INVERTEBRATE ZOOLOGY - H. I. Shcherbak - 2008

KINGDOM METAZOA

SUBKINGDOM EUMETAZOA

DIVISION TRIPLOBLASTICA, OR BILATERIA

SUBDIVISION SPIRALIA

PHYLUM MOLLUSCA

CLASS GASTROPODA

SUBCLASS OPISTHOBRANCHIA

Exclusively marine gastropods. The mantle cavity is small, shifted to the right side; the ctenidium, osphradium, atrium, and Kidney are unpaired. The apex of the ctenidium points backward (hence the subclass name); sometimes it is reduced and replaced by secondary gills, or Respiration occurs through the Skin surface (order Nudibranchia). The Nervous system lacks chiasmony (streptoneury). In many species, the mantle envelops the shell. The FOOT undergoes significant modifications. In some forms, it is reduced; in others, conversely, the sides of the foot expand into broad, wing-like lobes called parapodia, which serve for swimming, as, for example, in sea hares (genus Aplysia). Opisthobranchs are hermaphrodites, and their Reproductive System has a more complex Structure than that of prosobranchs. The shell in opisthobranchs undergoes considerable changes.

Most species of the order Tectibranchia possess a thin, reduced shell, often covered by the parapodia of the foot, as in sea hares (Fig. 243). Frequently, the shell becomes thin, transparent, and incapable of containing the entire body of the slug, while in most species, it disappears completely.

Actively swimming species of the order Pteropoda have adapted to a planktonic lifestyle: their bodies are transparent, the shell is absent, and the foot is modified into fins. A typical representative is the sea angel (Clione limacina), distributed in the cold waters of the Northern Hemisphere (Fig. 231, b).

The order Nudibranchia comprises crawling benthic forms that lack a shell and gills; instead, secondary gills develop on the dorsal surface, for example, in Aeolidia papillosa (Fig. 231, c). In some species, Branches of the Liver extend into the cavities of these secondary gills, and stinging capsules (nematocysts) from hydrozoan polyps—upon which these animals feed—accumulate at the tips of the gills. The stinging capsules are not digested by the Mollusks but serve them for defense.

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Fig. 243. Opisthobranch mollusks (from Vlastov and Matokin):

a - sea hare (Aplysia depilans); b - Bertelinia limax with a bivalved shell

Species of the genus Bertelinia (order Saccoglossa), distributed in tropical seas, are of particular interest (Fig. 243, b). They possess not a solid shell, but a bivalved one, complete with two adductor Muscles. Despite their external resemblance to bivalves, berteliniids feature a well-developed HEAD with tentacles, a gill, a radula, and other Organs typical of gastropods. The larva possesses a cap-shaped shell that subsequently transforms into a bivalved one, yet the spiral embryonic whorl is preserved at the apex of one of the Valves.