738 



ZOOLOGY 



is concealed. Sometimes only the apical portion of the shell 

 is spiral, the remainder being a straight or sinuous cylinder. The 

 spiral form of the shell and the parts enclosed in it, as well as the 

 direction of the spiral — whether dextral or sinistral — are, it may be 

 here pointed out, very fundamental features of the organisation of 

 the Gastropod, and are foreshadowed at an early stage in the 

 segmentation of the ovum. The mouth of the shell has usually 

 a prominent margin or peristome, which is 

 sometimes entire and continuous, sometimes 

 broken by a deep notch or a spout-like process 

 or canal, formed in connection with the de- 

 velopment of a spout-like prolongation of the 

 mantle, the siphon, which lies in it. The 

 mouth of the shell in many Gastropoda is 

 capable of being closed by means of an oper- 

 culum 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. The margin 

 of the mantle in some cases bears a series of 

 tentacles. Lateral folds of the mantle are in 

 some of the Gastropoda (Fig. 622) reflected 

 over the shell and may completely cover it. 

 In some cases these folds unite by their edge, 

 so that the shell comes to be enclosed in a 

 complete sac of the mantle ; such enclosed 

 shells are always imperfectly developed and 

 incapable of covering the body. Thus in 

 Aplysia and some other Opisthobranchs the 

 shell is greatly reduced, thin and horn-like, 

 and concealed within the mantle, while in the 

 nudibranch members of the same sub-order it 

 is entirely absent (Fig. 623). The shell is also 

 completely absent in some of the pelagic forms 

 ( Ileteropoda and Pteropoda) ; in others, though 

 present and external, it is too small to enclose 

 the animal (Fig. 624). In the slugs, among 

 the Pulmonata, the shell is vestigial and in 

 most cases is concealed by the mantle (Fig. 625). 



The foot varies in the extent of its development in the different 

 families of the class. It usually presents an elongated fiat ventral 

 surface on which the animal creeps by wave-like contractions 

 of the muscular tissue. An exceptional case is that of Caecum, in 

 which the creeping movement is entirely due to the action 

 of cilia covering the ventral surface. In the typical Gastropods 

 the foot is usually distinguishable into three parts, a middle part 



Fir.. 621.— Shell of 

 Terebra oculata. 



