ON THE ANCESTRAL FORM OF THE CHORDATA. 323 



remarkable structures in the trunk is the post-anal gut (fig. 197). 

 Its structure is fully dealt with in the chapter on the alimentary 

 tract, but attention may here be called to the light which it appears 

 to throw on the characters of the ancestor of the Chordata. 



In face of the facts which are known with reference to the 

 post-anal section of the alimentary tract, it can hardly be 

 doubted that this portion of the alimentary tract must have 

 been at one time functional. This seems to me to be shewn (i) 

 by the constancy and persistence of this obviously now function- 

 less rudiment, (2) by its greater development in the lower than 

 in the higher forms, (3) by its relation to the formation of the 

 notochord and subnotochordal rod. 



If the above position be admitted, it is not permissible to 

 shirk the conclusions which seem necessarily to follow, however 

 great the difficulties may be which are involved in their accept- 



FIG. 197. LONGITUDINAL SECTION THROUGH AN ADVANCED EMBRYO OF BOM- 



BINATOR. (After Gotte.) 



m. mouth; an. anus; /. liver; ne. neurenteric canal; me. medullary canal; 

 ch. notochord; pn. pineal gland. 



ance. These conclusions have in part already been dealt with 

 by Dohrn in his suggestive tract (No. 250). In the first place 

 the alimentary canal must primitively have been continued to 

 the end of the tail ; and if so, it is hardly credible that the 

 existing anus can have been the original one. Although, there- 

 fore, it is far from easy, on the physiological principles involved 

 in the Darwinian theory, to understand the formation of a new 

 anus 1 ; it is nevertheless necessary to believe that the present 



1 Dohrn (No. 250, p. 25) gives an explanation of the origin of the new anus which 

 does not appear to me quite satisfactory. 



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