Brooks.] 226 [February 2, 



several of our more common marine Gasteropoda; and the results 

 reached seem to point to the conclusion, which I believe has never 

 been pointed out, that although the Gasteropoda are much more 

 specialized and highly evolved than the Lamellibranchs, nearly all 

 their organs, excepting those of locomotion and relation, conform 

 much more closely to the embryonic type than do the same organs in 

 an adult Lamellibranch. The latter group must therefore be re- 

 garded as a side branch from the main stem, of which the Gastero- 

 poda are a much more direct continuation. 



I have already shown (Proc. Amer. Association, 1875) that the 

 embryonic shell of Anodonta is, at first, a cup covering what is to 

 become the dorsal surface of the embryo, and is therefore homologous 

 with the shell of a Gasteropod. This cup or hood soon folds down on 

 to the sides of the embryo, precisely as described in Dentalium by 

 Lacaze-Duthiers, and at a very early period splits along the dorsal 

 median line and becomes separated into the two halves of a bivalve 

 shell, which are thus shown to be together the homologue of the shell 

 of a Gasteropod exclusive of the operculum, which, as Selenka has 

 shown in his " Entwickelung von Tergipes claviger," is formed by a 

 split which extends across the long axis of the body, and therefore at 

 right angles to that which, in Anodonta, gives rise to the two valves. 

 The valves of an adult lamellibranchiate shell are a specializa- 

 tion of the embryonic shell; are bilateral in origin, and together 

 represent the dorsal or haemal cup or shell of a Gasteropod, a Poly- 

 zoon, or a Brachiopod ; while the ventral or neural operculum of a 

 Gasteropod corresponds to the neural valve of a Brachiopod or the 

 lid of a cheilostomatous Polyzoon, and is wanting in the Lamelli- 

 branchs. 



The digestive organs of an adult Lamellibranch, although they are 

 very much less specialized than those of a Gasteropod, seem to be 

 much more widely removed from the embryonic type. The stomach 

 of the Veliger of Astyris, like that of a Polyzoon, is divided by a 

 constriction into two chambers. (Compare also the figure of the 

 embryo of the Pteropod, Carollnia tridentata by H. Fol, and that of 

 Limnaea by Rabl.) In the embryo of Mytilus we have, according 

 to Lacaze-Duthiers, a similar stomach, and in the adult of Yoldia 

 we have the same a little modified; here the anterior portion of the 

 stomach receives the bile-tubes, and the posterior portion is pro- 

 longed so as to form a conical, somewhat twisted, intestine-like 

 pouch, from the bottom of which the small intestine originates. In 



