PLANORBIS ; LYMN^EA. NUDIBRANCHIATA. 



373 



species is one of those most remarkable for the decollation of its 

 shell. Another large snail of tropical climates, as Achatina, 

 which feeds on trees and shrubs, chiefly on the western coast of 

 Africa, and in the West Indies ; several of its species are dis- 

 tinguished by the beauty of their colours ; and some of them 

 are reversed. 



914. The aquatic Pulmonea have only two tentacula. From the 

 necessity of coming to the surface to breathe, they can only live 

 in water of inconsiderable depth ; and they chiefly inhabit ponds 

 and shallow streams, or the banks of rivers. Some, however, live 



on the sea-shore. 

 The Planorbis, the 

 shell of which is 

 quite flat, having 

 all its coils upon the 

 same level, is a very 

 common genus in 

 this country ; as is 

 also the LymncBa, 

 which feeds upon 

 seeds, as well as the 



FIG. 567. LYMNJEA STAGNALIS. 



softer parts of the 

 plants, and the stomach of which has a very muscular gizzard. 



ORDER II. NUDIBRANCHIATA. 



915. The animals of this Order, which might be designated 

 Sea- Slugs, are all marine ; and being adapted to breathe 

 water at any depths, and also (in many instances) to swim with 

 facility, they are often found at a great distance from land. 

 When they swim, it is usually in a reversed position, the foot 

 being turned upwards ; this is made concave by muscular action, 

 so as to serve as a kind of boat, the buoyancy of which keeps 

 the animal at the surface without effort. This Order is a very 

 numerous one ; and some of its species attain considerable size. 

 The number of those existing on our own shores is much greater 

 than has been usually supposed; a large number of species 



