No. I.] THE EMBRYOLOGY OF CREPIDULA. 107 



These cells, which are altogether characteristic in appearance 

 among the gasteropods, have been found in Neritina, Umbrella, 

 four species of Crepidula, Urosalpinx, and Fulgur. In all these 

 cases they are particularly notable because of their small size. 

 Cells of the same origin and position are found in Chiton, Unio, 

 Nereis, Amphitrite, Lepidonotus, and Clymenella. In all the 

 annelids mentioned it has been found that they form either the 

 whole or a part of the prototroch. In four species of Crepidula, 

 at least the two anterior ones form a portion of the preoral 

 velum, and this is probably true of the two posterior ones also. 

 In no other mollusk has their destiny been determined, but it 

 is highly probable that it is the same in all the gasteropods 

 mentioned, since these cells are wonderfully alike in origin, 

 position, size, and general appearance in all these cases. Con- 

 sidered in the light of their origin, history, and destiny, it is 

 almost certain that the ttirret cells of the gasteropods are homolo- 

 gous with the trochoblasts of the annelids. 



In all the annelids named these cells divide twice, and then, 

 according to Mead ('94), in Amphitrite, Lepidonotus, and Clyme- 

 nella, "stop dividing forever." In Umbrella, Unio, and Chiton 

 they have been seen to divide once only.^ In Crepidula I have 

 never seen them divide, though I believe the anterior ones do 

 divide at a late stage (Fig. 50). In the other forms their divi- 

 sions have never been seen. 



This is certainly a very remarkable history. Here are four 

 cells which divide at most two or three times, and then probably 

 never divide again, while adjoining cells divide many times and 

 continue this process for a long period. In Nereis while these 

 trochoblasts are producing sixteen cells, the apical cells produce 

 twenty-eight ; in Crepidula, during the time that they are pro- 

 ducing six or at most eight cells, the apical cells give rise to 

 forty-two. 



These cells are smaller when formed, and divide much more 

 slowly in the gasteropods than among the annelids. This, I 

 believe, is due to the fact that the velum is established rela- 

 tively much later among the gasteropods than is the prototroch 

 among the annelids. 



1 In Ishnochiton Heath has observed that they divide several times, before 

 entering into the formation of the velum. 



