Elements of comparative zoology (1904) Elements of comparative zoology elementsofcompar1904king Year: 1904 MOLLUSCS. 201 luscs (fig. 39)—are without such protection in the adult, although shells are present in the young. In the nudi- branchs there are commonly developed gills upon the dor- sal surface, and in the living condition these forms are, from their bright colors, among the most attractive of molluscs. Here, too, are forms (Pteropods) especially de- veloped for a life on the surface of the ocean, the foot being modified into a pair of wing-like structures. ORDER II.—PULMONATA. The gre
Elements of comparative zoology (1904) Elements of comparative zoology elementsofcompar1904king Year: 1904 MOLLUSCS. 201 luscs (fig. 39)—are without such protection in the adult, although shells are present in the young. In the nudi- branchs there are commonly developed gills upon the dor- sal surface, and in the living condition these forms are, from their bright colors, among the most attractive of molluscs. Here, too, are forms (Pteropods) especially de- veloped for a life on the surface of the ocean, the foot being modified into a pair of wing-like structures. ORDER II.—PULMONATA. The great majority of the land- and fresh-water snails and slugs belong here. In them gills have disap- peared, and the mantle-cavity has been modified into an organ (lung) for breathing air, the opening to which is to be seen on the right side of the body. Over six thousand species belong here, some (snails) having a well-developed spiral shell, while the slugs (fig. 40) are appar- ently shell-less; but in these slugs one can frequently find a rudimen- tary shell imbedded in the mantle. CLASS III.—SCAPHOPODA (TOOTH-SHELLS). In these the mantle edges are fused below, forming a tube, and as a result there is formed a tubular shell, open at both ends, in shape something like the tusk of an elephant. The foot is relatively large, and is adapted for digging in the sand in which these animals live. There is no distinct head, but the mouth is provided with a FIG. 40. — Slug (Limax campestris). s, respir- atory opening. From Ludwig's Leunis.
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