No. I.] THE EMBRYOLOGY OF THE UNIONIDAE. 65 



those who have studied the post-embryonic development, the 

 anterior part of this area is the basis for the formation of the 

 foot and pedal structures. 



Schierholz derives the rudiment of the foot from the median 

 portion of the cells of the larval mantle lying between the oral 

 plate and the anterior end of the ventral plate. That I am 

 unable to agree with him goes without saying. 



It is practically certain that the anus forms behind the ante- 

 rior limit of the blastopore, but still within the limits of the 

 area originally occupied by its posterior portion. 



It will suffice to merely mention the lateral pits {cf. Figs. 89 

 and 93 to 97) lying at the sides of the foot-fold, as we may now 

 call the area of the ventral plate. They are covered with active 

 cilia, which are in direct continuity with the cilia of the foot- 

 fold (Fig. 93). The structure of the walls is shown in the 

 sections (Figs. 94 to 97). Schierholz and Schmidt derive 

 the gill-filaments from the outer walls of these pits. Within 

 the pits, according to Schierholz, lie two or more rounded cells 

 which he regards as the basis of the otocysts. I have some- 

 times seen such cells in the stage of Fig. 79 lying on the 

 surface near the anterior end of the ventral plate, but it 

 seemed to me that they did not persist. In any case it is diffi- 

 cult to see how they could represent the otocysts. 



The cerebral ganglia have begun to form in some glochidia 

 of Anodonta in February. A section through their rudiment 

 is shown in Fig. 94, which is taken about 22 /i. in front of the 

 stomodaeum. 



The bristles which lie on each side of the thread-gland and 

 beneath the hooks of the shell have been considered sensory 

 by all authors who have mentioned them. I found that when 

 the glochidia were left for some time in a weak solution of 

 methylene blue the cells bearing these bristles were the only 

 ones in the embryo which took the stain. After fixing with 

 picrate of ammonia, long, stained protoplasmic processes of 

 these cells could be traced for some distance beneath the larval 

 mantle. In Fig. ^6 I have shown the course of the processes 

 of the lateral bristle-bearing cells. In no case was I able to 

 make out any coordinating structure with which the processes 



