248 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 Trigonia the gills are reduced to comb-like 



Fig. M)8.Mytilis edufis, common mussel, a, mantle ; J, foot; c, byssns ; d and e, 

 muscles retracting; the foot ; /, mouth ; a, palpi ; h t visceral mass i, inner gill ; j, 

 outer gill. From Brehm's " Thierlebem" t 



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. Trigonia is also capable of leaping a short distance ; 

 while Lima (Fig. 167) is an active flyer or leaper. Ostrea and 

 Cyclas are dioecious, while many mollusks are monoecious or 

 hermaphroditic. The foot varies much in form, in the 

 mussel (Mytilus, Figs. 168, 169), Pinna, Cydocardia (Car- 

 dita) (Fig. 170), and the pearl-oyster it is finger-shaped and 



