ON THE ANCESTRAL FORM OF THE CHORDATA. 



319 



While the embryological data appear to me to point to the existence of a 

 primitive suctorial mouth, very different conclusions have been put forward 

 by other embryologists, more especially by Dohrn, which are sufficiently 

 striking and suggestive to merit a further discussion. 



As mentioned above, both Dohrn and Semper hold that the Vertebrata 

 are descended from Chastopod-like forms, in which the ventral surface has 

 become the dorsal. In consequence of this view Dohrn has arrived at the 

 following conclusions : (i) that primitively the alimentary canal perforated 

 the nervous system in the region of the original cesophageal nerve-ring ; (2) 

 that there was therefore an original dorsal mouth (the present ventral mouth 

 of the Chaetopoda) ; and (3) that the present mouth was secondary and 

 derived from two visceral clefts which have ventratfy coalesced. 



A full discussion of these views 1 is not within the scope of this work ; 

 but, while recognizing that there is much to be said in favour of the inter- 

 change of the dorsal and ventral surfaces, I am still inclined to hold that the 

 difficulties involved in this view are so great that it must, provisionally at 

 least, be rejected ; and that there are therefore no reasons against supposing 



~-sd 



op 



FIG. 195. VENTRAL VIEW OF THE HEAD OF A LEPIDOSTEUS EMBRYO SHORTLY 



BEFORE HATCHING, TO SHEW THE LARGE SUCTORIAL DISC. 



m. mouth; op. eye; sd. suctorial disc. 



the present vertebrate mouth to be the primitive mouth. There is no 

 embryological evidence in favour of the view adopted by Dohrn that the 

 present mouth was formed by the coalescence of two clefts. 



If it is once admitted that the present mouth is the primitive mouth, and 

 is more or less nearly in its original situation, very strong evidence will be 

 required to shew that any structures originally situated in front of it are the 

 remnants of visceral clefts ; and if it should be proved that such remnants 

 of visceral clefts were present, the views so far arrived at in this section 

 would, I think, have to be to a large extent reconsidered. 



The nasal pits have been supposed by Dohrn to be remnants of visceral 



1 For a partial discussion of this subject I would refer the reader to my Monograph 

 on Elasmobranch Fishes, pp. 165 172. 



