Echinoderms

Echinoderms (Echinodermata), one of the types of the animal kingdom. All echinoderms are bottom-dwelling marine animals, mostly freely mobile, rarely sessile (some sea lilies). Modern echinoderms are divided into 5 classes: starfish (Asteroidea), brittle stars or ophiuroids (Ophiuroidea), sea urchins (Echinoidea), sea cucumbers (Holothurioidea) and sea lilies (Crinoidea). In fossil state, several other groups of sessile echinoderms are also known: cystoids (Cystoidea), blastoids (Blastoidea). Often these groups are combined together with sea lilies into the subphylum Pelmatozoa, in contrast to the other 4 classes, which form the subphylum Eleutherozoa. The body of echinoderms is built on the principle of radial, specifically five-rayed symmetry, most often consisting of a disk with 5 rays or radii extending from it; less frequently the body is spherical (sea urchins) or sausage-shaped (sea cucumbers). A main axis passes through the body, at the opposite ends of which lie the mouth and the anus. The oral side of the body is called the oral or ambulacral side; the side opposite to it is the aboral or antiambulacral side. In most cases, the main axis has a vertical position, with the mouth downward (starfish, sea urchins, ophiuroids) or with the mouth upward; only in one class (sea cucumbers) does the main axis lie horizontally with the mouth at the front and the anus at the back end of the axis. Beneath the outer epithelium lies a thick layer of connective tissue, in which calcareous plates develop, forming the skeleton of echinoderms. The skeleton consists either of separate microscopic bodies (sea cucumbers) or of large plates, sometimes surrounding the body with a continuous shell (sea urchins). The skeleton is provided with protective outgrowths: spines, special pincer-like structures (pedicellariae), etc. Echinoderms have a well-developed coelom and a complete, often long and winding (sea urchins, sea cucumbers), intestine. The nervous system is very primitive; its main part is the perioral ring, which sends 5 radial nerves into the rays; the nervous system lies either in the outermost epithelium or is submerged under the skin. The locomotory system is particularly distinctive, bearing the name ambulacral. It consists of a system of channels, beginning on the surface of the body with the perforated madreporite plate and ending in numerous hollow tube feet with suckers at their ends. The channels are filled with fluid, which is pumped into the tube feet and serves for their extension in the direction of movement. There is a rather complexly developed circulatory system; for respiration, hollow skin outgrowths - gills - are most often used, and less frequently (in sea cucumbers) special paired protrusions of the posterior part of the intestine - water lungs. There is no special excretory system: for excretion, amoeboid cells that accumulate waste products and exit the body through the skin serve. For the restoration of the number of amoebocytes, there are special lymph glands: the axial organ, Tiedemann's bodies. The reproductive system is very simple; it is mostly dioecious and consists of ovaries or testes, opening outward through short ducts. There are no copulatory organs: the sex cells are expelled from the body into the sea water, where fertilization and development take place. Most echinoderms reproduce only sexually, but in some starfish and ophiuroids, asexual reproduction is observed by transverse fission of the body into two or several parts, followed by regeneration of the missing organs in each piece. The development of echinoderms is characterized by complete and uniform cleavage, the formation of mesoderm through the protrusion from the walls of the midgut of paired coelomic sacs that separate from the intestine, and a complex metamorphosis. The young larva of echinoderms, unlike the adult organism, is built on bilateral symmetry and is called a dipleurula. It leads a free-swimming existence, later taking in different classes of echinoderms different, sometimes very bizarre forms. Only at the end of metamorphosis does the animal acquire radial structure, descends to the bottom, and passes to the definitive mode of life. Remains of echinoderms are found starting from the oldest Silurian deposits. Cystoids and blastoids become extinct already in the Carboniferous period; the others have survived to the present time. The history of development and morphology of echinoderms indicate that this type originates from free-swimming bilateral ancestors, which then transitioned to a sessile mode of life, and thereby acquired radial structure. Subsequently, most echinoderms (except sea lilies) returned to a free but crawling mode of life and began to lose radial symmetry, again replacing it with bilateral symmetry. The origin of echinoderms is not precisely clarified; it should only be noted that according to many developmental features (method of mesoderm formation, fate of the blastopore), echinoderms are closest to the type of chordates; they are found at various depths, but require water of high salinity, and therefore are almost or completely absent in seas with insufficient salinity, for example, in the Baltic and even the Black Sea. Within the USSR, echinoderms are abundantly represented only in the Northern Polar Sea and in the Far Eastern seas.