204 TORREY 



by Wilson ('92), Mead ('97), Treadwell ('01) and others. I 

 think it possible, however, that in the last analysis it will be 

 found that, in all cases where there is a trochophore stage, the 

 cells which will later form the growing point have shifted down- 

 ward from a position immediately below the prototroch on the 

 dorsal side toward the anal region and that the mechanical cause 

 of this shifting is the migration of cells from the prae-trochal 

 region through the dorsal gap and into the post-trochal region. 

 In the majority of forms this is not as clearly the case as it is 

 in Thalassona, since the process may become modified by the 

 early formation of the anus or anal region. Although Wilson 

 did not describe any migration of cells from the prae-trochal 

 region, it seems quite probable that the dorsal triangular space, 

 covered with small transparent cells, originated in this way and 

 that the growth of this area forces the teloblasts toward the 

 lower pole. In Auiphitrite it is possible that Mead has not 

 given the migrating cells credit for forming as much of the 

 dorsal region as they deserve. In this case, too, the shifting of 

 areas is not readily recognized, because of the absence of telo- 

 blasts. This possibility also applies to Arcnicola. In Podarke, 

 with the exception of the concrescence of the ventral plate the 

 shifting is exactly similar to that in TJialassevia and the grow- 

 ing point arises from the same cell region. 



These processes in TJialasscina are very closely paralleled by 

 those commonly occurring in molluscs. In Chiton (Metcalf, 

 '93) and Ischnochiton (Heath, '99) there is the same shifting of 

 the blastopore to the ventral side to form the stomodaeum, and 

 the anus also arises very late. In Teredo (Hatschek, '81) the 

 blastopore closes on the ventral side in the region of the future 

 mouth. In Ostrea (Brooks, '80), DentaluLiii (Kowalevsky, '83) 

 and Patella (Patten, '^6) the blastopore shifts toward the ventral 

 side and stands in the closest relation to the stomodaeum, but 

 not to the proctodaeum. 



4. Formation of the Enter on. 

 After sinking in, the entomeres become rounded and divide 

 rapidly in such a way that a very thick epithelium is formed 

 surrounding a small lenticular shaped cavity (Text-Fig. 6, A). 



