234 



SYSTEMATIC ZOOLOGY 



the sexes are separate as a rule. A protrusible proboscis is often 

 present, and frequently the edge of the mantle in front is drawn 

 out into an elongated groove or tubular structure, called the 

 siphon. By means of the siphon water may be drawn into the 

 mantle cavity. An operculum is often present, attached to 

 the dorsal side of the foot. In the mantle cavity there is 



usually only a single gill, 

 and in most cases a well- 

 developed penis in the 

 male. 



The shells vary from 

 simple conical caps to the 

 most elaborately coiled spi- 

 rals (Figs. 236 and 257). 

 and may be smooth and 

 polished, or roughened with 

 ridges, or provided with 

 long spines ; the colors are 

 often very beautiful in 

 tropical shells and some 

 have pearly linings. There 

 are some species in which 

 the spiral is so loosely 

 wound that adjacent coils 

 are not in contact with one 

 another (Fig. 238), so that 

 the shells closely resemble 

 the calcareous tubes of some 

 of the polvchsetous An- 

 nelida. Most of the spiral 

 shells have the coil running 

 from left to right ; such shells are called dextral in distinction 

 from those running toward the left, which are called sinistral. 

 The limpets are familiar examples of conical shells; they con- 

 stitute a conspicuous article of food in parts of Europe where 

 they are abundant. Sometimes the shell is closed at the tip, 

 sometimes there is an opening shaped like a keyhole, hence the 

 name keyhole limpets (Fig. 239). The slipper limpets ( Crepidula 

 fornicatd) (Fig. 240) are common, attached to other shells or to 



Fig. 236. Triton nodiferus, shell, natural size; 

 from New Zealand. (After Parker and Mas- 

 well.) 



