MOLLUSC A. 



there are sense-organs, usually eyes, ears, and some- 

 times tactile organs. The aquatic forms breathe by 

 gills, the terrestrial forms by lungs. The alimentary 

 canal is distinguished into throat, stomach, intestine, 

 and rectum : kidneys, comparable with the segmental 

 organs of worms, are always present, and a liver 

 usually. The blood system is not completely closed, 

 the vessels being in communication with " sinuses,' 

 i.e. irregular spaces containing blood ; but there is 

 always a heart. 



The true MoUusca consist of three classes — the 



Fig. M.— Veliger larva of a (lostcrojiod moUnso: S, shell; P, foot; T'el, 

 velum ; T, tentacles; Op., operculum, for the closure of the shell opening, 



Lameilibrahchiata, Gasteeopoda, and Cephalopoda. 

 The Pteropoda, nearly related to the fiasteropods, are 

 sometimes separated from them to form a fourth class. 



The Lamellibranchs (mollusca which bear gills in 

 the form of plates), also called Gonchifera, and Bivalve 

 Shell-fish, are without a definite head. Their shells 

 are placed to right and left of the body (not to back 

 and front, as are those of the Brachiopoda), and are 

 joined by an elastic hinge, and by adductor muscles. 



