GASTROPODA. 27 



giant Cryptochiton of Kaoitschatka the plates arc entirely covered 

 over by the thick leathery granular mantle, and in another set, 

 Chitonettus, which consists of long slug-like animal-, the plates are 

 very small; and placed at intervals along the back. 



Between three and four hundred living species are known, and 

 about one fourth that number has been found fossil from tin- 

 Silurian aa:e downwards. 



Opisthobram in VIA. 



This section contains a large and varied assemblage of mollusks Ca • 12, 

 in which, as already stated (see p. 14), the gills are not con- E ~ F> I 

 tained in a cavity over the neck, but placed towards the hinder 

 part of the body, and are either wholly exposed or partly covered 

 by the mantle. This group includes the " Bubble-shells " [Bul- 

 lid(c), the " Sea-Hares " (Aplysiida), the " Umbrella - shells " 

 (Umbrellidce), the Nudibranchs, and some others. 



The Sea-Hares, so called on account of a slight resemblance to 

 a crouching hare and not for their nimbleness of foot, are found 



Fig-. r>. 



Sea-Hare (Aplysia punctata) . British. 



a, labial tentacles ; b, \\\\\ ei tentacles or rhinophores ; c, siphonal fold 



of the mantle near the shell : d, eye. 



in most parts of the world, in pools at low water. At the hinder 



part of the back two flaps of the mantle partly conceal a thin 



horny shell which serves as a protection to the gills and vital 



organs beneath. When molested; these animals discharge a large 



quantity of a purple fluid, discolouring the surrounding water for 



a distance of more than a yard. 



D 



