INVERTEBRATE ZOOLOGY IN THREE VOLUMES - BOOK 3 - H.I. Shcherbak - 1997

PHYLUM MOLLUSCA

Molluscs are predominantly aquatic, more rarely terrestrial, free-living animals, with only a few species adapted to a parasitic lifestyle. The phylum comprises about 130,000 species. They are coelomate animals that stand at the same level of Organization as Arthropods.

Molluscs are bilaterally symmetrical animals, although some of them (Class Gastropoda) become asymmetrical due to the shifting of A number of Organs. The body of molluscs is unsegmented, and only a few representatives show certain signs of metamerism.

As a rule, the body of molluscs consists of three main regions — the HEAD, trunk, and FOOT, though 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 that performs a locomotor function. It typically appears as a flat sole or keel. However, in some molluscs, it is modified into a swimming or prey-capture organ, or is partially or completely reduced. The trunk lies above the foot and may expand dorsally into a more or less prominent hump.

A characteristic feature of molluscs is the mineral-organic shell, which in typical cases covers the entire body and serves a protective function. The shell may be univalve, bivalve, or composed of several plates. In many forms, the shell is reduced to a greater or lesser extent.

As a rule, three layers can be distinguished in the shell: the outer organic layer, or periostracum, consisting of an organic substance called conchiolin; the middle prismatic, or porcelain-like layer (ostracum), composed of calcium carbonate in the form of prismatic crystals oriented perpendicularly to the surface; and the inner nacreous layer (hypostracum), built of extremely thin plate-like crystals of calcium carbonate arranged parallel to the shell surface, which produces a characteristic pearly luster due to the uneven refraction of light On the surface of these plates. The crystallization of CaCO₃ occurs on an organic matrix of plates composed of Proteins and Polysaccharides.

Beneath the shell lies the mantle, a fold of Skin that hangs freely down the sides of the trunk and encloses its base. Between the trunk and the mantle lies the mantle cavity, which houses the respiratory organs — gills or Lungs, hypobranchial (mucous) glands, and organs of chemical sense (osphradia). The openings of the hindgut, Kidneys, and Reproductive System also empty here. All these structures are collectively referred to as the mantle complex of organs. In terrestrial gastropods, the mantle is modified into an organ of aerial Respiration, the lung. In some groups, such as most cephalopods, the mantle envelops the shell externally, making it an internal Structure. In cephalopods, the mantle has well-developed Muscles and participates in jet propulsion.

The mollusc shell is formed through the secretory activity of the epithelium along the mantle margin. Initially, a protein matrix is secreted, which then serves as the foundation for the crystallization of calcium carbonate. The source of the calcium deposited in the shell includes both 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, growing along the free margin and thickening over the entire inner surface.

The integument of molluscs consists of a single-layered epidermal epithelium and Connective Tissue (cutis). In addition to epithelial cells, There is a vast number of glandular cells that secrete abundant mucus. The epithelium varies across different Regions of the body. On the inner surface of the mantle, the gills, and frequently on the SOLE OF THE foot, the epithelium is predominantly ciliated (with cephalopods being an exception). The epithelium lining the shell lacks cilia; its cells secrete the shell material. The outer regions of the skin epithelium not covered by the shell secrete a thin cuticle, whereas in chitons and solenogasters, the cuticle is thicker and contains numerous calcareous spines and scales.

The musculature of molluscs is well developed, with the greatest Muscle mass located in the foot. In many body areas, especially the mantle and foot, the musculature is undifferentiated and closely resembles the skin-muscle sac of worms. However, specialized bundles of muscles differentiate in the body: muscles that retract the body or specific parts into the shell, muscles that close the shell Valves in bivalves, and those that provide mobility for the armor plates in chitons, among others. The musculature of the mouth apparatus and Pharynx is also well developed. Molluscan musculature is smooth, with only certain pharyngeal muscles of gastropods and the shell adductors of some bivalves being striated.

Molluscs are coelomate animals, although their coelom is not very large. It is non-metameric and mostly consists of two small cavities: the pericardial cavity, surrounding The Heart, and the gonadal cavity. Each coelomic compartment has a pair of coelomoducts. The coelomoducts of the pericardial coelom perform an excretory function and serve as kidneys; the coelomoducts of the gonadal coelom act as genital ducts. The spaces between Internal Organs are filled with parenchyma containing unepithelialized clefts — schizocoelic sinuses and hemolymph-filled lacunae. Thus, molluscs possess all possible types

of Body Cavities: parenchyma, schizocoel, and coelom, though their relative proportions vary among different classes.

The Digestive System begins with the mouth opening, which leads to the Oral Cavity, transitioning into the pharynx. At the boundary between the Oral Cavity and the pharynx lie the jaws — cuticular, horny thickenings whose shape and number vary among different molluscs. They are most strongly developed in cephalopods, where they serve for capturing and crushing prey. A characteristic feature of the molluscan feeding apparatus is the presence of a specialized organ, the radula (or ribbon), located on the odontophore — a muscular protrusion on the floor of the oral cavity. It consists of a cuticular ribbon whose surface is covered with numerous transverse rows of horny Teeth pointing backward. Thanks to specialized musculature, the Tongue with the radula can move forward and backward within the oral cavity or protrude slightly from the mouth. Using the radula, molluscs can scrape food, such as Algae, from the surfaces of submerged objects. In some predatory molluscs, the radula, together with the pharynx, serves to capture and hold prey. The ducts of the Salivary Glands open into the pharynx. The pharynx leads into the Esophagus, which may expand to form a crop. All these are PARTS OF THE ectodermal foregut. However, in bivalve molluscs, along with the reduction of the head, most foregut organs are lost: the jaws, pharynx, radula, crop, and salivary glands; only the oral cavity and a short esophagus remain.

The endodermal midgut consists of The Stomach, into which a large digestive gland (the so-called Liver) opens, and the Small Intestine. The structure of the stomach varies depending on the feeding habits. It is most complexly built in detritus-feeding molluscs. The liver secretes Enzymes that break down proteins, fats, and CARBOHYDRATES; its cells can also phagocytose small food particles. Absorption of digestive products and storage of nutrient reserves take place in the liver. The small intestine transitions into the ectodermal hindgut, or rectum, which opens into the mantle cavity. The shape of the intestine varies: in some molluscs, the mouth and anus are located at opposite ends of the body, while in others, the intestine forms a loop (anopedal flexure), bringing the mouth and anus close together.

The excretory system of molluscs consists of paired kidneys, the inner ends of which open via a ciliated funnel (nephrostome) into the Pericardium, and the outer ends into the mantle cavity. Based on their mesodermal origin and the presence of a ciliated funnel opening into the coelom (pericardium) at the inner end, the excretory organs of molluscs correspond to the coelomoducts of segmented worms.

Molluscs have an open Circulatory system, and only in most cephalopods does it become nearly closed. The CIRCULATORY SYSTEM OF molluscs is more complex than that of any other invertebrates. It includes a heart, Blood Vessels, and lacunae. The structure and position of the heart vary among different molluscs; it consists of 1 to 4 atria and 1 to 2 ventricles, and is surrounded by the pericardial coelom. Mostly two aortas (occasionally one) emerge from the ventricle and branch into Arteries; blood is then poured into lacunae and sinuses that lack proper walls and are surrounded by the Tissues and Organs of the body. Blood (hemolymph) returns to the atrium via efferent vessels from the respiratory organs (gills or lungs). Sometimes, in addition to the heart, there are auxiliary pulsating organs (branchial hearts in cephalopods). In most molluscs, venous vessels are few, being replaced by venous sinuses, and only in cephalopods is the Venous system fully formed.

The respiratory organs in most molluscs are represented by ctenidia, or true gills, which lie in the mantle cavity. These are skin outgrowths, each typically resembling a feather and consisting of a central axis with gill lamellae arranged on both sides. Blood vessels run through the axis. There may be a single pair of ctenidia or many, as in chitons, where their number can reach 80. In many gastropods, only one ctenidium remains. In most bivalves, the ctenidia enlarge and, aside from respiration, perform the Functions of locomotion and water filtration, transforming into a sieve-like filter that strains food particles.

Among molluscs, there are forms in which ctenidia have disappeared and been replaced by other respiratory organs that are physiologically equivalent to ctenidia, though not homologous to them; these structures are called secondary, or adaptive, gills. Respiration can also occur simply through the skin, especially across the mantle surface. In terrestrial and some freshwater gastropods, aquatic respiration has been replaced by aerial breathing, with the lung — a modified mantle cavity — serving as the respiratory organ.

The Nervous system achieves varying degrees of complexity across different classes of molluscs. In chitons, solenogasters, and monoplacoplhorans, it is poorly ganglionated and consists of a circum-esophageal nerve ring connected to two pairs of longitudinal trunks linked by transverse commissures (with cerebral ganglia differentiating in the latter two classes). In most molluscs, the concentration of Nerve Cells along the trunks forms several pairs of ganglia connected by commissures and connectives. This type of nervous system is termed scattered-ganglionic. Further concentration of ganglia leads to the majority of them becoming concentrated in the head to form a complex Brain.

Alongside the Central Nervous System, all molluscs possess a peripheral diffuse cutaneous plexus that resembles the nerve plexus of Cnidarians. It consists of nerve cells of all types and is capable of independent Reflexes. A complex plexus is also present in the internal organs of molluscs; it connects to the central nervous system via buccal and visceral ganglia.

Sense Organs are well developed in most molluscs. These primarily include a pair of eyes, whose complexity ranges from simple pits to vesicular eyes complete with a lens and vitreous body. The most complex structure is found in the eyes of higher cephalopods, which closely resemble those of mammals. Bivalves, having lost the head region, lack eyes, though some have secondarily evolved eyes that vary in structure and Location. Many molluscs bear tentacles on the head as organs of Touch. Organs of chemical sense are represented by osphradia, located in the mantle cavity near the Base of the gills. Most molluscs possess equilibrium organs, known as statocysts. Chitons are characterized by so-called aesthetes, which pierce the shell plates and respond to water currents and light.

Molluscs include both dioecious and hermaphroditic species. The Gonads are paired by origin, but many species retain only a single gland formed by the fusion of two or the reduction of one. The genital ducts develop from coelomoducts, and their complexity depends on the mode of Fertilization. Species with external fertilization have simply structured reproductive ducts, whereas species with internal fertilization feature significantly more complex efferent ducts.

The embryonic development of molluscs is very similar to that of polychaete Annelids. Typically, the eggs contain a moderate amount of yolk and undergo spiral determinate Cleavage; Gastrulation occurs via invagination. The blastopore in molluscs, as in polychaetes, elongates into a slit and closes from back to front; subsequently, its anterior part transforms into the mouth, while the anus forms at the site of the posterior part. Then a larva, the trochophore, is formed. In most molluscs, the trochophore develops into a more complex larva called a veliger, which bears the rudiments of characteristic molluscan organs — the shell and foot — as well as a more developed prototroch, specifically known as the velum. Sometimes a veliger hatches directly from the egg, or all larval stages are completed inside the egg, with a fully formed mollusc hatching out, but this is a secondary phenomenon. In cephalopods, the eggs are very rich in yolk, cleavage is discoidal, and development is direct.

THE PHYLUM MOLLUSCA includes seven classes: Polyplacophora (or Loricata), Aplacophora (or Solenogastres), Bivalvia, Monoplacophora, Gastropoda, Scaphopoda, and Cephalopoda.

Traditionally, the phylum is divided into two subphyla: Amphineura (including the first two classes) and Conchifera (comprising the remaining classes). Recently, alternative Classification schemes for the phylum have been proposed.

Based on their body plan, the classes of Mollusks can be divided into two groups (Fig. 10). The first group includes mollusks in which the mouth and anus are located at opposite ends of the body; the shell, when present, consists of several plates (dorsal or lateral); and during metamorphosis, the larva grows uniformly in length. This group comprises the classes Polyplacophora, Aplacophora, and Bivalvia. In contrast, the second group of classes features a mouth and anus positioned close together, a visceral hump on the dorsal side accompanied by an anopedial intestinal flexure, and a single continuous shell. During larval metamorphosis, a dorsal hump forms, into which the intestinal loop (anopedial flexure) is drawn. These mollusks appear as if folded in half, with the posterior end of their body actually representing the middle of the dorsal hump. This group includes the classes Monoplacophora, Gastropoda, Scaphopoda, and Cephalopoda.

Fig. 10. Body plans of mollusks from various classes:

a–c — without a visceral hump; d–f — with a visceral hump; 1 — mouth; 2 — shell; 3 — anus; 4 — foot