28O THE PROTOCHORDATA. 



remain behind. This is, as we should expect, the exact 

 reverse to what obtains in Amphioxus. 



In connexion with the evolution of the praeoral lobe, 

 we thus have an excellent example of repeated change of 

 function. 



We may conclude, therefore, that the prasoral lobe, 

 which, in the Invertebrates, is above all the bearer of the 

 cerebral ganglion, and in the Protochor dates is released 

 from this function and becomes in part a locomotor 

 (Balanoglossus, Cephalodiscus) fixing (Ascidian) and bur- 

 rowing (Amphioxus) organ, is represented in the craniate 

 Vertebrates by the prcemandibular head-cavities, whose 

 walls give rise to most of the eye-muscles. 



THE MOUTH OF THE CRANIATE VERTEBRATES. 



In consequence of the increase in the size of the brain, 

 its forward extension and its cranial flexure, together with 

 the relative reduction of the head-cavities, it is obvious 

 that the mouth has been carried round from its primitively 

 dorsal position to its final position on the ventral side of 

 the head in the craniate Vertebrates. (Cf. Fig. 91.) This 

 would have been all that need be said about the mouth 

 were it not for the fact that the view, originally started by 

 DOHRN, that the Vertebrate mouth was a new formation 

 resulting from the fusion of two gill-slits, has received such 

 wide support and still in a measure holds its own. 



Since the Annelid mouth perforates the central nervous 

 system in passing through the circumcesophageal nerve- 

 collar, it was necessary to frame a theory which would 

 get over the difficulty that nothing of the kind occurs in 

 the Vertebrates. Accordingly Dohrn supposed that the 

 old Annelid mouth had become aborted, and was replaced 



