250 



SUBKLtfGDOM MOLLUSCA. 



CLASS III. LAMELLIBRANCHIATA. 



Fig. 



Atya arenaria, Clam.t 



General Characteristics. 

 The Lamellibranchs* (lamellated 

 gills) are all bivalves. The 

 two parts of the shell are con- 

 nected by a hinge, usually with 

 interlocking teeth. To hold 

 them open that being always 

 the unconstrained position an 

 elastic ligament is fixed to the 

 hinge, if inside, acting by expan- 

 sion, if outside, by contraction. 

 The valves are closed by muscles, 

 ordinarily two, but sometimes 

 only one. The respiratory organs 

 are leaf-like gills, to which in the 

 lower forms the water is brought 

 by a double tube, one orifice for 

 ingress, and the other for egress, 

 after it has laved the gills and 

 circulated around the mouth 

 which filters out the food. Most 

 move by means of a foot, which 

 is often very powerful, and, in the 

 boring species, is covered with 

 flinty particles. 



f SIPHONS ) ,. ( Clam, 



PRESENT. } S ' Ph0nata - 1 etc. 



* All this class are headless, and hence they are sometimes termed " Acephala." 

 They are also called Conchifers (shell-bearers). 



t All the viscera, right valve and half the siphon retained, g. Gills ; s. Siphon 

 introducing water to the gills and mouth ; s'. Siphon carrying out the water after it has 

 bathed the gills and surrendered the food which it contained to the mouth, o ; t. Four lip 

 tentacles ; h. Heart, the intestine \ passing through it ; m. Mantle ligament uniting the 

 valves around their edges ; p. Orifice in the mantle through which the foot f protrudes ; 

 p. Cloaca.; a, a'. Muscles closing the valves by their contraction. 



