324 



POST-ANAL GUT. 



vertebrate anus is a formation acquired within the group of the 

 Chordata, and not inherited from some older group. This 

 involves a series of further consequences. The opening of the 

 urinogenital ducts into the cloaca must also be secondary, and it 

 is probable that the segmental tubes were primitively continued 

 along the whole post-anal region of the vertebrate tail, opening 

 into the body cavity which embryology proves to have been 

 originally present there. They are in fact continued in many 

 existing forms for some distance behind the present anus. If 

 the present anus is secondary, there must have been a primitive 

 anus, which was probably situated behind the post-anal vesicle ; 

 and therefore in the region of the neurenteric canal. The neur- 

 enteric canal is, however, the remnant of the blastopore (vide 

 p. 277). It follows, therefore, that tJie vertebrate blastopore is 

 probably almost, if not exactly identical in position with the primi- 

 tive aims. This consideration may assist in explaining the 

 remarkable phenomenon of the existence of the neurenteric 

 canal. The attempt has already been made to shew that the 

 central canal of the nervous system is really a groove converted 

 into a tube and lined by the external epidermis. This tube (as 

 may be concluded from embryological considerations) was pro- 

 bably at first open posteriorly, and no doubt terminated at the 

 primitive anus. On the closure of the primitive anal opening, 

 the terminal portions of the post-anal gut and the neural tube, 

 may conceivably have been so placed that both of them opened 

 into a common cavity, which previously had communication with 

 the exterior by the anus. Such an arrangement would neces- 

 sarily result in the formation of a neurenteric canal. It seems 

 not impossible that a dilated vesicle, often present at the end of 

 the post-anal gut (vide fig. 28*, p. 58), may have been the com- 

 mon cavity into which both neural and alimentary tubes opened 1 . 



1 As pointed out in Vol. II. p. 255, there is a striking similarity between the history 

 of the neurenteric canal in Vertebrates, and the history of the blastopore and ventral 

 groove as described by Kowalevsky in the larva of Chiton. Mr A. Sedgwick has 

 pointed out to me that the ciliated ventral groove in Protoneomenia, which contains 

 the anus, is probably the homologue of the groove found in the larva of Chiton, and 

 not, as usually supposed, simply the foot. Were this groove to be converted into a canal, 

 on the sides of which were placed the nervous cords, there would be formed a precisely 

 similar neurenteric canal to that in Vertebrata, though I do not mean to suggest that 

 there is any homology between the two (vide Hubrecht, Zool. Anzeigcr, 1880, p. 589). 



