THE GASTROPODA 85 
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 Pyrula (Fig. 71), Aplysia (Fig.154), many Bullidae, 
and various Pulmonates, such as Vitrina, Parmarion, Hemphilia, 
Homalonyzx, 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 (Cypraca), 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 forma closed sac containing the 
4 
Animal and shell of Pyrula laevigata, seen from above. a, siphon; 6, head-tentacles ; (, 
head ; d, foot, expanded as in crawling; i, 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, Doridiwm). 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 
