334 MARINE INVERTEBRATES 



specimen, break away the shell with a hammer, using care not to 

 lacerate the soft, fleshy portion within ; when this is accomplished, 

 wash the animal carefully to remove the slimy exudation. Note 

 the " visceral hump," which is spiral, and which formerly occu- 

 pied the upper-whorl portion of the shell. The thin skin covering 

 it is the mantle, which below is greatly thickened and free, lying 

 about the foot like a heavy fleshy flap. 



MANTLE 



The mantle-edge in both of these examples is simple ; that is to 

 say, it possesses no fringe of tentacles, nor is it supplemented by 

 extra processes, characters which mark many genera of marine 

 Gasteropoda. In Fulgur and Buccinum the mantle-edge does not 

 protrude below the edge of the shell ; but in many genera, especially 

 those which possess smooth, glossy shells, like the cowries (Cyprcea) 

 and the graceful Oliva, the mantle is proportionately very much 

 larger. In these two genera, when the animal is extended, as in 

 crawling about the sand, the mantle curves upward and incloses a 

 large portion of the shell itself. Indeed, in some genera the shell 

 is almost entirely concealed by this extension of the mantle (Siga- 

 retus, Natica, etc.). 



When the shell is removed, the folding of the mantle which 

 constitutes the siphon can be plainly seen. The office of the siphon 

 has already been referred to, also the fact that the presence of a 

 siphon in the gasteropod mollusk may always be determined by 

 merely glancing at the shell alone, for a notch at the base of the 

 aperture indicates the place through which the siphon passed. 

 In Buccinum this is merely a notch, but in Fulgur the siphonal 

 canal of the shell is much longer. Just why the long siphons of 

 some mollusks should be naked and exposed to danger, while 

 others are so carefully protected by elongated portions of the 

 shell, is a mystery, but nature is full of such contradictions. 



THE GASTEROPOD FOOT 



The foot is long, broad, and flat on the under side, like a 

 disk. The variations in the gasteropod foot are almost infinite. 



