The Structure and Life History of Some Minute Prosobranchs of Rock Pools:Skeneopsis Planorbis(Fabricius),Omalogyra Atomus(Philippi),Rissoella Diaphana(Alder) andRissoella Opalina(Jeffreys)

Abstract

The external features of Skeneopsis planorbis (Fabricius) are described and compared with those of Omalogyra atomus (Philippi), Rissoella diaphana (Alder) and R. opalina (Jeffreys).The foot has a large posterior mucous gland (Figs. 1 and 4, Pm; PI. IV, figs. 1 and 2, Al, Pl), its secretion forming a thread on which the mollusc can climb from one level to another.Correlated with their small size are modifications of the pallial organs. Skeneopsis, the largest and least specialized, has a bipectinate osphradium, but the gill is reduced to nine filaments; the anus lies well within the mantle cavity. In the other genera osphradium and ctenidium are lost, though the latter may be represented in Rissoella by a small tract of ciliated epithelium. In the absence of a ctenidium the animals depend entirely upon pallial respiration and the stream of water through the mantle cavity is maintained by other means: from the anus strips of ciliated epithelium pass forward to the mouth of the mantle cavity, causing a strong exhalant stream and carrying away the faecal pellets. There is a compensating inhalant flow. The kidney (PI. IV, figs. 1 and 2, K), with its rich vascular supply, has migrated into the tissues of the mantle, increasing its respiratory efficiency.

Observations on the opisthobranch mollusc Acteon tornatilis (L.).

“…As a matter of convenience, therefore, it seems appropriate to leave the animal where it has long been placed, as the most primitive type of living opisthobranch. This work on Acteon, and recent work on Siphonaria (Hubendick, 1947), on the ellobiid pulmonates (Morton, 1954),on Onchidella (Fretter, 1946),on Omalogyraand Rissoella (Fretter, 1948),on the pyramidellids (Fretter & Graham, 1949)and on other tectibranchs (Lloyd, 1952), demonstrate, however, with great emphasis, how close the relationship of all the three major groups of gastropods is at some central point such as this. It seems likely that further research will show that it is from some archaeogastropod nucleus, represented to-day by such molluscs as the trochids, that all these groups have arisen.…”

“…As in other tectibranchs like Onchidella (Fretter, 1943), Aplysia (Eales, 1921), Philine and Scaphander (Lloyd, 1952) the albumen gland is an appendage of the female tract: this contrasts withprosobranchs where the eggs traverse the cavity of the gland.…”

“…8 illustrates such a gland in Siphonaria. Onchidella celtica (Joyeux-Laffuie, 1882;von Wissel, 1898;Fretter, 1943) possesses very similar ones except that they discharge by nger ducts. Many limpet-like prosobranchs, too, in which the conical shape f the shell and the absence of operculum prevent the animal from shutting up ntirely when disturbed, possess similar or even more elaborate glands.…”

“…The important feature of these cells is that they can ingest spermatozoa; within the cytoplasm ingested gametes lose their identity and are apparently digested. Absorption of effete sperm by the epithelium of the vesicula seminalis has been recorded for Littorina (Linke, 1933), the Stenoglossa (Fretter, 1941) and the tectibranch Onchidellaceltica (Fretter, 1943).The two branches of the hermaphrodite duct, after its bifurcation into male and female channels, embrace the duct of the receptaculum seminis as it opens to the ventral channel. The prostate (P) constitutes the broad pallialregion of the male duct.…”

Three new species of Ammonicera from the Eastern Pacific coast of North <br />America, with redescriptions and comments on other species of Omalogyridae (Gastropoda, Heterobranchia)

“…Sars, 1878 are arguably the smallest known gastropods (Bieler & Mikkelsen, 1998;Gofas & Warén, 1998;Habe, 1972). Omalogyrids live in shallow marine environments worldwide, commonly dwelling and grazing on algae (Bieler & Mikkelsen, 1998;Fretter, 1948). Two species of the family are known to be hermaphrodites Simone, 1997), and the two whose mode of development is known hatch as crawling juveniles from egg capsules (Franc, 1948;Fretter, 1948;Gofas, 2011).…”