288 



MANUAL OF ZOOLOGY 



SECT. 



being closed by means of an operculum borne on the 

 foot. In some terrestrial forms in which an operculum is 

 absent, the opening may be closed up during winter by a 

 layer of hardened mucous matter 

 to which the name of epiphragm 

 is applied. In some of the Gastro- 

 poda (Fig. 164) lateral folds of the 

 mantle are reflected over the shell, 

 and may completely cover it. These 

 folds may unite by their edges, so 

 that the shell comes to be enclosed 

 in a complete sac of the mantle : 

 such enclosed cells are always im- 

 perfectly developed and incapable 

 of covering the body. Thus in 

 Aplysia (the " Sea-Hare ") and other 

 allied forms the shell is greatly re- 

 duced, thin and horny, and con- 

 cealed within the mantle, while in 

 certain members (the Nudibranchs) 

 of the same sub-order it is entirely 

 wanting (Fig. 165). The shell is 

 also completely absent in some of 

 the pelagic forms (Heteropoda and 

 Pteropoda) in others, though pres- 

 ent and external, it is too small to 

 enclose the animal. In the Slugs 

 the shell is vestigial, and is con- 

 cealed by the mantle. 



The Gastropoda have a well- 

 marked /lead, separated from the body by a constriction or 

 neck. The mouth, situated at the anterior end of the 

 head on its ventral aspect, is in many instances provided 



FIG. 163. Shell of Terebra 

 oculata. 



