94 



THE AMERICAN NATURALIST 



[Vol XL 



Let us assume, with Haeckel, that the ProspondyUa are the stock 

 from which the Leptocardia and the Archicrania both arose. 



From the latter hypothetical group are developed all the Craniate 

 forms which, down to the present time, have been classified in two 

 main divisions: the Cyclostomata and the Gnathostomata. 



In 1894, I showed that the so called tongue apparatus of the 

 Cyclostome fishes, particularly of the Myxinoids, was developed 

 by a transformation of the jaw apparatus from the maxillo-man- 

 dibular apparatus of some Gnathostome ancestor, and these views, 

 together with the anatomical evidence supporting them, were 

 printed in the Journal of Morphology, vol. 17, and in the Bulletins 

 of the University of Cincinnati, vol. 1, nos. 1 and 2. 



The development of the mouth of Bdellostoma and the pre-oral 

 and postoral bars (the maxillary and mandibular arches) respec- 

 tively in the early stages of Bdellostoma, before the formation of 

 the tongue apparatus, adds further corroboration of the accuracy 

 of the interpretation of the homologies of the cranio-facial appa- 

 ratus of the Marsipobranchi. 



With the discovery of the jaw apparatus in the Cyclostomes, 

 the most essential character used by systematists for the separa- 

 tion of this group from the Gnathostomes disappears. 



Many other characters, however, of which perhaps the absence 

 of paired appendages is the most noteworthy and important, re- 

 main as a sufficient ground for a very distinct separation of these 

 forms from the rest of the vertebrates. 



But the group Gnathostomata must now include the Marsipo- 

 branchi as well as all the forms hitherto included, so that, as our 

 classification now stands, all the Craniata are Gnathostomes, and, 

 as before, the only living Acraniate is amphioxus. 



The solution of the problem of the origin of the cranio-facial 

 apparatus is thus pushed back upon the extinct vertebrate forms 

 which fill in the gap between the common ancestor of amphioxus 

 and Craniata. Possibly paleontology may bring us the needed 

 information, or it may be that the embryology of some form yet 

 unstudied will disclose the method of the transformation of the 

 acraniate or agnathous into the craniate or gnathostome head. 



