[3] DEVELOPMENT OF THE OYSTER. 781 



effected, as iu the adult, by an adductor muscle, am, the fibers of which 

 pass from one valve to the other. But it is a very curious circumstance 

 that this adductor muscle is not the same as that which exists in the 

 adult. It lies, in fact, in the fore part of the body, and on the dorsal 

 side of the alimentary canal. The great muscle of the adult, on the 

 other hand, lies on the ventral side of the alimentary canal and in the 

 hinder part of the body. And as the muscles, respectively, lie on op- 

 posite sides of the alimentary canal, that of the adult cannot be that 

 of the larva which has merely shifted its position ; for, in order to get 

 from one side of the alimentary canal to the other, it must needs cut 

 through that organ, but as in the adult, no adductor muscle is discover- 

 able iu the position occupied by that of the larva, or anywhere on the 

 dorsal side of the alimentary canal ; while, on the other hand, there is 

 no trace of any adductor on the ventral side in the larva, it follows 

 that the dorsal or anterior adductor of the larva must vanish in the 

 course of development, and that a new ventral or posterior adductor 

 must be developed to play the same part and replace the original muscle 

 functionally, though not morjihologically.* 



" This substitution is the more interesting since it tends to the same 

 conclusion as that towards which all the special peculiarities of the 

 oyster lead us ; namely, that, so far from being a low or primitive form 

 of the group of lamellibranchiate moUusks to which it belongs, it is 

 in reality the extreme term of one of the two lines of modification 

 which are observable in that group. The Trigoniw, the arks, the 

 cockles, the fresh-water mussels and their allies, constitute the central 

 and typical group of these mollusks. They possess two subequal ad- 

 ductors, a large foot, and a body which is neither very deep nor very 

 long. From these, the series of the boring bivalves exhibits a gradual 

 elongation of the body ending in the ship-worm (Teredo) as its extreme 

 term. While, on the other hand, iu the sea-mussels, the Aviculce and 

 the scallops, we have a series of forms which, by the constant shorten- 

 ings of the length and increase of the depth of the body, the reduction 

 of the foot, the diminution of the anterior of the two adductors, and 

 the increase of the posterior, until the latter becomes very large and 

 the former disappears, end in the oyster. 



" And this conclusion that the oysters are highly specialized lamelli- 

 branchs, agrees very well with what is known of the geological history 

 of this group, the oldest known forms of which are all dimyary, while 

 the monomyary oysters api)ear only later. 



" When the free larva of the oyster settles down into the fixed state 

 the left lobe of the mantle stretches beyond its valve, and applying itself 

 to the surface of the stone or shell to which the valve is to adhere, 



* The larva of the cockle has at first, like the oyster larva, only one adductor, 

 ■which answers to the anterior of the two adductors which the cockle possesses in the 

 adult state. 



