Lecture notes on invertebrate zoology

Semantic module 4. Phylum Mollusca and Phylum Echinodermata

Lecture topic 16. Systematics of mollusks

Class Bivalvia (pelecypods). Structural features: bivalve shell, mantle cavity, Digestive System. Reproduction and development. Bivalves in aquaculture: oysters, mussels, scallops, and monadacnas. Significance in aquatic biocenoses. Class Cephalopoda. Features of highest Organization in Morphology, internal Structure, and biological characteristics. Main commercial species.

Class Lamellibranchia, or Bivalvia. The class unites about 20,000 species of marine and freshwater Mollusks with a bivalve shell covering the body on both sides. A characteristic feature of these soft-bodied animals is the reduction of the HEAD; most species possess a pair of ctenidia transformed into lamellar gills. The body of mollusks is elongated, laterally compressed, and bilaterally symmetrical. The Mouth is located at the anterior end of the trunk. The FOOT in bivalves is laterally compressed and pointed at the free edge, forming a keel; it is adapted for digging in the substrate rather than for locomotion. The body is covered by the mantle, which hangs down on the sides, forming two mantle folds. Between the folds and the body lies the mantle cavity containing the mantle complex of Organs. Sometimes the mantle grows at the anterior end, forming small tentacles and even eyes. Often, the edges of the mantle fuse, forming 2-4 siphons. The body of mollusks is protected by two Valves connected on the dorsal side by a ligament and a hinge. Each valve is formed by three layers: the outer organic (conchiolin) layer, the middle prismatic (or porcelain) layer, and the inner nacreous (mother-of-pearl) layer. The shell valves are closed by adductor Muscles. The digestive system begins with the mouth located at the anterior end of the body above the Base of the foot. Around the mouth are 2 pairs of long labial palps with cilia. Due to head reduction, mollusks lack a Pharynx, radula, jaws, and Salivary Glands. The mouth opens into a short Esophagus, followed by The Stomach and the midgut. The opening of a sac-like appendage, in the cavity of which the crystalline style is formed, opens into the posterior part of the stomach. Surrounding the stomach is a paired Liver, the ducts of which empty into the stomach; the midgut branches off from the stomach into the base of the foot, makes several bends, and runs along the DORSAL SIDE OF the trunk to the posterior end, piercing the ventricle of The Heart. The Nervous system consists of three pairs of ganglia (cerebropleural, pedal, and visceroparietal) connected by long connectives. Sense Organs are poorly developed. Osmphradia are located at the base of the gills, and two statocysts are situated near the pedal ganglia.

Respiratory organs are represented by modifications of typical ctenidia. The Circulatory system is open, and the heart is located on the dorsal side of the body within the pericardial cavity (Pericardium); it consists of a ventricle and two atria. Anterior and posterior aortas branch off from the ventricle. The anterior aorta extends forward, supplying arterial Blood to the Internal Organs, foot, and the anterior part of the mantle. The posterior aorta is directed backward and divides into two mantle Arteries. Venous blood collects in lacunae, becomes oxygenated in the gill filaments, and enters the atrium via the afferent gill vessel. The excretory system consists of a pair of Kidneys located in the posterior half of the body on the sides and below the intestine. The kidneys open into the pericardium by one end and into the mantle cavity by the other. Lamellibranchs are predominantly dioecious creatures; their Gonads are paired, lying in the anterior region of the trunk and extending into the base of the foot. Most mollusks of this class possess special oviducts and sperm ducts that open to the outside on the sides of the base of the foot. Fertilization is mostly external. Egg Cleavage is spiral. A trochophore larva is formed, which transforms into a veliger As a result of a series of changes. The veliger possesses a velum, an apical plate with a tuft of cilia, a bivalve shell, a foot rudiment, mantle, nervous system ganglia, stomach, liver, etc. The excretory organs are also protonephridia. The class of Lamellibranchia mollusks unites 3 superorders: Protobranchia, Eulamellibranchia, and Septibranchia.

Gastropods are animals whose body is distinctly divided into a head, trunk, and foot. A characteristic feature of all gastropods is structural Asymmetry, which manifests itself, firstly, in the reduction of the right Organs of the mantle complex and the enhanced development of organs on the left side, and secondly, in the spiral coiling of the visceral sac (clearly expressed in the shell shape). The body of most gastropods is elongated and dorsal-convex; the head bears the mouth, 1-2 pairs of tentacles, and eyes. The foot is a muscular ventral outgrowth with a flat sole. In most species, the trunk hangs over The Foot as a more or less spirally coiled sac. A mantle fold forms on the trunk, beneath which lies the mantle cavity with the mantle complex of organs. The mollusk's body can be fully or partially retracted into the spirally coiled shell. In some species, the shell has the shape of a large cap or is reduced to separate calcareous particles in the integument (slugs). The digestive system begins with the mouth on the lower side of the anterior end of the head. The pharynx contains a Tongue covered with a cuticle and transverse rows of Teeth (radula). Salivary glands empty into the pharynx. They are followed by the esophagus, which in some mollusks forms an expansion—the crop. The anterior end of the midgut forms a sac-like expansion—the stomach—into which the liver ducts empty. The stomach is followed by the Small Intestine, which forms several loops, turns forward, and transitions into the hindgut ending in the anus. Most gastropods breathe by means of gills. Terrestrial mollusks lack gills; Lungs have formed in their place.

In connection with body asymmetry, Changes in the heart have occurred in gastropods, manifesting as the gradual Atrophy of the right atrium. In many species, the right atrium completely disappears. The heart is located above the hindgut and surrounded by the pericardium. An aorta departs from the ventricle and divides into two trunks: the cephalic aorta and the aorta supplying blood to the internal organs. Venous blood collects in lacunae. The blood is predominantly colorless and contains amebocytes.

The nervous system of gastropods is formed by five pairs of nerve ganglia (cerebral, pedal, pleural, visceral, and parietal) connected by commissures. In some mollusks, the number of ganglia may increase to 7 (or decrease to 3). The Functions of tactile organs are performed by the head tentacles, mantle edges, etc. Organs of chemical sense include osphradia and the anterior pair of head tentacles. Organs of equilibrium are represented by a pair of statocysts. Visual organs are eyes of varying complexity. The excretory system consists of a pair of kidneys, of which only the left one persists. Among gastropods, there are both dioecious and hermaphroditic species. There is always a single gonad. In hermaphroditic forms, the hermaphroditic gland produces both spermatozoa and eggs. The common hermaphroditic duct gradually widens and divides into an oviduct and a sperm duct. The oviduct widens into a Uterus, which opens into the genital cloaca. The sperm duct opens to the outside near the anterior end of the body. In addition, There is a copulatory organ.

The class Cephalopoda unites about 700 large marine mollusks that swim freely, less frequently crawl, and lead a predominantly predatory lifestyle. Their body is bilaterally symmetrical with a distinct division into a head and trunk. The foot is transformed into tentacles or arms; the second part of the foot forms a funnel located on the ventral side near the entrance to the mantle cavity. The head bears a mouth surrounded by 8-10 tentacles, as well as a pair of large eyes. The trunk is covered on all sides by the mantle, which on the ventral side is separated from the trunk by the mantle cavity containing the mantle complex of organs: the anus, genital and renal openings, and ctenidia. The shell is predominantly reduced and well-developed only in the most ancient forms (Nautilus). In the remaining species, it has been preserved as a plate beneath the integument on the dorsal side of the trunk. Besides the external Skeleton, cephalopods possess a special internal skeleton in the form of a cartilaginous capsule around the nerve ganglia, eyes, and statocysts, as well as supporting cartilages in the locks, at the base of the tentacles, and inside the fins. The digestive system begins with the mouth leading into a muscular pharynx containing a tongue with a radula. 1-2 pairs of salivary glands empty here. The esophagus passes into a muscular stomach, into which the ducts of the bilobed, less commonly simple, liver empty. The duct of the ink sac empties into the hindgut before the anus. The nervous system in cephalopods is the most complex among all mollusks. Its ganglia are large and form a general circumesophageal nerve mass. Several ganglia can be distinguished: paired cerebral and visceral, and an unpaired pedal ganglion, which breaks down into two nerve nodes—the tentacle ganglion and the funnel ganglion. Two large mantle nerves depart from the posterior section of the ganglionic mass, forming two large stellate ganglia on the inner surface of the mantle.