466 TREADWELL. [Vol. XVII. 



found in the other cases mentioned. This difference is slight, 

 but enough to disprove a complete homology. 



In Nereis, according to Wilson, while the cells correspond- 

 ing to the primary trochoblasts of the other genera appear in 

 each quadrant, the upper sinistral one is pushed out of the 

 prototroch ring, and the completed organ is composed of only 

 twelve cells. The origin of the secondary trochoblasts was 

 not determined. 



Mead has thrown doubt on the accuracy of these observations 

 and a reinvestigation of the subject is perhaps to be desired. 

 I can discover no internal evidence of their incorrectness. If 

 true, they strengthen the position here taken and show a lack 

 of complete cell homology between the prototroch of Nereis 

 and the other annelids. 



Conklin also has argued for a very complete homology 

 between the trochoblasts of Crepidula and those of annelids, 

 but a comparison of his description of the velum of Crepidula 

 with the annelid prototroch seems to me not to bear out the 

 precise similarity which he maintains. There are in Crepidula 

 four cells corresponding in origin to the four primary prototroch 

 cells of Annelida. Of these, two — those of the ventral surface 

 (quadrants A and B) — enter into the formation of the velum, 

 but the posterior ones do not, unless the whole head vesicle is a 

 part of the velum, for they lie in front of the functionless pos- 

 terior branch of the velum, which Conklin considers the homo- 

 logue of the velum in other mollusks and the prototroch of 

 annelids, and behind the functional anterior branch which he 

 considers a new formation. The velum is completed in the 

 ventral portion at least by cells which have the same origin as 

 the secondary trochoblasts of Amphitrite, but proportionally 

 more cells from the second quartette come in, and it is not 

 impossible (p. 134) that some of the third quartette cells may 

 also form a part of the prototroch. The organ occupies a cor- 

 responding position in annelids and in mollusks, and I have no 

 doubt that it is homologous in the two cases. The close cor- 

 respondence in the mode of origin is additional evidence for 

 this homology, but the point I wish to emphasize here is that 

 this homology is not a complete homology, nor does it seem to 



