pedicellaria reach such complexity that it is difficult to imagine their origin from the most ordinary needle of the skeletal plate. So, in starfish, along with the simplest pedicellaria, sitting directly on the body of an animal and representing 2 needles inclined towards each other, or two (bivalve pedicellaria) or several valves, often arranged in a row (comb pedicellaria), there are pedicellaria with more elongated, very mobile valves (Fig. 142), equipped with teeth, outgrowths and sitting on a special base. Such pedicellariae often form tufts or ridges around the spines and can be attached to the animal's body with a soft neck or rod. They are very mobile, equipped with muscles that slam and open the valves. The main function of such pedicellaria is to clean the skin from foreign particles, protect the soft parts of stars from damage by various small animals. Often pedicellaria also serve to capture prey, but sometimes they are used as organs of attack or defense, and then they are equipped with a poisonous gland. Poisonous pedicellaria are found in sea urchins along with other types of pedicellaria, which have received further development and complication of their organization compared to starfish pedicellaria.

Skeletal plates of echinoderms very often have a radiant arrangement. The radiant structure in echinoderms is manifested not only in external forms or in the placement of skeletal plates, but also in the location internal organs. Thus, the nervous and circulatory systems of echinoderms, like the ambulacral system discussed above, are built according to the radiant type and have a similar arrangement.

The nervous system of echinoderms is rather primitive, consisting of three sections, two of which, called the hyponeural and apical systems, are predominantly motor. They are developed very differently and sometimes very weakly in different classes. of this type animals, while the third, sensitive section (ectoneural system) is very constant and well developed in all echinoderms. All three parts of the nervous system are closely interconnected, since with such a distribution of functions, without this connection, it would not be possible to transfer the irritation perceived from the outside to the muscles, glands and other organs. Each of the named departments of the nervous system consists of a nerve ring and radial nerve trunks extending from it, the number of which corresponds to the number of ambulacres (Fig. 124). The ring and nerve trunks consist of nerve cells and nerve fibers and are nerve centers that perceive and conduct stimuli. The most superficially located on the oral side of the animal is the sensitive (ectoneural) part of the nervous system, which in sea lilies and stars lies directly in the outer epithelium of the ambulacral grooves, and in the serpentine, sea urchins and holothurians - in the epineural canals formed by immersing the walls of the ambulacral grooves inward and closing them into tubes. The nerve ring of the ectoneural system surrounds the pharynx or esophagus, and the radial trunks in most forms reach the end of the ambulacra, repeating the complex branching of the rays and sending branches to various organs. Deeper than this system and separated from it only by a thin layer of connective tissue is the second, motor section of the nervous system - the g and p neural system. In holothurians, the ectoneural and hyponeural systems are so closely adjacent to each other that for a long time they were not even distinguished. This section of the nervous system is most significantly developed in sea lilies and brittle stars due to the increased mobility of the rays, and in brittle stars the radial nerves form ganglionic thickenings that send nerves to the muscles of the vertebrae of the rays. The third section - the apical entoneural system - is located even deeper than the previous one, on the side opposite the oral one, and is associated with the epithelium of the body cavity. This motor system is absent in holothurians, poorly developed in starfish, hedgehogs, brittle stars, and only in sea lilies has received sufficient development. In crinoids, it controls the movement of rays and their parts and has a complex structure.

Due to this structure of the nervous system, when the ectoneural section is predominantly developed, many echinoderms become very sensitive to external stimuli, although their sense organs are generally poorly developed. Numerous sensory cells, single or collected in groups, found on the ambulacral legs of starfish, brittle stars, sea urchins and sea cucumbers, the skin of the hands of sea lilies, mouth tentacles, etc., serve as organs of touch. Special organs of vision are found in relatively few echinoderms, although all these animals have considerable photosensitivity. The most developed eyes, built according to the type of eye fossae (Fig. 125), are found in most starfish. They are located at the end of each beam at the base of the terminal tentacle of the radial channel of the aquifer system. More poorly developed eyes are found in the anterior part of the body of some holothurians. Such eyes determine only the degree of brightness, but do not distinguish between individual objects. The pigment spots of sea urchins, located, like the eyes of starfish, above the ends of the radial channels of the aquifer system, are also very sensitive to light and can be considered as reduced eyes. Despite the fact that light irritation is carried out mainly through the eyes, removing them does not deprive the animal of photosensitivity. So, the very common starfish Asterias rubens in our White Sea with the tips of the rays cut off together with the eyes continues to crawl away from very bright light and very intense darkness. Yes, and the rest of the echinoderms, such as brittle stars, which do not have organs of vision, react strongly to changes in illumination, since the entire skin of echinoderms has a diffuse susceptibility to light.

The senses of smell and taste in echinoderms are well developed. They can perceive taste stimuli even at considerable distances, although their organs of smell and taste are poorly developed and are represented by numerous sensitive cells located on the ambulacral legs, tentacles of brittle stars, mouth legs of some hedgehogs, and on the tentacles of individual holothurians. Some zoologists also consider small, barely visible formations, spheridia, found in almost all sea urchins, as taste organs. They are usually located close to the mouth in the middle of the ambulacra and consist of a dense, highly refractive substance covered with ciliated epithelium. Spheridia (Fig. 125) are connected to the shell by a short stalk, at the base of which are sensitive cells and nerve plexuses. The function of these organs has not been fully elucidated. Their similarity to the static organs of jellyfish suggests that in sea urchins they can rather play the role of organs of sense of balance than organs of taste.

The static organs that allow the animal to determine where is up and where is down are otocysts (Fig. 125), which are found only in two families of holothurians, whose representatives either live at great depths or burrow into the sand. Otocysts are vesicle-shaped containing several or many small otoliths and are connected directly to the radial nerves.

The circulatory system of echinoderms is also built according to the radiant type. It consists of a perioral ring and five radial blood vessels, going to the ambulacra between the radial nerve trunks and the channels of the aquifer system (Fig. 126). In starfish, urchins and brittle stars, on the side opposite the oral one, there is a second ring of the circulatory system, which gives lateral branches to the sex glands and is connected to the first ring by a special axial organ, characteristic only of echinoderms. Its structure will be discussed further. However, the circulatory system in echinoderms is a system of lacunae rather than blood vessels. It is devoid of its own walls and is formed by gaps in loose connective tissue. The fluid of the circulatory system is similar in composition to the fluid of the body cavity or ambulacral system. The circulatory system of echinoderms is rather comparable to the lymphatic system of vertebrates, because its main purpose is to carry nutrients throughout the body. Therefore, in the thickness of the intestinal wall there are blood gaps or, as in holothurians and sea urchins, well-developed dorsal and abdominal blood vessels that can periodically contract, which causes a weak movement of "blood". Echinoderms do not have pulsations, which are common for the circulatory system, but there is a pulsation of the pericardium, the walls of which, like in chordates, form an invagination - the heart. The pulsation of the pericardium causes a weak movement of the blood fluid, but the correct movement of "blood" in echinoderms is not, and gas exchange, so characteristic of the circulatory systems, has not been established in them. The function of gas exchange is performed by the fluid of the body cavity.

Very closely related to the circulatory and nervous system is the system of canals lined with epithelium and developing from isolated areas of the common secondary cavity of the body. This canal system surrounds the blood vessels and is therefore called the pseudohemal (perihemal) system. It consists of a ring around the pharynx or esophagus, located between the ambulacral ring and the perioral nerve ring, and five radial canals entering the ambulacra and passing between the radial nerves and the ambulacral canals. The fluid of the pseudohemal system corresponds to the fluid of the body cavity, contains a large number of amoebocytes, and the close location of this system to the nervous system suggests that it serves to nourish the nerve cords and protect them from compression. The pseudohemal system is developed differently in different echinoderms. Thus, in sea urchins and holothurians, there are only radial "channels" of this system, while in crinoids, as a rule, it is generally absent.

The radiant plan of the structure of echinoderms is partly manifested in the location of the genital organs. The reproductive system consists of the sex cord and the sex glands, which form as outgrowths of the first. The reproductive system of sea lilies has the most peculiar and primitive structure. Their sex cords in the form of five dense cellular ribbons pass into the rays and branch, repeating the branching of the latter. Their terminal branches go into special outgrowths on the rays, called pinnules, and take the form of hollow blind bags. Sexual products develop from the cells of the genital sacs inside the pinnula and are brought out through the rupture of their walls. In starfish, brittle stars and hedgehogs, the genital cord has the form of a pentagonal ring, the walls of which, growing, give rise to the sex glands located interradially. The gonads of starfish have the appearance of five pairs of branching, grape-shaped sacs that are located at the base of the rays or go far into them. They open outwards with short channels between the rays. In brittle stars, numerous small sex glands are located on the inner side of five pairs of special brood burs (burs). The sex glands open directly into the genital bags, which communicate with the external environment through narrow slits located on the oral side of the disk at the base of the rays. In sea urchins, five penniform gonads are located in the upper part of the shell, and in many wrong hedgehogs the five-dimensional structure is disturbed by the disappearance of the posterior gland. In holothurians, the correct radiant arrangement of the elements of the reproductive system is completely lost. They have a single sex gland, consisting of a bundle of simple or branching tubules. All tubes join into one genital duct, which in most forms opens outward in the anterior part of the body, in the dorsal interradius.

Violation of five-ray symmetry in modern echinoderms is quite common. Thus, all holothurians and irregular sea urchins are bilaterally symmetrical animals and clearly possess visible anterior and posterior ends of the body. And in all other echinoderms, except for some multi-beam stars, branched brittle stars and lilies, in which an increase in the number of stony channels and madrepore plates is observed, in fact, only one plane of symmetry can be drawn, despite the radiant structure, due to the position of the axial complex of organs located only in one of the interradii. This feature of modern echinoderms was inherited by them from bilaterally symmetrical ancestors, while the bilateral symmetry of holothurians and irregular hedgehogs should rather be considered as a secondary phenomenon associated with the loss of the radial symmetry of the ancestors in connection with their transition to a crawling and burrowing lifestyle.

The axial complex of organs is very characteristic of the structure of echinoderms and is a spatial association of very heterogeneous organs. It is formed by the ambulacral, circulatory and reproductive systems, as well as areas of the secondary body cavity. The main components of this complex are a stony canal, a madreporous plate with pore canals, an axial organ that runs along the stony canal in the form of an oblong sac, two separate sections of the secondary body cavity (left and right axial sinuses), containing the axial organ, and, finally , sexual cord, placed in a special area of ​​\u200b\u200bthe body cavity - the sexual sinus. The axial organ is formed by a plexus of blood vessels extending from the oral blood ring, pericardium (right axial sinus) and a significant amount of connective tissue, as well as mobile cells - amoebocytes. The function of the axial complex is quite diverse, but not yet sufficiently elucidated. With the help of the pulsating pericardium, the movement of fluid in the circulatory system is caused; through inflow and outflow sea ​​water through the madrepore plate, the constant hydrostatic pressure in the ambulacral system is regulated and unnecessary substances are removed; the axial organ serves, on the one hand, as a place of accumulation and excretion of decay products from the body, on the other hand, as a lymphatic gland that forms amoebocytes - cells that move freely in the fluid of the body cavity. It is possible that the axial organ associated with the sexual cord is also an organ of internal secretion. The axial complex of organs is present in starfish, urchins, and brittle stars, although its position and the ratio of organs in it are not the same in different classes due to the displacement of the madreporite. Sea lilies and holothurians do not have an axial complex of organs, but only stony and pore canals, which very often open directly into the common body cavity.

The whole of echinoderms in most forms reaches a significant volume, is lined with ciliated epithelium, contains the intestines and most of the viscera covered with this epithelium. Only in sea lilies, due to the development of a significant amount of connective tissue mesodermal strands that fill the body cavity, and in a number of brittle stars, which have special ectodermal bags (bursae) that protrude deeply into the body cavity, its volume decreases sharply. The secondary body cavity arises from two posterior coelomes of the larva, and the mesenterium, which separates the right and left coelomes of the larva, is greatly reduced in many adult echinoderms and is preserved only in holothurians, following the bends of the intestine. The body cavity is filled with cavity, or perivisceral fluid, very transparent and close in composition to sea water, but containing an admixture of proteins and a large number of cellular elements. This cavity fluid is in constant motion caused by the ciliary epithelium lining the body cavity and plays a very important role in metabolic processes. Cellular elements (amoebocytes) contained in the perivisceral fluid are involved in the distribution of nutrients and in the release of decay products.

The cavity fluid is also essential in the processes of respiration: in some echinoderms, the perivisceral fluid contains red blood cells with hemoglobin; in addition, many respiratory organs are formed by the walls of the body cavity. Thus, the outer perioral, highly branched (peristomal) gills of most sea urchins are protrusions of a separate section of the secondary body cavity - the oral, or peripharyngeal, coelom located around the esophagus. Through the walls of the gills, covered with an extremely thin layer of skin, oxygen dissolved in water penetrates into the cavity fluid. Skin gills, or papules, in the form of thin-walled contractile very small vesicles are found in starfish. Along with the gills, the ambulacral legs and other thin-walled appendages of the ambulacral system obviously play an important role in respiration, since a pigment close to hemoglobin was found in the fluid cells of the ambulacral system in a number of forms. Most echinoderms are very oxygen hungry and die quickly in poorly ventilated aquariums. However, their respiratory organs are poorly developed and the chemistry of respiration is still poorly understood. Real respiratory organs - water lungs - are available only in holothurians. In those holothurians that do not have aquatic lungs, the respiratory function can obviously be performed by the oral tentacles, the intestines, the relatively thin skin of the body, and, finally, the dorsal appendages of the ambulacral system, through which oxygen penetrates. In brittle stars, breathing occurs through thin-walled sac-like chambers-bursae located at the base of the rays and periodically filled with sea water due to contractions of the dorsal wall of the disc and filling and emptying of the stomach. The physiology of respiration in echinoderms has not been sufficiently studied, but it is likely that various organ systems may take part in respiratory processes along with their other functions.

The digestive organs are not equally developed in different classes of echinoderms, which is associated with differences in the methods of nutrition and with a different structural plan of representatives of individual classes.

The digestive system begins with the mouth, which, depending on the methods of nutrition, may be provided with a skeleton or devoid of it. In holothurians, the mouth, surrounded by tentacles that help in catching prey, passes, like in sea urchins and lilies, first into a short ectodermal pharynx, and then into a long narrow endodermal intestine, the length of which significantly exceeds the length of the body of the animal. The intestine therefore forms loops (Fig. 127) lying in the secondary body cavity and suspended from the body wall by means of the mesenteric membranes. In representatives of these three classes, one can distinguish between the anterior, middle and posterior sections of the intestine. The posterior end of the intestine opens outward through the excretory (anal) opening, which in sea lilies is located near the mouth, in hedgehogs - on the side opposite the mouth, or on the edge of the shell. In holothurians, the hindgut expands into a rather voluminous, muscular cloaca, which in most species opens outwards at the posterior end of the body. Starfish and brittle stars do not have a pharynx and the mouth leads directly to a voluminous sac-like stomach attached to the body wall by mesenteric bands. The stomach of the brittle stars is folded, with radial and interradial outgrowths, ends blindly, since the brittle stars have neither hepatic appendages, nor the hindgut, nor the anus. In starfish, the stomach has 5 pairs of hepatic protrusions, which extend into the rays and abundantly secrete digestive juices. Thus, the intestinal canal of echinoderms retains the features inherited by modern animals from bilaterally symmetrical ancestors. It develops almost entirely from the primary intestine and does not have a radiant structure. The influence of five-ray symmetry affects only the intestines of starfish, which has five pairs of hepatic processes.

The processes of digestion in echinoderms are not well understood. It is known that the walls of the intestine contain a large number of cylindrical cells that secrete digestive juices, as well as many free amoeboid cells. Enzymes have been found in them: lipase, amylase, maltase, invertase, trypase and some others, but it has not been established where the various enzymes are formed.

Very little is known about the processes of excretion of decay products by echinoderms, especially the processes of excretion of liquid substances. Echinoderms do not have special excretory organs, and the function of excretion is performed by numerous amoebocytes, which are found in large numbers everywhere in the coelomic fluid, so the excretion is diffuse. In one case, amoebocytes are thrown out by breaking the walls of the body, and places with the thinnest covers are chosen. In another case, amoebocytes, together with decay products, are deposited in different parts of the body, forming large colored masses.

From the characteristics of echinoderms just described, it follows that their radiant structure is to some extent an apparent phenomenon, since the position of the axial complex of organs, the structure of the intestinal canal, and the reproductive system of holothurians testify to the primary bilateral symmetry of these animals. The study of the development of echinoderms also gives grounds to believe that they originated from a bilaterally symmetrical free-living ancestor.

The development of echinoderms is always accompanied by a complex transformation with the passage of the stage of a pelagic two-sided larva. In all classes, development in the early stages proceeds more or less similarly, but then significant differences are observed. In most forms, reproduction is only sexual and eggs are laid directly into the water, where they are fertilized and develop. After crushing, which is usually complete, of a radial type, a typical blastula is formed, covered with cilia. At one pole of this blastula, a deep invagination occurs, giving rise to the endodermal midgut, and the blastula turns into a gastrula. The first important change in the structure of the gastrula occurs with the formation of protrusions of coelomic pockets on the sides of the primary intestine. The order of their formation is very variable in different larvae, but the most common and primary for them is the independent formation of three pairs of lateral protrusions, the subsequent lacing of which from the primary gut gives 3 pairs of coelomic sacs. At this stage of development, larvae of echinoderms are most similar to ctenophores, a comparison with which was first made by Mechnikov (1874). Later, unlike most worms, arthropods, and mollusks, the primary mouth of the larva, or blastopore, turns into an anus, and the oral opening is formed a second time by invagination of the ectoderm of the ventral side towards the curving end of the primary intestine. A similar formation of the mouth is also characteristic of pogonophorans, hemichordates, and chordates, which brings them closer to echinoderms and allows them to be attributed to the group of deuterostomes.

After the anus is displaced to the ventral side, the echinoderm larva becomes completely bilaterally symmetrical, with a convex dorsal and depressed ventral side, a near-oral rim of cilia and three pairs of coelomic sacs. This stage of larval development is called dipleurola and is common to most echinoderms. With further development in different classes of echinoderms, dipleurol undergoes significant changes, manifested especially in the modification of the external shape and shape of the perioral corolla of cilia. The form of dipleurola changes least of all in holothurians. Their larval stage, called auricularia (Fig. 128), has oval shape, a mouth lying deep in the abdominal depression and surrounded by one corolla of ciliated cilia, as well as a skeleton in the form of small microscopic bodies. The larvae of starfish are called bipinaria, ophiur-ophiopluteus, and sea urchins are called chinopluteus. All of them are characterized by the presence on the body of special cords equipped with cilia, with which they can swim freely. The larva of sea lilies - doliol yar and I - differs from the rest in that it is devoid of a mouth and has a barrel-shaped body, girded not with a continuous ciliary cord, but arranged in the form of five ciliary rings. In the future, all lilies go through another stage of a larva of the Pentacrinus type, attached by a stem to the substrate.

The size of echinoderm larvae is very small, their length is less than 1 mm. The period of their life in plankton is very different and is determined not only by the type of animal, but also by the surrounding conditions. So, some larvae of the same species during reproduction in summer are in plankton for 2-3 weeks, but in winter this stage can be delayed up to 2-3 months. Then, in order to turn into an animal similar in appearance to their parents, echinoderm larvae undergo even greater changes. In sea urchins, stars and brittle stars, most of the body of the larva and its organs are destroyed and die. But in holothurians, during development, only a small anterior part of the larva atrophies, and the rest passes into the body of an adult. Echinoderms grow very slowly, throughout their lives, reaching puberty in a few years. The types of larvae discussed above are the main ones for various classes of echinoderms, however, it should be noted that appearance larvae and the method of their transformation into an adult organism vary not only from family to family, but also from species to species.

While in plankton, the larvae can travel great distances from the habitat of adult forms. On the one hand, this is a positive fact that contributes to the dispersal of the species. On the other hand, as planktonic organisms, echinoderm larvae are eaten by other animals, but this is compensated by the production of an extremely large number of eggs by echinoderms, which ensures the survival of the species.

Very often, echinoderms take care of their offspring (Table 21). In such cases, the mother covers the laid eggs with her body or bears them. Usually, juveniles are hatched in certain places of the mother's body, less often they crawl all over the body. In echinoderms, cases of live birth are known, when the embryo develops inside the maternal organism, being placed in the body cavity, either in the genital tubes, or in bursal protrusions (in brittle stars) and receives from it the nutrition necessary for development. Live birth is especially common in Arctic and Antarctic species. In this case, development is direct, without the planktonic larval stage.

Along with sexual reproduction in echinoderms, asexual reproduction also exists in some cases, when the body of an adult organism is divided in half or into several pieces. Each half or piece restores the lost pieces. This method of reproduction is observed in some starfish, brittle stars and individual holothurians.

In a certain connection with asexual reproduction is the phenomenon of regeneration, which is very characteristic of echinoderms, that is, the ability to restore lost individual organs or parts of the body. Very often, under adverse conditions or when attacked by an enemy, echinoderms discard (autotomize) rays or parts of the body, throw out the insides, and sometimes even fall into pieces. The rate of recovery of lost parts is different and very dependent on temperature. Thus, regeneration proceeds faster in tropical forms in summer than in arctic and boreal forms or in winter. The restoration of lost parts in young individuals is faster than in old ones, however, echinoderm litans have a very low regenerative capacity. This protective useful device is very poorly developed in sea urchins. Their body is covered with a strong shell that protects against external damage, therefore, under adverse conditions, sea urchins can discard and then restore only pedicellaria, spines and ambulacral legs.

There is little information regarding the lifespan of echinoderms, but it is likely that they live a long time. On the plates of the skeleton of some echinoderms, annual growth rings are installed. According to the calculations of these rings, the age of large sea urchins is estimated at 35 years. Some starfish only reach their normal size by the age of 14. The brittle stars may live 3 to 4 years or a little more.

All echinoderms live exclusively in the seas and are very sensitive to the slightest desalination of water. They are completely absent in the Caspian Sea, in the Baltic they are represented by only 1 species, and in the Black by only 8 species, but in the Barents, Kara, Chukchi and Okhotsk Seas they make up the bulk of bottom animals. Relatively few species are able to tolerate rather significant salinity decreases - up to 20-24 g of salts per 1 kg of water (20-24°/00) and are even sometimes found in estuarine areas; on the contrary, other species feel great in water with extremely high salinity. An example is the Red Sea, which has a salinity of 43-47 g of salt per 1 kg of water and is abundantly populated by a wide variety of echinoderms. Echinoderms live in the seas and oceans of all latitudes of the globe, on a wide variety of soils and depths - from the tidal zone to maximum depths oceanic depressions, which are deeper than 10,000 m. More often they are confined to certain depths and are found either exclusively in the littoral zone, or in the sublittoral or bathyal zone, or they live only in the abyssal zone, and within individual groups echinoderm species diversity sometimes increases with depth. However, among echinoderms, a significant number of species turn out to be eurybath1 forms, the range of vertical distribution of which sometimes exceeds 5-7 thousand m. Such species, which can occur at the most diverse depths, are especially numerous among brittle stars. For a number of echinoderms, seasonal migrations from the depths to the shores and, conversely, associated with breeding periods and temperature changes were noted.

The degree of confinement of echinoderms to certain soils is different. Many holothurians, sea urchins, and brittle stars live exclusively on muddy or sandy bottoms, but some members of these classes prefer rocky bottoms. However, it is likely that the nature of the soil does not play a significant role in the distribution of echinoderms. Obviously, a more important role in their distribution is played by the salinity and salt composition of the water, as well as depth, currents, temperature, the history of the development of the seas and oceans, and a number of biological factors. Among echinoderms there are species that prefer a certain temperature. Thus, arctic forms are found only at temperatures close to 0 ° C, and high arctic forms are found exclusively at negative temperatures, while arctic-boreal species are more eurythermal, i.e., they can tolerate significant temperature fluctuations. Among the boreal species, there are also eurythermal forms, which even in winter remain in the littoral zone, under conditions of snow cover, but the bulk of the species migrate to deeper waters with the onset of hulods. Echinoderms are especially well represented in the fauna of tropical seas and coastal zones, which is associated with more constant high temperatures and with the most diverse habitats in these areas compared to cold or temperate seas or with great depths. Nevertheless, even in the last two regions, echinoderms are quite numerous and often take part in the formation of certain communities, or biocenoses, being the leading forms of most of them. The fauna of the Pacific Ocean is rich in echinoderms, while the fauna of the Atlantic Ocean is much poorer. Some species of echinoderms have a fairly wide distribution throughout all oceans, others have broken ranges (bipolar, amphiboreal and amphipacific species), and still others are confined only to tropical, boreal arctic or antarctic regions. Many species, genera, and families of echinoderms are endemic to certain areas; they perfectly characterize both the littoral and deep-sea faunas of various regions. Therefore, the zoogeographic significance of echinoderms is very great.

Echinoderms also play an important role in the general economy of the sea, being its numerous and typical inhabitants. Like mollusks and other animals, they are involved in maintaining a certain salt composition of sea water, consuming large amounts of salts in the process of feeding, especially calcium carbonate, from which the skeleton is built. Echinoderms have the ability to extract radioactive substances from the water, which primarily accumulate in their body cavity fluid. In addition, echinoderms serve as food for various animals. For example, cod, haddock, flounder, catfish and other fish eat a significant amount of brittle stars and sea urchins. Sea urchins are also eaten by the sea otter-sea otter. They hunt echinoderms and some large mollusks and crustaceans. On the other hand, many echinoderms are predators and eat various animals themselves. Particularly predatory are starfish, which can eat fish that are larger than their body, and often attack oyster banks and fishing nets. Some echinoderms are used as food by humans. Among them, the most important are edible holothurians - sea cucumbers, of which there are more than 40 species and varieties. The oldest consumers of trepangs are the inhabitants of China, Japan, the islands of the Indo-Malay archipelago. However, even in these countries, trepangs are rather a tasty dish, since the high price and limited productivity of fishing grounds prevent the wider consumption of these animals. In China, according to their healing properties, trepangs are compared with ginseng root and are called the "root of the sea." In addition to holothurians, along the shores of the Mediterranean Sea, certain types sea ​​urchins, whose caviar is used for food. Echinoderms are an excellent object for experimental embryological research, and in last years their eggs, especially sea urchins, were often used in space flights to reveal the influence of cosmic and other rays on living organisms. Fossil echinoderms are also useful for humans, since some Construction Materials, such as Derboshire and Belgian marble, trachyte limestone, consist mainly of the remains of these animals. In addition, a number of fossil echinoderms, especially sea urchins, are used as guide forms important for stratigraphy.

Among modern echinoderms, only stalked crinoids are fixedly attached to the substrate, while all other animals move freely. Usually, echinoderms crawl along the ground both along horizontal and vertical surfaces, less often burrow into the ground, and only a few species of holothurians lead a pelagic lifestyle and can be found in the water column. Burrowing into the soil is quite widespread in all groups of echinoderms, except for lilies, in which, due to the peculiar way of feeding, the ambulacral furrows must always be clean. Most forms burrow into the ground to a shallow depth, leaving part of the body or part of the rays above the ground. Digging into the ground in many echinoderms is associated with nutrition. Such animals swallow the soil randomly, and the capture of the soil with the nutrients in it occurs mechanically together with progressive movement animal forward in the ground. Ground beetles are found among many holothurians, sea urchins and some stars. Most echinoderms feed on animal food, but some of them feed exclusively on coastal algae or even the remains of terrestrial vegetation.

Regarding the origin of echinoderms and phylogenetic relationships within this type, there are still only numerous conjectures and hypotheses. This is due to some extent to the fact that the appearance of echinoderms dates back to ancient times, since at the end of the Cambrian and at the beginning of the Silurian, over 500 million years ago, there were already representatives of almost all classes of echinoderms known to us, with the exception of holothurians, reliable remains of which were found only from carbon. It is assumed that echinoderms originated in the Precambrian and the most ancient of them probably had an underdeveloped skeleton, which did not contribute to their preservation in the fossil state. On the basis of embryology, comparative anatomy, and paleontology, it is believed that the ancestors of echinoderms apparently had a bilaterally symmetrical, elongated body, equipped with five pairs of tentacles, a tubular intestine, and three pairs of coelomes. Their mouth was on the front of the body, the anus on the back.

These animals led a benthic lifestyle, which led them to some asymmetry in the structure. The closest to such an ancestor were representatives of the primitive classes of the Pelmatozoa subtype - heterosteles and cystids, within which a transition from bilateral to radial symmetry was already outlined. Attachment to the substrate by the right side of the anterior end of the animal's body, which took place in representatives of this subtype, caused the mouth to shift to the left and upward, the anus to the right, and the acquisition of a radiant body plan. Thus, the evolution of pelmatosa proceeded in the direction of the development of radial symmetry, and in the higher representatives of this subtype, a number of sea lilies, in which an increase in the number of stony canals is observed, it covers almost their entire organization, with the exception of the intestines and the method of discharge of the adductor grooves from the mouth.

Concerning origin subtype Eleutherozoa there is no consensus. However, most zoologists believe that they are descended from pelmatozoa, since the organization of the latter has many features from which the structure of the former can be deduced. The evolution of the Eleutherose subtype proceeded in the direction of restoring bilateral symmetry due to stem atrophy, with a transition to a mobile lifestyle and active food intake.

Causes controversy and the question of the origin and relationship of the classes of this subtype. It is likely that the different classes of Eleutherozoa do not have a common ancestor, but originated from different groups of Pelmatozoa. Among the Eleutherozoa, starfish and brittle stars are the closest to each other. The former are known from the Cambrian, the latter from the Silurian. Of all the classes of pelmatozoans, the closest to the stars and ophiurae are the edrioasteroids in terms of the structure of their ambulacres. Therefore, based on the data of comparative anatomy and embryology, it is assumed that stars and brittle stars, as two close and partially parallel trunks, separated from the Early Cambrian edrioasteroids. Among modern starfish, the most primitive are considered clearly plate stars(Phanerozonia). Which of the detachments of modern brittle stars is more primitive has not yet been clarified, but many believe that it is the most primitive detachment of branched brittle stars(Euryale).

Concerning class of sea urchins, the first representatives of which are known from the early Silurian, then such features as the presence of an axial set of organs and a sex cord make them very close to stars and brittle stars. Their similarity with brittle stars is evidenced by the morphology of the larvae and some biochemical properties. Therefore, it is quite possible that hedgehogs evolved from forms close to the early edrioasteroids. However, some zoologists, in particular Mortensen, believe that they have a difinitic (double) origin. The most ancient hedgehogs of the Bothriocidaridae family are associated with Cystidia, and the rest of the hedgehogs with Edrioasteroids. Among modern hedgehogs, the most primitive are considered detachment of elastic hedgehogs(Lepidocentroida), most of whose forms are extinct.

Holothurians stand somewhat apart from other echinoderms, but the presence of a number of primitive features in them, for example, the structure of the reproductive system, indicates their ancient origin and similarity to Cystidae. Due to the poor development of the skeleton, the paleontological history of holothurians is almost unknown. Many authors agree only that holothurians belong to the Eleutherozoa subtype, but they occurred independently of other classes of this subtype. considered more primitive detachment of holothurians with tree-like tentacles(Dendrochirota).

Animal life: in 6 volumes. - M.: Enlightenment. Edited by professors N.A. Gladkov, A.V. Mikheev. 1970 .

• Biological encyclopedic dictionary

ECHINODERMS- (Echinodermata), one of the types of the animal world. All I. are demersal marine animals, mostly free-moving, less often sessile (part of sea lilies). Modern I. are divided into 5 classes: starfish (Asteroidea), serpentine, or brittle stars (Ophiuroidea), ... ... Big Medical Encyclopedia

- (Echinodermata) a type of animal with apparent radial (usually 5-radial) symmetry of the body, a hard calcareous external skeleton, separate circulatory and digestive, as well as nervous and ambulacral systems. They make up one of the most... Encyclopedia of Brockhaus and Efron

- (Echinodermata) a type of invertebrate deuterostome with five-beam symmetry and a special aquifer (ambulacral) system. Most of their representatives have a skeleton (made of aragonite), consisting of plates fused together and ... ... Geological Encyclopedia

A term introduced by Blainville (1816) and then applied to the divisions established by Cuvier (see Theory of T.). Currently, the following T. are accepted: 1) The simplest (Protozoa) unicellular animals or representing a colony completely ... ... encyclopedic Dictionary F. Brockhaus and I.A. Efron

- (Echinodermata) type of invertebrates; marine free-moving or attached, secondarily radially symmetrical animals with a calcareous skeleton and with an ambulacral system (See Ambulacral system). Type I. refers together with ... ... Great Soviet Encyclopedia

- (Echinodermata) a type of marine invertebrates with a calcareous skeleton with needles or spines; includes classes of sea lilies, holothurians, sea urchins, etc.; certain tropical species of I. are poisonous and can be dangerous for swimmers; ... ... Big Medical Dictionary

Deuterostomes ... Wikipedia

Systematics of the phylum Echinodermata:

Subphylum/Subdivision: Eleutherozoa Bather, 1900 = Free-moving, or Eleutherozoans

Subphylum/Subdivision: Homalozoa = Homalozoa †

Class: Ctenocystoidea = †

Class: Homoiostelea Gill et Caster, 1960 = †

Order/Order: Soluta Jaekel, 1901 = †

Class: Homostelea = †

Class: Stylophora = †

Subphylum/Subdivision: Pelmatozoa Leuckart, 1848 = Attached

Class: Blastoidea = Sea buds †

Class: Cystoidea = Balloons, or Sea Bubbles †

Class: Edrioasteroidea = Edrioasteroidea †

Class: Eocrinoidea = †

Class: Glyptocystida = †

Class: Paracrinoidea = †

Class: Rhombifera =

Echinoderms are secondary cavitary animals, in the adult state having a radial symmetry of the body. In most species, the organs are arranged along five radii, but in some the number of rays is different. If in coelenterates the radial symmetry of the body is primary, then in echinoderms it will be secondary, since their ancestors had bilateral body symmetry. Free-swimming larvae of echinoderms are bilaterally asymmetrical. Echinoderms are characterized by the presence of an ambulacral system that serves for movement and participates in the processes of respiration and excretion. The secondary cavity of the body is well defined and filled with abdominal fluid. Echinoderms are inhabitants of the sea. These are predominantly benthic animals capable of slow movement along the substrate, rarely attached to it. Some echinoderms serve as an object of fishing.
Echinoderms, as first shown by the studies of I. I. Mechnikov, are interesting for revealing the phylogenetic relationships of invertebrates with representatives of the Chordata type. Despite the radial symmetry of the body of adults, the organization and development of echinoderms have many common features with chordates. The secondary body cavity in them, like in chordates, is formed by separating the mesodermal sacs from the intestine. Like chordates, they are secondary cavitary animals in which, in the process of development, the gastropore overgrows or turns into an anus, and the mouth of the larva is formed anew. Representatives of both types have a two-layer skin and skeletal elements of a mesodermal nature. These similarities suggest that the lower chordates are phylogenetically related through common ancestors with echinoderms. Remains of echinoderms found in sediments Paleozoic era.
Structure and life functions. The integument of echinoderms consists of two layers: the outer, which has the character of a single-layer epithelium, and the inner, formed by fibrous connective tissue. Various elements of the calcareous skin skeleton develop in the inner layer. In starfish, they look like calcareous plates arranged in longitudinal (along the rays) rows and usually bearing spines protruding outwards. In sea urchins, the body is enclosed in a calcareous shell of rows of tightly connected plates with long needles sitting on them. In holothurians, small calcareous bodies of various shapes are scattered in the skin.
musculature developed to varying degrees depending on the mobility and nature of the skin skeleton. It is composed of individual muscles and muscle bands.
The ambulacral system begins with a porous madrepore plate located on the dorsal side of the body. From it, a stony canal extends deep into the body, which opens into an annular canal surrounding the esophagus. The annular channel gives radial channels to each ray of the body. Short tubules branch off from the radial canals in both directions, from which contractile vesicles - ampoules - extend into the cavity, and contractible tubular ambulacral legs with suction cups at the ends go outward. The ambulacral system is filled with water entering through the madrepore plate. With the contraction of the ampoules, the water from them passes into the cavity of the legs, which is why they lengthen and stretch. The suction cups located at the ends of the legs are sucked to the substrate, after which the length of the legs is reduced, since water from their cavity is discharged back into the ampoule. By the joint efforts of many simultaneously contracting legs, the body of the echinoderm is pulled up, and the animal slowly moves along the bottom. Thanks to the suction cups of the ambulacral legs, echinoderms can crawl even along the vertical surface of the rock.
Nervous system echinoderms has a radial structure. Radial nerve cords depart from the peripharyngeal nerve ring according to the number of rays of the body.
sense organs poorly developed. Primitive eyes are located in starfish at the ends of the rays, and in sea urchins - on the upper body. There are also organs of touch, etc.
Digestive system. The mouth opening is located in most of them in the middle on the lower surface of the body. The mouth leads into a short esophagus, followed by the midgut and short hindgut. Some have no anus.
Respiratory organs starfish and urchins have skin gills - thin-walled outgrowths on the upper side of the body. Apparently, the ambulacral system also takes part in the respiratory process. In a number of echinoderms, breathing occurs through the integument of the body.
Circulatory system usually consists of two annular vessels, one of which surrounds the mouth and the other the anus, and radial vessels, the number of which coincides with the number of rays of the body. Both annular vessels are connected by a hematopoietic axial organ penetrated by a network of blood vessels.
excretory organs. Echinoderms have no special excretory organs. The release of dissimilation products occurs through the walls of the channels of the ambulacral system and with the help of special amoeboid blood cells migrating inside the body.
Sex organs have a different structure. Most echinoderms are dioecious, but there are also hermaphroditic forms.
Development goes through a series of complex transformations. Bilaterally symmetrical larvae of echinoderms swim in the water column.
Many echinoderms have an amazing ability to regenerate body parts. For example, one ray of a starfish can restore an entire animal.

Brief description of the classHolothurians, or sea pods:

Sea capsules or sea cucumbers are called animals whose body is strongly compressed at the slightest touch, after which, in many forms, it becomes like an old capsule or fresh cucumber. About 900 species are known.
Name " sea ​​cucumbers"Given to these animals by Pliny, and the description of some species belongs to Aristotle, so long ago these animals attracted attention.
Holothurians, or sea capsules, are not only interesting for their external features, bright color, amusing way of life and some habits, but also have a rather significant economic value. Over 40 species and varieties of holothurians are used as food by humans. Edible holothurians, which are called trepangs, have long been valued as a very nutritious and healing dish, so their fishing has been practiced since ancient times. The main trepang fisheries are concentrated mainly in tropical areas: in the waters of the Indo-Malay archipelago, the Pacific Islands, the Philippine Islands, off the coast of China and Japan.
Less significant fisheries are carried out in Indian Ocean, in the Red Sea, off the coast of America, in the region of Africa and Italy. In our Far Eastern seas, two types of edible holothurians are mined, which are used for the preparation of canned food and dried products. Holothurians are more often eaten in the form of broths and stews and their boiled skin, previously subjected to lengthy processing and drying. Some modern European firms make various canned food from holothurians, which are in great demand. In Italy, fishermen eat fried holothurians without subjecting them to complex pre-processing, and the inhabitants of the Pacific Islands eat raw caviar and aquatic lungs of these animals. The extraction of trepang in the Pacific Ocean is about 10,000 centners per year.
Holothurians are rather large animals, the average size of which is from 10 to 40 cm. However, among them there are also dwarf species, barely reaching a few millimeters, and real giants, whose body length with a relatively small diameter, about 5 cm, can reach 2 m or even more. The body shape of holothurians is very diverse.

Literature: Course of zoology. B. A. Kuznetsov, A. Z. Chernov, L. N. Katonova. Moscow, 1989