AQUATIC. 

 Class I. CONCHIFEKA, 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 rudimentary eyes, which are set in the inter- 

 stices of those filaments where the mantle is open. The/bo 

 is tongue-shaped, and sometimes capable of considerable 

 extension. It is used by the animal for creeping or attach- 

 ing itself to other bodies by a byssus or bundle of muscular 

 threads. Reproductive system similar to that of the moncecia 

 among plants both sexes being united in the same in- 

 dividual, 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 LAMELLIBRANCHIATA*. 



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. SPBLEBIID^E. 



BODY subglobular : mantle open in front, and forming at 

 the posterior side a cylinder, which is often divided near 

 its opening into two tubes. The cylinder or tubes are 



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



