INVERTEBRATE ZOOLOGY - H. I. Shcherbak - 2008

KINGDOM MULTICELLULAR ANIMALS (METAZOA)

SUBKINGDOM TRUE MULTICELLULAR ANIMALS (EUMETAZOA)

DIVISION TRIPLOBLASTIC (TRIPLOBLASTICA) OR BILATERAL (BILATERIA) ANIMALS

SUBDIVISION SPIRALIANS (SPIRALIA)

PHYLUM MOLLUSCS (MOLLUSCA)

CLASS GASTROPODS (GASTROPODA)

SUBCLASS PROSOBRANCHS (PROSOBRANCHIA)

The largest and most diverse group of gastropods. It includes marine and freshwater forms, with some species having transitioned to life in moist terrestrial habitats. They are characterized by the anterior position of the mantle complex, chiastoneury (streptoneury), and separate sexes. The shell is spirally coiled or cap- and saucer-shaped. Ctenidia are present. In most species, the FOOT bears an operculum, which tightly seals the shell aperture (Fig. 231, a).

Species of the order Archaeogastropoda are characterized by the presence of two or one ctenidia, two Kidneys, and two atria. The shell features one or several apertures through which Water exits the mantle cavity. The mantle organ complex is symmetrical only in a few species, such as Puncturella (Fig. 235); in most, however, it is asymmetrical, with the right ctenidium being smaller than the left. These include the well-known abalones (genus Haliotis), which inhabit the intertidal and subtidal zones of tropical seas; they possess a flattened, ear-shaped shell perforated with several small holes (Fig. 240, b). Abalone shells feature a striking, iridescent nacreous layer, making them popular for jewelry and ornaments. Their meat is highly nutritious and flavorful. Small specimens of Theodoxus fluviatilis (family Neritidae) are found in freshwater bodies, whereas the majority of species in this family are marine.

Class="center">

Fig. 240. Prosobranch Molluscs (from Vlastov and Matyokin):

a - Fissurella; b - Haliotis; c - Littorina littorea; d - Cypraea moneta; e - Murex; f - Rapana thomasiana

A distinct, specialized group of gastropods comprises the so-called limpets (families Patellidae, Tecturidae, and Lepetidae). They possess a cap-shaped shell lacking apertures; their ctenidia are reduced, and instead, numerous secondary (adaptive) gills develop on the ventral surface of the mantle (Fig. 237). These molluscs inhabit coastal rocks, occasionally crawling above the waterline. Patella tarentina, widespread in the Black Sea, is a typical dweller of the marine surf zone and has also invaded the Sea of Azov.

The order Mesogastropoda includes molluscs that retain only the Organs of the left side: the left ctenidium, atrium, and Kidney. The shell is predominantly spirally coiled, without additional apertures, and the aperture is closed by an operculum in most species. Mesogastropods live primarily in marine environments, though freshwater and even terrestrial species inhabiting tropical and subtropical forests are also known.

In shallow tropical seas, numerous species of small yet exceptionally beautiful molluscs of the family Cypraeidae can be found; the most notable among them belong to the genus Cypraea, characterized by an involute shell. Typical inhabitants of the coastal marine zone include species of the family Littorinidae. One of its species, the common periwinkle (Littorina littorea), is among the most widespread molluscs of the Northern Hemisphere (Fig. 240, c).

A peculiar group is formed by pelagic predatory heteropods (Heteropoda), which feature a glassy, transparent body. Their shell is either rudimentary or entirely absent, and the foot is modified into fins—a keel-shaped ventral fin and a caudal fin (e.g., Pterotrachea) (Fig. 241).

Fig. 241. The heteropod Pterotrachea coronata (from Vlastov and Matyokin):

1 - Mouth; 2 - caudal appendage; 3, 4 - posterior and anterior PARTS OF THE foot, respectively

Freshwater bodies harbor Representatives of the family Hydrobiidae. Bithynia tentaculata and B. leachi are common, with the latter species well-known as an intermediate host for the cat Liver fluke, Opisthorchis felineus. Snails of the family Viviparidae are also abundant. The most frequently encountered are river snails of the genus Viviparus, which feature a spirally coiled shell with an operculum; their gills are reduced, and Respiration is cutaneous (Fig. 242). They brood eggs within the mantle cavity, from which tiny young snails emerge. Tropical freshwater habitats are home to large apple snails (family Ampullariidae), familiar to aquarists worldwide. They are characterized by dual respiration: when near the water's surface, an apple snail extends a long breathing tube formed by the rolled edge of the mantle to breathe atmospheric air; when submerging deeper, it switches to gill respiration.

Fig. 242. Freshwater prosobranch molluscs

(from Vlastov and Matyokin): a - Bithynia (Bithynia leachi); b - river snail (Viviparus contectus)

The order Stenoglossa comprises marine, predominantly predatory snails whose anterior HEAD region is elongated into a proboscis. They possess a single ctenidium, a single atrium, and one kidney. Most species bear a horny operculum. Secretions from their Salivary Glands assist them in boring through the shells of bivalves and the tests of Echinoderms, which form their prey. This group includes some of the largest gastropods with striking shells used for souvenirs and jewelry, such as Strombus, Murex, Cassis, Conus, and others (Fig. 240). The veined rapa whelk (Rapana thomasiana), introduced from the Pacific Ocean, is now found in the Black Sea.