The Development of Ischnochiton. 649 



chiton, as far at least as the stage represented in diagram E, b. And 

 it is a noteworthy fact that the concrescence of the blastopore, 

 lacking in Ischnochiton, is very intimately related to the concrescence 

 of the first somatoblast — an obvious secondary modification — and 

 it remains to be demonstrated that the Gastrula-raphe is not directly 

 dependent upon the rapid growth of the first somatoblast and will be 

 lacking when this latter feature is wanting. 



Also upon this hypothesis it is possible to explain the non-seg- 

 mented character of the ventral nerve cord in the Archiannelida and 

 the slowness with which it becomes segmented in some other cases. 

 It is taken for granted that since the trochophores of the Annelid 

 and Chiton are nearly related organisms their respective nervous 

 systems are homologous. In the Chiton the pedal cords are relatively 

 far apart and they may arise in part or entirely from the third 

 quartette in the foot, the position of which is probably represented 

 in diagram E, f. And originally they may have formed from the 

 third quartette on the ventral surface in the Annelid but in the 

 enormous development of 2d these cells have become so shifted that 

 they no longer form a conspicuous part of the ventral area. And if 

 the nerve cords ever did arise from them they have become shifted 

 from this position and now arise from the median non-segmented 

 portion of the first somatoblast. 



Assuming that the trunk of the Archiannelida is formed in the 

 same way as in Amphitrite it may be claimed that the influence of 

 the neighboring segmented portion of the somatic plate never extends 

 to the non-segmented part from which the ventral cord arises. In 

 other forms the evidences of metamerism appear but slowly and where 

 it early appears it may be looked upon as a modification of the 

 primitive condition. 



The more detailed studies in Annelid development serve to show 

 that during ontogeny the mesoblast is not necessarily the first layer 

 to exhibit signs of metamerism. In Nereis dumerilU according to 

 VON WiSTiNGHAUSEN ('91) metamerism arises simultaneously in the 

 ecto- and mesoderm. Wilson ('92) in speaking of Nereis limbata 

 says: "As elongation begins, the seta-sacs make their appearance 

 and with them is given the first indication of metamerism." It seems 

 from Hatschek's figures of the Eehiurus trochophore that segmentation 

 first appears in the etoderm. Mead states for Amphitrite: "the first 

 indication of metamerism is a groove which appears a little behind 



42* 



