246 



SYSTEMATIC ZOOLOGY 



can break away from its threads and form a new attachment 

 elsewhere. Organs of special sense are variously present in 

 different groups, in the form of palps and tentacles as tactile 

 organs, eyes, situated on the edges of the mantle or the tip of 

 the siphon, and otocysts. The alimentary canal terminates in 

 an anus near the posterior adductor muscle. The sexes are 

 usually separate, but sometimes united in one individual, and in 

 the marine genera there is a larva resembling the trochosphere. 

 The further classification of the Lamellibranchia is somewhat 

 disputed. They are sometimes divided into two subclasses, the 

 Asiphonia, those without a siphon, and the Siphoniata. those 

 having a siphon. For convenience we may consider some of the 



more important mem- 

 bers under these two 

 groups, noting particu- 

 larly some of economic 

 importance. The oyster 

 is one of the most im- 

 portant members of the 

 class from this Doint of 



i 



view. The edges of the 

 mantle are not fused, 



Fig. 255. Mitylus edulis. By, byssus, attached to a tnei 'G IS no siphon, the 

 stick; /■, foot; \ exhalant siphon. (From the Cain- foot IS VCl'V rildimentai'V 

 bridge Natural History.) 



or entirely absent, and 

 the two valves of the shell are of unequal size. The left valve is 

 concave and attached to some foreign object, the right is smaller 

 and flatter. They are hermaphroditic and reproduce in June 

 and July. In Europe the species principally eaten are Ostrea 

 edulis and Ostrea angulata. In America the most common is 

 Ostrea virginiana, which occurs along the whole eastern coast 

 from the Gulf of St. Lawrence to the Gulf of Mexico except on 

 the eastern shores of New England. On the western coast oi 

 the United States two species which are eaten, occur. Ov 

 beds are now planted artificially. The eggs are often hatched 

 in ponds of salt water, and the young oysters when they first 

 attach themselves are called spat ; later, when old enough to be 

 transplanted to the shore beds, they are termed seed oysters. 

 Our largest oysters are from ten to fifteen centimeters long ; in 



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