228 



ZOOLOGY. 



mia the shell is inequilateral, one, usually the lower, being 

 fixed to some object, and the intestine does not pass through 

 the ventricle ; in Area the ventricle is double. In Lucina 

 and Corbis there is but one gill on each side, and in Pecten, 

 Spondylus and Tngonia the gills are reduced to comb-like 



Fig. 162. Mytilis edulis, common mussel, a, mantle ; k. foot; c, byssns ; rt and t, 

 muscles retracting the toot ; /. mouth ; a, palpi ; A. visceral mass z, inner giii ; j, 

 outer gil]. From Brehm's ' Thierleben. 



processes. There are usually no eyes present ; in the scallop 

 (Pecten), however, there is a row of bright shining eyes 

 with tentacles along the edge of the mantle, and contrary 

 to the habits of most bivalves, the scallop can skip over the 

 surface of the water by violently opening and shutting its 

 shell. Triqonia is also capable of leaping a short distance ; 

 while Lima (Fig. 161) is an active flyer or leaper. The Ameri- 

 can oyster* is dioecious, while most mollusks are monoecious 

 or hermaphroditic. The foot varies much in form; in the 

 mussel (Mytilus, Figs. 162, 163), Pinna, Cyclocardia (Car- 

 dita) (Fig. 164), and the pearl-oyster it is finger-shaped and 



* The European oyster is clearly hermaphroditic (Ryder). 



