282 THE PROTOCHORDATA. 



two halves of the mouth coalesce. In view of the pre- 

 vious existence of the gill-slit theory of the mouth, some 

 such theory being a necessary accessory to the Annelid- 

 theory, it is not surprising that this undoubted double 

 origin of the mouth in Teleosteans should be regarded as 

 a striking confirmation of Dohrn's hypothesis. And yet, 

 occurring as it does only in the Teleosteans, whose devel- 

 opment is admittedly in many respects highly modified, 

 the interpretation which Dohrn and his followers have 

 placed upon this observation must always have been open 

 to doubt. The simplest explanation of the double origin 

 of the Teleostean mouth is that, owing to certain condi- 

 tions (possibly mechanical) of development, the two angles 

 of the mouth develop before the median portion. This is 

 the conclusion which H. B. Pollard has also reached in 

 his recent studies on the development of the head in the 

 Teleostean fish, Gobiiis capita. 



According to the standpoint I have adopted in the fore- 

 going pages, there is no a priori reason for doubting that 

 the Vertebrate mouth is completely homologous with the 

 Protochordate mouth ; and that the latter in its turn is 

 the direct descendant of the typical Invertebrate mouth. 



Again, the anatomy and development of the Protochor- 

 dates and of the Cyclostomi (Ammocoetes) show no indica- 

 tion whatever of a discontinuity in the evolution of the 

 most highly elaborated mouth of the gnathostomous or 

 jawed Vertebrates. 



We conclude, therefore, that the ventral mouth of the 

 craniate Vertebrates is the homologue of the primordial 

 dorsal mouth as we find it in the Protochordates, and that 

 its direction of evolution has been, as was so ably main- 

 tained by Balfour, from the cyclostomous to the gnatho- 

 stomous condition. 



