GASTROPODA. 27 



giant Cryptocliiton of Kamtschatka the plates are entirely covered 

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

 Chitonellus, which consists of long slug-like animals, 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 the 

 Silurian ac;e downwards. 



Opisthobranchiata. 



This section contains a large and varied assemblage of mollusks [Case 12, 

 in which, as already stated (see p. 14), the gills are not con- E_F - 1 

 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- 

 lidce), the "Sea-Hares" (Aplysiidae), the " Umbrella - shells " 

 (Umbrellida) , 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 



Fiff. 15. 



Sea-Hare {Aphjsia punctata). British. 



«, labial tentacles ; b, upper tentacles or rhinopliores ; 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 



