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 single. 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 body surface (order Nudibranchia). The Nervous system lacks chiastoneury. In many species, the mantle envelops the shell. The FOOT undergoes significant modifications. In some forms, it is reduced, whereas in others, the sides of the foot expand into broad wing-like lobes, the so-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 accommodating the entire slug's body, 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 true gills; instead, secondary gills develop on their dorsal surface, for example, in Aeolidia papillosa (Fig. 231, c). In some species, hepatic branches extend into the cavities of these secondary gills, and stinging capsules (nematocysts) from the 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 as a means of defense.

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

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

Of particular interest are species of the genus Bertelinia (order Saccoglossa), distributed in tropical seas (Fig. 243, b). They possess not a solid shell, but a bivalve one, complete with two adductor Muscles. Despite their superficial resemblance to bivalves, bertelinias 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 bivalve shell, yet the spiral embryonic whorl is preserved at the apex of one of the Valves.