BIVALVES. 245 



horny ligament. Their name aptly expresses the character 

 of the breathing organs, which are in general large, vas- 

 cular, and crescent-shaped, placed on either side between 

 the body and the mantle. A perfect system of circulation 

 is performed by means of a small heart composed of a 

 single ventricle, and the lobes of the mantle are fringed 

 round the edge with numerous tentacular filaments, very 

 sensitive and irritable to the touch, and which, being in 

 constant activity, draw a current of water for the ready 

 capturing of their prey. 



The LamellibrancJiiata form a shell composed of two 

 valves joined together by a horny ligament, with mostly 

 a rather complicated hinge. They include two orders — 

 Unimusculosa and Biimisculosa. 



In the first, the points of muscular attachment are always 

 more or less discernible on the internal surface of the shell ; 

 the animal is often affixed by a byssus, or beard. 



The first order is divided into five families, as follows : — 

 Ostracea, Pectinacea, Avicidacea, Mytllacea, and Triclac- 

 nacea, — better known as Oysters, Mussels, Clams, &c. 



