THE GASTROPODA 



mantle are reflected over the shell and cover a greater or less part 

 of its external surface. This condition is found in various Fissurel- 

 lidae (Fissurellidea), in Marsenina, in many Cypraeidae (Fig. 70) and 

 Marginellidae, in Fyrula (Fig. 71), Aplysia (Fig. 15 4), many Bullidae, 

 and various Pulmonates, such as Vitrina, Parmarion, Hemphilia, 

 Homalonyx, Amphipeplea, etc. The portion of the internal surface of 

 the mantle that has thus become external may bear more or less well- 

 developed and ramified appendages (Cypraea), and the other surface 

 of the reflected mantle may sometimes secrete an external coating 

 of enamel over the portion of the shell to which it is applied. The 

 borders of the mantle, extending more and more over the shell, 

 may finally meet, unite, and thus form a closed sac containing the 



FIG. 71. 



Animal and shell of Pyrula laevigata, seen from above, a, siphon ; I, head-tentacles ; C, 

 head ; d, foot, expanded as in crawling ; h, the mantle-skirt reflected over the sides of the 

 shell. (From Lankester, after Owen.) 



shell, which, together with the visceral sac contained in it, suffers 

 a diminution, or even an almost complete disappearance, of its 

 spiral form, so that the animal appears to be quite naked. This 

 condition may be seen in Pupilia among the Fissurellidae, in the 

 majority of the Lamellariidae, in Pustularia among the Cypraeidae, 

 in many Tectibranchs, such as Notarchus, Doridium, Gastropteron, 

 Philine, Pleurobranchus, and in sundry limaciform Pulmonates. In 

 some cases the shell-sac remains in communication with the exterior 

 by means of a fine ciliated canal, situated at the posterior end of 

 the body (Philine, Doridium}. In the Tectibranchs the internal 

 shell is often very slightly calcified, and at the same time the 

 pallial cavity becomes more and more reduced. Finally, the shell 

 and the shell-cavity disappear, leaving the mantle absolutely naked 



