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 ofSkeneopsis planorbis(Fabricius) are described and compared with those ofOmalogyra atomus(Philippi),Rissoella diaphana(Alder) andR. 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 inRissoellaby 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.