BASIC LECTURE NOTES IN ZOOLOGY

Module 4. MOLLUSCS, CHAETOGNATHS, PHORONIDS, BRYOZOANS, BRACHIOPODS, POGONOPHORANS, HEMICHORDATES, ECHINODERMS

Content Module 4.2. Hemichordates, echinoderms

Topic 4.2.2. Phylum Echinodermata

General characteristics of the phylum. Origin, evolution, palaeontological data, modern diversity, geographical distribution, role in nature and human life. Overview of the main taxonomic groups important for human life and the functioning of natural ecosystems. Subphylum Crinozoa. Class Crinoidea. Subphylum Echinozoa. Class Holothuroidea. Class Echinoidea. Subphylum Asterozoa. Class Asteroidea. Class Ophiuroidea.

A phylum of animals that emerged approximately 580 million years ago. Most species are extinct (about 13,000 fossil species), with over 6,000 extant species. Exclusively marine benthic animals, highly sensitive to decreases in salinity. They inhabit the seas and oceans of all latitudes across the globe, ranging from the intertidal zone to the maximum depths of oceanic trenches. Free-living animals displaying radial, predominantly five-rayed Symmetry with elements of bilateral symmetry; their body is differentiated into an oral side, bearing the Mouth opening, and an opposite, aboral side. Radial symmetry is a secondary phenomenon, as evidenced by certain fossil forms, the bilateral symmetry of echinoderm larvae, and The Structure of several Organ Systems in modern species. The external body shape may resemble a flower, a star, a sphere, a cucumber, etc. Sizes typically range between 5 and 50 cm, although both smaller species and those reaching several metres in length are known. Externally, Echinoderms are covered by a single-layered ciliated epithelium (except for holothurians, which lack cilia), rich in glandular Cells that secrete mucus, as well as adhesive and toxic substances. Pigment cells are present, accounting for the diverse and striking coloration of many echinoderms. Beneath the epithelium lies a well-developed Connective Tissue layer (cutis) of mesodermal origin, lined internally by peritoneal epithelium that bounds the coelom. The internal Skeleton is laid down within the plasma of cells in the outer layer of the cutis as microscopic biycrystals (composed of calcite and organic matter). These biocrystals grow, acquire a specific shape, and are extruded from the cells. Here, they interconnect to form small plates of various shapes or fuse at their edges to form large spongy plates. Various outgrowths may form on their surface: spines (hence the name of the phylum), prickles, specialised Organs such as pedicellariae, etc. Musculature is poorly developed; it is located primarily in the arms, where skeletal elements are movably articulated, and also operates movable body outgrowths: tube feet, skeletal spines, and pedicellariae. Only holothurians possess a well-developed body wall (dermomuscular) sac. All Muscles are smooth. They are coelomate animals. The coelom develops as three pairs of coelomic pouches; besides the general body cavity, these give rise to the canals of the Water-vascular and perhaemal systems, axial sinuses, the ampulla of the axial organ, and the genital strand. The general body cavity achieves significant development, except in sea lilies, where its volume is greatly reduced due to a Abundance of connective tissue strands, and in ophiuroids, due to The Development of specialised ectodermal invaginations called bursae projecting into the body cavity. The coelom is lined with ciliated epithelium, which also covers The surface of all internal

organs, and is filled with a transparent fluid whose composition is close to that of seawater, but with added Proteins. This fluid also contains A large number of cellular elements (coelomocytes, of which 14 types are distinguished) involved in nutrient distribution, Respiration, excretion, and the Protection of the Organism against Bacteria and parasites. Notably, it was experiments with starfish larvae that led É. Metchnikoff to formulate the phagocytic theory of Immunity.

Class SEA LILIES (Crinoidea).

About 4,000 fossil species and over 620 extant species, distributed in the tropical Regions of the Indian and Pacific Oceans, although they also occur in Antarctic and Arctic waters. Sizes range from a few centimetres to 2.5 m. Some crinoids remain attached to the substrate throughout their lives or may detach for short periods to move over small distances. Others lead a free-swimming or crawling lifestyle, always keeping the oral surface facing upwards; periodically, they halt and attach to the substrate. The body consists of a small, calyx-shaped body trunk from which five long rays (arms) extend; these fork near their base to become ten, though most arms branch extensively (up to 200). A single-layered epithelial covering is present only in young individuals, whereas in adults all external skeletal plates are naked. Beneath the epithelium, connective tissue is found in areas devoid of a skeleton. On the flat oral side of the trunk, large skeletal plates occur only in some juveniles and a few species throughout life; however, the Skeleton of the oral side is underdeveloped and represented by small calcareous sclerites.

Class Holothurians, or Sea Cucumbers (Holothuroidea).

Less demanding regarding water salinity; some apodous holothurians can even inhabit the quite brackish waters of mangrove swamps. There are 1,100 known species, with 8 in the Black Sea, the most common being *Stereoderma kirschbergi* (formerly *Stereodeima kirschbergi*). The average size is 10–40 cm, though some species exceed 2 m in body length. Most are coloured brown, dirty white, or grey, while others exhibit bright coloration. The body is elongated along the oral-aboral axis, with most species resembling thick worms. Nearly cylindrical, fusiform, spherical, or other shapes occur. One can distinguish the anterior end, bearing a crown of tentacles around the mouth, and the opposite, posterior end, where the anus is located. The oral-aboral axis of the body lies parallel to the substrate rather than perpendicularly, so the animals rest as if on their side. The side facing the substrate is conventionally termed the ventral side, and the opposite one the dorsal side. The ventral side is flattened to varying degrees, while the dorsal side is convex. The body is covered by a non-ciliated epithelium. The integument is soft due to a significant reduction of the skeleton, which is represented solely by microscopic calcareous bodies of various shapes scattered throughout the cutis and its diverse outgrowths. In addition to this peculiar external skeleton, holothurians possess an internal skeleton—a circumoral calcareous ring comprising five large radial and numerous small interradial sclerites. It serves as an attachment site for various muscles and protects the circumoral nerve ring. The musculature is highly developed. A continuous layer of circular musculature lies beneath the Skin, underneath which runs the longitudinal musculature arranged in five bands. At the anterior end of the body, these bands diverge from the body wall and attach at an angle to the margin of the mouth opening. Contraction of these muscles causes the anterior end of the body to retract along with the tentacles. A series of muscles attached at one end to the circumoral calcareous ring and at the other to the anterior end of the body produce the reverse movement. Contraction of the circular musculature leads to body elongation, whereas contraction of the longitudinal muscles results in rapid shortening. Beneath the Muscle layers lies the ciliated peritoneal epithelium, which lines the voluminous body cavity containing the Internal Organs. The body cavity performs a mechanical supporting function for the soft body walls, which lack a rigid skeleton.

Class Starfish (Asteroidea).

Inhabitants of seas and oceans, never found in waters with a salinity below 30 ‰. Only a single species, *Marthasterias glacialis*, occurs in the Black Sea near the Bosphorus. Over 300 fossil and about 1,500 extant species are known. Sizes range from 1 to 80 cm or more. They are often brightly colored, sometimes variegated. The body is flattened along the oral-aboral axis, comprising a central disc that gradually transitions into radial rays, or arms. The arms may be very short (as in *Patina pectinifera*), projecting only slightly along the margins of the disc, or they may be longer, with the body radius (the distance from the centre to the tip of a ray) significantly exceeding the interradius length (the distance from the centre of the disc to its margin between the rays). Starfish always face the substrate with their oral side. A five-rayed structure is typical for starfish, although species with six or more (up to 50) rays are quite numerous.

The body surface is covered with a thin cuticle, beneath which lies a single-layered ciliated epithelium containing glandular cells. The bulk of the body wall (skin) consists of connective tissue, within the thickness of which lie calcareous skeletal plates and muscle fibers. Internally, the skin is lined with peritoneal epithelium bounding the coelom. The body is not enclosed in a continuous shell. The skeleton consists of numerous calcareous plates interconnected by connective tissue and muscles. The skeleton of the oral side is more strongly developed. Along the oral and partially lateral sides of each ray run eight rows of skeletal plates. Along the margin of each ray lie one or two rows of marginal plates, medial to them are rows of adambulacral plates, and centrally lie two rows of ambulacral plates positioned at an angle to each other, forming a gabled roof over the ambulacral groove. Each ambulacral plate features a small depression through which the canal of the tube FOOT passes. The STRUCTURE OF THE skeletal plates in the circumoral region of the body is somewhat different: here, the first ambulacral plates are significantly broadened and transformed into oral plates, while the first adambulacral plates, also widened, bear pointed processes known as dental ossicles directed toward the mouth opening. The skeleton is composed of narrow calcareous plates that can form an irregular meshwork or cover the entire surface.