SUMMARY NOTES OF ZOOLOGY LECTURES

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

Content Module 4.1. Molluscs, Chaetognaths, Phoronids, Bryozoans, Brachiopods, Pogonophorans

Topic 4.1.1. Phylum Mollusca

General characteristics of the phylum. Origin, evolution, paleontological Background, modern diversity, geographic distribution, and their role in nature and human life. List of major taxonomic groups important for human life and the functioning of natural ecosystems. Class Polyplacophora (chitons). Class Aplacophora. Class Bivalvia. Class Monoplacophora. Class Gastropoda. Class Scaphopoda. Class Cephalopoda. Biological diversity of Dnipropetrovsk Region.

Predominantly aquatic, rarely terrestrial free-living animals; only a few have adapted to a parasitic lifestyle. Approximately 130,000 species. Coelomate animals. Bilaterally symmetrical animals, although some of them (class Gastropoda) are asymmetrical due to the displacement of certain Organs. The body is unsegmented, with only a few representatives showing signs of metamerism. The body consists of three main regions: the HEAD, trunk, and FOOT, although the head may be partially or completely reduced. The head bears the Mouth, tentacles, and eyes. The foot is a muscular, thickened outgrowth of the ventral body wall (performing a locomotor function). The foot appears as a flat sole or keel, and is sometimes modified and transformed into an organ

for swimming, capturing prey, or is partially or completely reduced. The trunk is positioned above the foot and may expand dorsally into a hump. The mineral-organic shell typically covers the entire body of the mollusc and performs a protective function. The shell can be continuous, bivalve, or composed of several plates. In many forms, the shell is reduced. The shell consists of three layers: the outer conchiolin layer (periostracum), made of the organic substance conchiolin; the middle prismatic or porcelain layer (ostracum), made of calcium carbonate in the form of prismatic crystals perpendicular to the surface; and the inner nacreous layer (hypostracum), made of the finest plate-like crystals of calcium carbonate parallel to the shell surface, possessing a pearly luster. Beneath the shell lies the mantle (a Skin fold that hangs freely along the sides of the trunk and encloses its base). Between the trunk and the mantle is the mantle cavity (housing respiratory organs such as gills or Lungs, hypobranchial (mucous) glands, chemical Sense Organs (osphradia), and the openings of the hindgut, Kidneys, and Reproductive System, collectively known as the mantle complex of organs). In terrestrial gastropods, the mantle is transformed into an organ of aerial Respiration, the lung. In some groups, such as most cephalopods, the mantle externally envelops the shell, which becomes internal. In cephalopods, the mantle is highly muscular and participates in jet propulsion.

The shell is formed through the secretory activity of the epithelium at the mantle edge. The source of calcium deposited in the shell is twofold: calcium absorbed in the intestine and transported to the mantle by the Blood, and calcium absorbed directly from the Water by mantle Cells. The shell increases in size throughout the mollusc's life by expanding along the free edge and thickening across the entire inner surface. The integument consists of a single-layered epidermal skin and Connective Tissue (cutis). There are numerous glandular cells. The epithelium varies across different Regions of the body. It is ciliated on the inner surface of the mantle, gills, and often the SOLE OF THE foot (with cephalopods being the exception). The epithelial cells lining the shell secrete the shell material. The outer areas of the skin epithelium free from the shell secrete a thin cuticle; in chitons and solenogastres, the cuticle is thicker and contains numerous calcareous spines and scales.

The musculature is well-developed, with the most Muscles concentrated in the foot. In many PARTS OF THE body, the musculature is undifferentiated and resembles a skin-Muscle sac. Specialized muscle bundles differentiate within the body, including muscles that retract the body or specific parts into the shell, muscles that close the Valves in bivalves, and those ensuring the mobility of shell plates in chitons. The musculature of the oral apparatus and Pharynx is well-developed. Molluscan musculature is generally smooth, though individual pharyngeal muscles in Gastropoda and shell adductors in some bivalves are striated.

Class Polyplacophora, or Chitons (Polyplacophora, or Loricata)

Exclusively marine animals, inhabiting mainly the littoral zone, with some extending to greater depths. Sluggish, mostly herbivorous animals that attach themselves to rocks and hard substrates using a muscular foot. About 1,000 species, with 3 found in the Black Sea. Small animals, with a body length ranging from 1 to 30–40 cm, and a mass of up to several kilograms. The dorsal surface is colored in ochre-yellow or brown tones with spots of various colors. The body shape is elongate-oval, dorsoventrally flattened. The body consists of a head, trunk, and foot. Most of the dorsal side is covered by a shell composed of eight separate, movably connected plates. Along the periphery of the dorsal side, free from the shell, lies the marginal zone of the mantle. On the anterior portion of the ventral side is the head with the mouth opening, separated from the foot by a transverse furrow. There are no sense organs on the head. The foot occupies most of the ventral surface and features a large, flat sole. The head and foot are bordered by a broad muscular fold, the mantle. Between the head-foot and the mantle lies a deep mantle furrow (mantle cavity). Situated within the mantle furrow is the mantle complex of organs: gills (ctenidia), osphradia, the anus, and paired reproductive and excretory openings.

Class Bivalvia

Exclusively aquatic animals inhabiting marine and freshwater bodies. Body sizes range from a few millimeters to 1.5 meters (Tridacna gigas). They lead a benthic lifestyle and are found at various depths. Most are slow-crawling forms, while some are immovably attached to the substrate. Approximately 20,000 species. About 150 species inhabit the freshwater bodies of Ukraine, and over 100 species live in the Black and Azov Seas. Bilaterally symmetrical Molluscs with a bivalve shell enclosing the body laterally. The body is elongated, laterally compressed, and consists of a trunk (filling the upper part of the shell) and a foot (on the

ventral side). The head is reduced. The mouth is located at the anterior end of the trunk, and the anus at the posterior end. The left and right shell valves are movably connected on the dorsal side by an elastic ligament made of the organic substance conchiolin. When the adductor muscles relax, the valves open automatically. In most bivalves, the two valves are interlocked by means of a hinge consisting of tooth-like outgrowths on the inner surface of the dorsal margin of one valve that fit into sockets on the opposite valve. Some species, such as the freshwater swan mussel (Anodonta), lack a hinge, and the valves are held together solely by the ligament. Shells vary greatly in shape. In most species, both valves are identical, but some species have asymmetrical valves. In oysters, for example, the left valve—used to attach to the substrate—is much larger and more convex, housing the entire body of the mollusc, while the right valve acts merely as a lid. In some forms, such as the shipworm Teredo, the shell is reduced, covering only 1/20 of the body length.

Class Gastropoda

Approximately 90,000 species. Over 500 species are found in Ukraine. They inhabit various biotopes of the World Ocean—from the coastal zone to depths exceeding 10,000 m; in freshwater bodies and on land; from polar latitudes to the tropics; and from plains to mountain peaks (over 5,000 m above sea level). A small number of species lead a parasitic lifestyle. Sizes range from 2–3 mm to several tens of centimeters. The largest include: Hemifusus proboscidiferus (shell length 60 cm); the sea hare Aplysia depilans (40 cm); some African land snails of the genus Achatina reaching up to 25 cm in length; and the worm-like holothurian endoparasite Parenteroxenos dogeli, measuring 128 cm in length. Asymmetry in Structure is manifested in the shell shape, the reduction of mantle complex organs on one side (usually the right), and the enhanced Development of the corresponding organs on the other side (usually the left). The body comprises a head, a foot, and a trunk that forms an outgrowth called the visceral mass, covered by a continuous cap-like or spirally coiled shell. The head is distinctly separated from the body, bearing the mouth, one or two pairs of tentacles, and a pair of eyes. In some forms, the head forms a snout, while in certain predatory and parasitic Prosobranchia, it transforms into a muscular proboscis that can be inverted or everted to capture prey. The foot is well-developed and typically features a sole adapted for crawling. In swimming forms, the lateral edges of the sole expand to form wide lobes (Aplysia, order Tectibranchia) or swimming fins (Clione limacina, order Pteropoda). In parasitic forms, the foot is reduced. Positioned above the foot is the sac-like trunk, or visceral mass, covered by the shell. In lower gastropods, it is symmetrical and indistinctly separated from the foot. However, in most gastropods, the trunk protrudes above The Foot as a large, spirally coiled sac. The shell is continuous, cap-like, or most commonly coiled in a spiral in a clockwise direction (dextral shells); left-coiled (sinistral) shells occur less frequently. The whorls of the shell fit tightly against one another and may fuse along their inner walls to form a solid central Column (columella), or they may remain slightly separated, forming a narrow opening along the axis of the shell known as the umbilicus.

Class Cephalopoda

Exclusively marine organisms, most abundant in tropical and subtropical waters, but also inhabiting temperate and polar seas. They occur from the surface down to the greatest ocean depths. Forms include benthic (bottom-dwelling), nektonic (active free-swimming), and planktonic types. They generally inhabit oceans and seas with a salinity of no less than 33 ‰. Only a few coastal squid species of Loliginidae near the coasts of Central America can tolerate temporary brackish water. Around 650 species are known, though this is not their final count. They represent the most highly organized group of molluscs and rank among the most advanced invertebrates. An ancient group of molluscs known since the Early Paleozoic. There are about 10,000 fossil species of cephalopods. Sizes vary greatly—from 1 cm (some cuttlefish) to 18 m (the giant squid Architeuthis dux). Exclusively predatory, highly active animals. Bilaterally symmetrical animals with an external shell (subclass Nautiloidea) or an internal, rudimentary shell (subclass Coleoidea). The body consists of a head and a trunk covered by a mantle. In cuttlefish, the trunk is flattened; in squids, it is cylindrical and tapered at the posterior end; and in octopuses, it is sac-like. The head is large, well-differentiated from the trunk, and bears eyes, a crown of appendages (arms or tentacles) surrounding the mouth, and a funnel (siphon). Coleoids possess either 8 or 10 appendages; squids and cuttlefish invariably have 8 arms and a pair of tentacles, whereas octopuses have only 8 arms. Nautiloids possess about 90 arms. The inner surface of the arms (except in Nautilus) is covered with

suckers arranged in one to four, rarely more, longitudinal rows. The largest suckers are located in the middle of the arms or closer to the base, while the smallest are at the arm tips.