372 



SECTION A. PECTINIBRANCHIATA. 



985. The animals of this section, which may be regarded as 

 the most characteristic of Gasteropods, have all two tentacles 

 and two eyes ; the latter being sometimes mounted on footstalks, 

 as in the snail. The mouth is prolonged 

 into a sort of proboscis ; and the foot 

 nearly always bears an operculum. In a 

 considerable number of the species there 

 is a tubular prolongation of the mantle, 

 termed the siphon, for the purpose of con- 

 veying water into the branchial cavity, so 

 that the animal can respire without putting 

 forth its body from its shelter ; and the 

 existence of the siphon is indicated in the 

 shell by a notch or canal at the anterior- 

 margin. The animals in which this con- 

 formation prevails are also furnished with 

 a retractile proboscis, and are for the most 

 part carnivorous in their habits. The ac- 

 companying figure of a Pleurotoma, a 

 shell belonging to this group, is introduced, 

 not only to show this structure, but also 

 for the purpose of explaining the names 

 which are given by Conchologists to differ- 

 ent parts of a shell, and which are made 

 use of in scientific descriptions. At a is 

 seen the canal for the reception of the 

 siphon ; b, a hollow, here nearly closed up, 

 termed the umbilicus ; c, the internal edge, 



or left lip, which is partly formed by the columella ; d, the ex- 

 ternal or right lip, the edge of which is free ; e, the notch, or 

 slit, which is characteristic of the genus Pleurotoma ; f, the sinus, 

 or posterior canal ; g, part of the last turn of the spire, which is 

 called the venter or belly ; h, h, the turns or whorls of the spire ; 

 t, the sutures, or lines where these are united to each other. 



FIG. 663. Pr.Fur.oTOMA 



1UBYI.ONKA. 



