THE MORPHOLOGICAL COMPOSITION OF ANIMALS. 125 



very complex. We find in them no trace of the union of 

 gemmiparously-produced individuals. Neither the molluscous 

 nor the vertebrate animal shows the faintest trace of a seg- 

 mentation affecting the totality of its structure; and we see 

 good grounds for concluding that such segmentation as ex- 

 ceptionally occurs in the one and usually occurs in the other, 

 is superinduced. 



[Note : — A critic calls in question the statement on p. 121 

 respecting the Amphioxus. At the outset, however, he 

 admits that in the Amphioxus "the central nervous system 

 and the notochord are not segmented." In the Annelid, 

 however, the central nervous system is segmented, and there 

 is segmentation of the part which, as a supporting structure, 

 is analogous to the notochord in respect of function — the 

 outer part which represents the exo-skeleton in contrast to 

 the endo-skeleton. He goes on to say that "the gut is not 

 involved [in the segmentation] and exhibits in Amphioxus 

 just as it does in worms differentiations entirely independent 

 of the segmentation of the mesoblast." Part of this state- 

 ment is, I think, not congruous with all the facts. In Proto- 

 drilus, one of the lowest of the Archiannelida, " the intestine 

 is moniliform, there being a constriction between each seg- 

 ment " and the next. (Shipley.) Complete segmentation of 

 the intestine is obviously impossible, since, were the canal 

 divided into portions by septa, no food could pass. But the 

 fact that the gut has these successive expansions and con- 

 strictions, corresponding to the successive segments, and 

 giving to each segment a partially-separate stomach, shows 

 that segmentation has gone as far as consists with the 

 carrying on of the lives of the segments. No such partial 

 segmentation exists in the Amphioxus. Thus, then, three 

 fundamental structures — the directive structure, the sup- 

 porting structure, and the alimentary structure — are respec- 

 tively simple in the lowest vertebrate and segmented, or 



