268 J. T. CUNNINGHAM. 



pits originally belonging to the pair of gill-clefts which has been 

 transformed into the hypophysis. The function of the nasal 

 duct in the adult is apparently to draw in water in order 

 that it may reach the olfactory organ and then expel it ; it is 

 probably, to use an undigniiied word, a sniffing organ, neces- 

 sitated by the disconnection of the mouth from the function of 

 respiration. This new function of the hypophysial gill-cleft 

 could easily be derived from its original one. 



Visceral Arches of Elasmobrauchs. 



In order to demonstrate the fallacy of the argument that 

 the external branchial cartilages of Selachians were the repre- 

 sentatives of a primitive " external'^ branchial skeleton retained 

 in the existing Cyclostomata, it would have been sufficient, says 

 the beginning of the fourth study, to describe the development 

 of these two cartilages (two to each arch) in Selachians and 

 compare it with the quite difiPerent history of the branchial 

 skeleton in the Lamprey. But it seemed advisable to give a 

 complete account of the development of the Elasmobranch 

 gill-arch, as previous results were fragmentary. 



It is to be understood that a typical arch such as the first, 

 second, or third branchial, is under consideration, not the hyoid 

 or the posterior, which are either modified or reduced. In a 

 horizontal section of the arch towards its middle the cavity of 

 the arch surrounded by its epithelial cells (head-cavity of 

 course the connection between the pharynx and the nasal pits in Myxine is 

 formed by the hypophysis and not by a nostril properly so named. The hypo- 

 physial invagination in the embryo of Petromyzon comes into very close 

 relation with the pharynx as well as with the infundibulum, and on the hypo- 

 thesis which I have supported in my paper on Kupffer's vesicle, &c., that the 

 infundibulum represents the original mouth, it is easy to understand how a 

 separation between infundibulum and pharynx might occur in either of two 

 ways, by leaving the hypophysis connected only with the infundibuluai as 

 in Petromyzon, or by leaving the communication between hypophysis and 

 pharynx still open as in Myxine. In other Vertebrates again the hypophysial 

 invagination has been absorbed into the stomodseum, and reaches from thence 

 to the infundibulum, but has not retained a connection with the pharynx. 

 These speculations can of course only be tested by examination of the deve- 

 lopment of Myxine, 



