. Natural history. Zoology. 640 MOLLUSCA—CLASS IV.—GASTROPODA. In the genus Tormdhm, the shell is cylindrical, the apex of the short spire being sinistral; in Volvidella the spire is no longer raised, the shell being convolute, , wound on itself like a measuring tape; in Bulla and Bullinella [ = Cylichna] it is involute, , the spire is sunk, leaving an aperture, whilst the columella is less pronounced ; in Scaphander the latter has disappeared and the shell is without any central axis, the whole interior being visible from the front ; in Philine this is carried still further, the shell
. Natural history. Zoology. 640 MOLLUSCA—CLASS IV.—GASTROPODA. In the genus Tormdhm, the shell is cylindrical, the apex of the short spire being sinistral; in Volvidella the spire is no longer raised, the shell being convolute, , wound on itself like a measuring tape; in Bulla and Bullinella [ = Cylichna] it is involute, , the spire is sunk, leaving an aperture, whilst the columella is less pronounced ; in Scaphander the latter has disappeared and the shell is without any central axis, the whole interior being visible from the front ; in Philine this is carried still further, the shell, which is internal, being widely open—unrolled, as it were. To the Bulloidea is now referred that section of the old class Pteropoda, or sea-butterflies, in which the animal retains its shell in the adult state. The chief peculiarity of the Pteropoda lies in the conversion of the foot into a pair of wing-like fins. Their shells vary greatly in shape, whilst their general appearance will best be understood from the foregoing figures (Pig. 19). The following families are referred to the Bulloidea:—Actfeonidso, Torna- tinidfe, Scaphandridre, Bullidre, Aplustridie, Ringiculidas, Gastropteridse, Philinidse, Doridiidae, Runcinidse, Oxynoidaa, Limacinidoe, Cymbulidae, Cavolinidfe. B.—Aplysioidea. In the Aplysioidea the shell is either very greatly reduced or wanting altogether ; the head carries two pairs of tentacles. The Ai', or sea-hares, have large bodies of greenish or oiive-greeii hue, with a pair of lappet-like extensions of the foot folded over the back ; the shell, concealed beneath the mantle-folds, is thin, covered with a horny periostracum, triangular in shape, and flexible when first removed from the animal (Fig. 18 D). The shell-less sea-butterflies are now placed with this group, which includes the following families :—Aplysiidse, Pneumonodermatidas. Clionop- sida3, Clionidfe, Notobranchseidse, and Eurybiidse. C. —Pleukobranchoidea. The Pleurobran
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