SHELLS. 227 



adhering to the surface of the water with the shell 

 downwards, and crawl in that direction with as 

 much apparent ease as on a solid surface, and they 

 will occasionally let themselves down gradually by 

 a thread. This power of crawhng under water 

 against its surface is not wholly confined to this and 

 the preceding species. The Physa fontinalis is a 

 representative of the genus. 



3. Genus Auricula. — The name is derived from a 

 fancied resemblance to the ears of some animals. 

 They are for the most part covered with an epidermis, 

 but some are often dehcately sculptured. One 

 species is found here, near the sea-shore, Auricula 

 Midee. 



II. Gasteropoda Nudilranchiata. — These have 

 neither a shell nor a pulmonary cavity, but their 

 branchi^, or gills, are exposed naked upon some part 

 of the back. The animals of this order are remark- 

 ably elegant in their forms, which present great 

 variety. Their motions aje graceful and lively, their 

 colours peculiarly brilliant, and their history and 

 economy marked by points of great interest. They 

 often swim in a reversed position, the foot appHed 

 against the surface, and made concave like a boat, 

 and they assist their progress by using the edges 

 of the cloak and the tentacula as oars. 



15 



