INVERTEBRATE ZOOLOGY - H. I. Shcherbak - 2008

KINGDOM MULTICELLULAR ANIMALS (METAZOA)

SUBKINGDOM TRUE MULTICELLULAR ANIMALS (EUMETAZOA)

SECTION TRIPLOBLASTIC (TRIPLOBLASTICA) OR BILATERAL (BILATERIA) ANIMALS

SUBSECTION SPIRALIA (SPIRALIA)

PHYLUM MOLLUSCS (MOLLUSCA)

Predominantly aquatic, less commonly terrestrial free-living animals, with only a few species adapted to a parasitic lifestyle. The phylum comprises about 130,000 species.

Molluscs (from Lat. mollis - soft) are bilaterally symmetrical animals, although some of them (Class Gastropoda) become asymmetrical due to the displacement of certain Organs. The body of molluscs is unsegmented, with only lower representatives showing some traces of metamerism.

The body consists of three main regions: the HEAD, the visceral mass (trunk), and the 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 that performs a locomotor function. The visceral mass is located above the foot and may expand onto the dorsal side as a more or less elevated hump.

A characteristic feature of molluscs is a mineral-organic shell that, in typical cases, covers the entire body and performs a protective function. The shell may be univalve, bivalve, or composed of several plates. In many forms, the shell is more or less reduced.

Three layers can be distinguished in the shell: the outer conchiolin layer (periostracum), which consists of an organic substance called conchiolin; the middle prismatic or porcelain-like layer (ostracum),

which contains calcium carbonate in the form of prismatic crystals arranged perpendicularly to the surface; and the inner nacreous layer (hypostracum), built of ultra-thin lamellar crystals of calcium carbonate arranged parallel to the shell surface, producing a characteristic iridescent pearlescent luster due to the uneven refraction of light On the surface of these plates.

Beneath the shell lies the mantle—a fold of Skin that hangs freely along the sides of the trunk and envelops its base. The mantle secretes the shell from the exterior. Between the trunk and the mantle lies the mantle cavity, which houses the respiratory organs (gills or lung), hypobranchial (mucous) glands, and chemical Sensory Organs (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.

The integument consists of a single-layered epidermal epithelium and Connective Tissue (cutis). On the inner surface of the mantle, the gills, and often on the SOLE OF THE foot, the epithelium is ciliated (with cephalopods being the exception). External areas of the epithelium free from the shell secrete a cuticle of varying thickness. In addition to epithelial Cells, the skin contains A large number of glandular cells that produce mucus.

The musculature is well developed, with the greatest Muscle mass concentrated in the foot. There are also Muscles that retract the body or specific parts into the shell, close the Valves in bivalves, ensure the mobility of shell plates in chitons, etc. The musculature of the oral apparatus and Pharynx is likewise well developed. The musculature is smooth, except for certain pharyngeal muscles in gastropods and the shell adductors in some bivalves, which are striated.

The body cavity, or coelom, in most molluscs consists of two small cavities: the pericardial cavity surrounding The Heart, and the gonadal cavity, which houses the gonad. Each coelomic compartment has a pair of coelomoducts, which function as excretory organs (kidneys) in the pericardial cavity and as genital ducts in the gonadal cavity. 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 cavity formations: parenchyma, schizocoel, and coelom, although their proportions vary across different classes.

Digestive System. The ectodermal foregut forms the Oral Cavity, the pharynx (into which Salivary Glands open), and the Esophagus, which may expand into a crop. The endodermal midgut comprises The Stomach, the digestive gland (hepatopancreas, commonly referred to as the Liver), and the intestine, which transitions into the ectodermal hindgut, or rectum. Jaws—horny thickenings of the cuticle—are located at the border between the Oral Cavity and the pharynx.

A characteristic feature of molluscs is the presence of a specialized organ—the radula (or ribbon-like rasp)—equipped with numerous transverse rows of horny Teeth, situated on the odontophore (a muscular protrusion on the floor of the oral cavity). The shape of the digestive tract varies: in some molluscs, the mouth and anus are located at opposite ends of the body, whereas in others, the gut forms a loop (anopedial flexure), bringing the mouth and anus close together.

The excretory system consists of paired kidneys. These are the aforementioned coelomoducts, whose inner ends open into the Pericardium via a ciliated funnel, and whose outer ends open into the mantle cavity.

The Circulatory system is open, meaning that Blood (hemolymph) flows not only through Blood Vessels but also through lacunae and sinuses. There is a heart, which varies in Structure and position among different molluscs; it consists of one to four atria and one to two ventricles, and is enclosed within the pericardial coelom. Most commonly, two aortas (occasionally one) emerge from the ventricle and branch into Arteries, after which the hemolymph is poured into lacunae and sinuses that lack proper walls and are bounded instead by body Tissues and organs. Venous vessels are generally few, replaced by venous sinuses, with a fully formed Venous system present only in cephalopods.

The hemolymph supplies tissues with oxygen and nutrients while removing Metabolic waste products and carbon dioxide. In most molluscs, the hemolymph also acts as a hydroskeleton against which muscles exert force during foot movements.

The respiratory organs in most molluscs are represented by feather-like gills, or ctenidia, situated within the mantle cavity. In some species, other respiratory structures have developed in place of ctenidia; these are known as secondary, or adaptive, gills. Respiration can also occur through the skin, particularly across the mantle surface. In terrestrial and certain freshwater gastropods (subclass Pulmonata), aquatic respiration has been replaced by aerial breathing, with the lung—a modified mantle cavity—serving as the respiratory organ.

In primitive molluscs, The Nervous System consists of a circumesophageal nerve ring and two pairs of connected nerve cords linked by transverse commissures. In most species, the cords bear several pairs of ganglia interconnected by commissures and connectives. This type of nervous system is termed scattered-ganglionic (or decentralized). As a result of ganglion concentration, the majority of them become aggregated in the head region to form a complex Brain, as seen, for example, in cephalopods.

Sensory organs in most molluscs are well developed. These primarily include a pair of eyes, The complexity of which ranges from simple pigment pits to vesicular eyes complete with a lens and vitreous body. Many molluscs bear tactile tentacles on the head. Chemical Senses are mediated by osphradia, located in the mantle cavity near the Base of the gills. Most molluscs also possess equilibrium organs known as statocysts.

Reproductive system. Molluscs include both dioecious and hermaphroditic species. Gonads are paired by origin, though many species retain only a single gonad. The genital ducts originate from coelomoducts, and their complexity correlates with the mode of Fertilization. In species with external fertilization, the genital ducts have a simple structure, whereas in species with internal fertilization, the efferent ducts are significantly more complex.

Reproduction. Molluscs reproduce exclusively sexually. Cleavage is spiral and determinate; Gastrulation occurs via invagination. The coelom and mesoderm form teloblastically.

Postembryonic development generally proceeds with metamorphosis. In some molluscs, a trochophore larva hatches from the egg, whereas in most species, the trochophore stage takes place inside the egg, and a more complex larva emerges—the veliger. The veliger possesses the rudiments of characteristic molluscan organs, such as the shell and foot, along with a more developed prototroch, specifically termed the velum. In some cases, all larval stages occur within the egg, and a fully formed mollusc hatches directly.

THE PHYLUM MOLLUSCA comprises seven classes. Two of them—Gastropoda and Bivalvia—account for 98% of all known extant species.