278 LIFE BY THE SEASHORE. 



edible oyster (Ostrea edulis) is related to the scallops, but 

 differs in its peculiarly sedentary habit with which is asso- 

 ciated the distorted and ugly shell, in the entire absence of 

 foot and byssus, and in some other characters. Just as 

 many domestic animals acquire their culinary value at the 

 expense of almost all the qualities which make them in- 

 teresting to the naturalist, so the oyster pays for its valued 

 qualities by the absence of the beautiful shell, the power of 

 active locomotion, the quick senses, and the other qualities 

 which make the Pectens and Limas so fascinating. Those 

 who are not epicures may perhaps be forgiven for regarding 

 a luscious oyster as about as attractive as a prize pig, 

 while . those to whom it. appeals as an article of diet will 

 probably mourn with the famous conch ologist that oysters 

 grew and died in countless numbers before ever men 

 existed to enjoy them. There can be no difficulty in recog- 

 nising an oyster if one should be found, which is not very 

 likely. The chief point of interest is the curious shapes 

 which the shells assume when subjected to pressure by sur- 

 rounding objects. The animals are incapable of locomotion, 

 and^are attached by the surface of the lower valve, for the 

 ftyssjijs gland has been completely lost. 



tf -'* r FtS. third order of Bivalves, the Eulamellibranchia, in- 

 cludfe$ the greater number of living forms. Its members 

 are classified according to the presence or absence of the 

 siphons, the amount of union of the mantle-folds, the 

 characters of the gills, and some other points. Most of 

 them live buried in sand or mud, and the development of 

 siphons is an adaptation to this habit. Their degree 

 of development is reflected in the shell in the condition 

 of the pallial sinus (see p. 268), and where, as in the My as, 

 they reach a great size, they cannot be completely with- 

 drawn into the shell, and this "gapes" permanently. From 

 the habitat usually sand or mud these Bivalves are rarely 

 conspicuous on the shore rocks ; many may be obtained by 

 systematic digging near low-tide mark, others are tossed on 

 the beach after storms, but the majority, even of the 

 shallow-water forms, are familiar only in the condition of 

 shells. As our concern here is rather with living animals 

 than with "shells," we shall describe relatively few Bivalves, 

 chiefly those which may be hoped for in the living condition. 



