No. I.] THE EMBRYOLOGY OF THE UNIONIDAE. 67 



and 84). It is already beginning to stretch forward towards 

 the oral plate above the cells of the larval mantle. It is 

 no doubt the mechanical cause of the splitting of the larval 

 mantle, which permits the ventral plate to come in contact 

 dorsally with the entoderm sac and to fuse anteriorly with 

 the oral plate. In the glochidia of Unio, which do not as 

 such reach so advanced a stage of development as those of 

 Anodonta, the entoderm remains in this state till the post- 

 embryonic development begins {cf. Fig. 89). In those glochidia 

 of Anodonta, however, which have wintered in the maternal 

 gills, the entoderm has already begun its differentiation. It 

 has taken on the form of a sac which in one case ran through 

 seven sections of 73^ mm. each (Figs. 95-97). Lateral expan- 

 sions in the middle of its course I took for the liver diverticula 

 from comparisons with Schmidt's sections of parasitic larvae 

 (Fig. 96 l.c). Anteriorly it was connected with the stomo- 

 daeum (Fig. 95) and posteriorly the end-gut was indicated 

 (Fig. 97). 



General Remarks. 



One cannot view such a remarkable and unusual series of 

 phenomena as accompanies the transformation of the young 

 larva into the glochidium without asking one's self what is 

 the reason of it all.? Why, fo/ instance, should the thread- 

 gland be formed so far from its definitive position ? The most 

 natural explanation is that the primitive function of the organ 

 in question has changed, and that a new position seemed more 

 favorable for the discharge of the new function. On such an 

 hypothesis there is nothing wonderful in such phylogenetic 

 changes of position being repeated in ontogeny. We can 

 easily apply this to the explanation of the displacements of 

 the thread-gland. In its definitive position practically in the 

 centre of the ventral surface, it manifestly occupies the best 

 position for the discharge of its present function, which, as we 

 have seen, is probably to assist the glochidium in attaining 

 parasitic attachment to its host. For if the thread becomes 

 attached, for instance to a fish's fin, the larva is pulled on to 

 the fin ventral surface down ; when the muscle contracts the 



