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 Arca the ventricle is double. In Lucina 
and Corbis there is but one gill on each side, and in Peeten, 
Spondylus and Trigonia the gills are reduced to comb-like 
Fig. 162.—Mytilts edulis, common mussel. @, mantle; 5, foot: ¢. byssus; d and 4, 
muscles retracting the foot 5 7, mouth 3g palpi ; 4, visceral masa - 2, inner gil; J, 
outer gill.—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. Trigonia is alsc capable of leaping a short distance ; 
while Lima (Fig. 161) is an active flyer or leaper. The Ameri- 
can oyster* is dicecious, while most mollusks are moneecious 
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). 
