AQUATIC. 



Class I. CONCHIFERA, or BIVALVES. 



Body of an oval form, and usually compressed at its sides : 

 mantle divided into two lobes which correspond with the valves 

 of the shell. It has no distinct head : but inside the mantle, 

 and within its folds, is contained a mouth ; and the edges of 

 the mantle in those bivalves which have it open, or of the 

 tubes which are formed by it in those which have it closed, are 

 often fringed with short filaments, which serve the purpose of 

 tentacles or feelers. Some kinds have also imperfect or rudi- 

 mentary eyes, which are set in the interstices of those fila- 

 ments where the mantle is open. The foot is tongue- shaped, 

 and sometimes capable of considerable extension. It is used 

 by the animal for creeping or attaching itself to other bodies 

 by a byssus or bundle of muscular threads. Reproductive 

 system similar to that of the monoecia among plants — both 

 sexes being united in the same individual, which is capable of 

 fertilizing itself. The whole, or most important part, of the 

 body is covered by a shell, formed of two valves which are 

 connected behind by a hinge or ligament. Respiratory organs 

 consisting of gills. 



Order LAMELLIBEANCHIATA *. 



Gills 4, semicircular or leaf-shaped, arranged in pairs on 

 each side of the body. 



This Order comprises all the freshwater bivalves of 

 Great Britain ; and they are divided into three families. 



Family I. SPH./ERIID^. 



Body subglobular : mantle open in front, and forming at the 

 posterior side a cylinder, which is often divided near its open- 

 ing into two tubes. The cylinder or tubes are contractile and 



* So called from tlie leaf-like form of the gills. 



B 



