AMPHIOXUS AND AMMOCCETES. 163 



AMPHIOXUS AND AMMOCCETES. 



We will now pass on to consider what new light the 

 larval development of Amphioxus throws on its relation- 

 ship to the craniate Vertebrates. 



As a type of the latter with which to make the com- 

 parison, we will select Ammoccetes, the larva of the lamprey, 

 Petromyzon, which is the nearest relative of Amphioxus 

 among the Craniota. 



Nervns BrancJiialis Vagi. 



Although Ammoccetes possesses an organisation which, 

 especially in virtue of its nervous system and sense- 

 organs, entitles it to an undoubted place among the 

 craniate Vertebrates, yet, on the whole, its structural ele- 

 ments remain in such a relatively simple condition of 

 elaboration that it readily adapts itself to a comparison 

 with Amphioxus. 



At the same time the system of ganglia and peripheral 

 cranial nerves indicated in Fig. 91 will show what a great 

 gap there is between the two forms. Nevertheless, a 

 nerve corresponding to that which lies over the gill-slits 

 in Fig. 91, the ncrvus brancJiialis vagi, has recently been 

 discovered in Amphioxus by VAN WIJHE, so that there 

 need be no difficulty in comparing the pharyngeal tract 

 of Ammoccetes with that of Amphioxus. 



It may be added here that the nerve-supply of the 

 pharynx of Amphioxus was described as a branchial plexus 

 by ROHON and FUSARI, but the origin of the nerves which 

 gave rise to the plexus was not satisfactorily determined, 

 beyond the fact that they arose from the rami viscerales 

 of the dorsal spinal nerves. VAN WIJHE also was not 



