Fam. 4. Clionidae.—Body long, angulated behind, proboscis short, mouth with two or three pairs of appendages, no jaw, no gills.
Clione limacina is so abundant in Arctic seas as at times to colour the surface for miles. Each of the cephalic appendages has about 60,000 minute pedicellated suckers.
Fig. 291.—A, Anterior portion of Pneumoderma; B, Clione limacina Phipps; C, Halopsyche Gaudichaudi Soul.; f, f, fins; h.s, h.s, hook-sacs; l.f, lobe of the foot; s, s, suckers; o, posterior genital orifice; t, t, tentacles. (After Souleyet.)
Fam. 5. Halopsychidae.—Body ovate, thick, rounded behind, no gill or proboscis, fins long, narrow, broadened at the ends, epidermis sub-cartilaginoid.
Halopsyche (= Eurybia) has the power of withdrawing its head completely into a sort of pocket, which is closed by an anterior fold of the mantle. There are two long non-retractile buccal appendages.
Order IV. Pulmonata
Gasteropoda with two pairs of tentacles, visceral loop euthyneurous, ganglia concentrated round the oesophagus; breathing air by a pallial cavity formed by the union of the front edge of the mantle with the cervical region, sexes united, shell present or absent, no operculum[407] (except in Amphibola).
Sub-order I. Basommatophora.—Eyes generally at the base of the tentacles, which are not retractile, male and female genital orifices separate, radula (p. [235]) multiseriate, shell always present, external. Fresh water or quasi-marine.
Fam. 1. Auriculidae.—Breathing organ a pulmonary sac or true lung; shell spiral, conoidal, internal partitions usually absorbed, aperture more or less strongly toothed. Jurassic——. Genera: Auricula, Carychium, Scarabus, Alexia, Tralia, Plecotrema, Cassidula, Melampus, Leuconia, Pedipes (Fig. [292]).