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THE AMERICAN NATURALIST [Vol. XL 



organ rudiment entirely absent, so far as we yet know, from the 

 tunicates. So that the absence of an ear from amphioxus is fully 

 accounted for, at least in so far as relation to the tunicates is con- 

 cerned. The tunicate ear is, in a strict sense, an otocyst, and 

 not an ear. 



With reference to the segmentation of the body of amphioxus, 

 all the evidence seems to point to the ancestral character of this 

 segmentation in relation to mesodermic segmentation of the 

 higher vertebrate forms, with the exception of one peculiarity, 

 which is probably palingenetic in its nature, but which, so far as 

 we know, does not occur unmodified in any other vertebrate. I 

 refer to the origin of the mesodermic segments from two bilaterally 

 placed hollow pouches pushed out from the mesenteron. 



From the many indications which have been discovered by 

 numerous investigators, the mesoderm in the higher forms follows 

 this plan of origin, but the architecture of the transformation is 

 coenogenetically very much shortened and changed, as in the 

 case of many other organs of the body. 



There is no occasion to dismiss all the pertinent indications 

 preserved in the higher forms which indicate that this method 

 of origin was the primitive one, simply because complete and 

 well formed diverticula are absent from the ontogeny of the 

 mesoderm in all vertebrates above amphioxus that have yet been 

 investigated. 



Regarding the suggested affinity between amphioxus and the 

 annelids in this matter of the segmentation of the mesoderm, 

 the unprejudiced mind will not hesitate to make the conclusion 

 that it is far less intimate than the relationship already described. 



The difficulties surrounding the establishment of the homology 

 of the reproductive organs of amphioxus with those of the higher 

 vertebrates are certainly not solved by any reference of the verte- 

 brate stock to the annelids as ancestors, for the difficulty com- 

 plained of by :\Iiiu)t that the reproductive organs appear seg- 

 mentally in ;un})lii«)xiis, but non-segmentally in other vertebrates, 

 is only iiin-c I by can yi tig the ancestral vertebrates back to 

 the annelids, for here the segmental arrangement of the gonads 

 is even more primitive and is accompanied by many annelidan 

 characters of the other organs of the body, which carry us farther 



