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.
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