434 GENERAL ZOOLOGY 



Examples: Chaetopleura apiculata, rock sucker; Chaeto- 

 derma nitidulum, lacks a shell. 

 CLASS II. SCAPHOPODA. Have tubular shells. A radula is 

 present. Sexes are separate. All marine. 



Example: Dcntalium entale, tooth shell. 

 CLASS III. GASTROPODA. Snails and slugs. Asymmetrical 

 body with four parts well developed; a creeping foot; an 

 unpaired mantle, nephridium, and gonad. The shell is usually 

 coiled spirally. A radula is present. Aquatic forms breathe 

 by gills, the land forms with lungs. The shell is reduced or 

 lacking in slugs. 



Examples: Lymnaea palustris, a fresh-water snail; Polynices 

 duplicata, a marine snail (Fig. 254); Busycon canaliculatum, 



Fig. 254. — Polynices, a marine snail with a highly developed mantle and foot. 



F, foot; M, mantle; S, syphon. 



a whelk, marine; Helix pomatia, French snail, edible; Limax 

 flavus, a land slug ; Dendronotus frondosus, a sea slug. 



CLASS IV. PELECYPODA. Lack a head and cephalic append- 

 ages. Have a bivalve shell consisting of right and left halves; 

 with paired mantle, gills, nephridia, and gonads. Frequently 

 the hinder edges of the mantle are modified to form incurrent 

 and excurrent siphons. The sense organs are poorly developed. 

 Sexes are usually separate. 



Examples: Anodonta grandis, a fresh- water clam; Ostrea 

 virginica, the American oyster; Venus mercaneria, the hard- 

 shelled or little-neck clam; My a arenaria, the soft-shelled or 

 long-necked clam (Fig. 255). 



CLASS V. CEPHALOPODA. Active, carnivorous, marine 

 forms, including the largest and most highly organized molluscs. 

 As a rule, eight or ten tentacles surround the mouth which with 

 the siphon represent the foot region. A pair of sharp chitinous 

 jaws lie just back of the lips in the mouth. The siphon is an 



