290 



SCOTT. [Vol. I. 



son, it can correspond only to the nasal portion of it, and it will 

 be somewhat remarkable to find the organ of Jacobson so low 

 in the vertebrate series. 



Is the unpaired condition of the olfactory organ in the Cyclos- 

 tomata to be regarded as a primitive or as a secondary charac- 

 ter? I think there can be no hesitation in accepting the latter 

 alternative. If the organ were primarily a single one, it would 

 be impossible to account for the fact that the olfactory nerves 

 are paired from the time of their first appearance. Were the 

 olfactory lobes and nerves unpaired and median, there would be 

 good grounds for considering the single nasal organ as the 

 primitive condition, but not otherwise. The fact that the olfac- 

 tory organ is unpaired at its first appearance and only in later 

 stages exhibits a tendency towards the formation of a paired 

 structure, is no ground for assuming that the unpaired condition 

 is the primitive one, for such abbreviations of development are 

 exceedingly common. Primarily, then, we have every reason 

 to believe that the olfactory organs, like all the other organs of 

 higher sense, were double. The reason for the coalescence of 

 the paired nasal pits into a single median pit is, I think, to be 

 found in the development of the type of suctorial mouth pecu- 

 liar to the Cyclostomata. We have already seen that the posi- 

 tion of the olfactory epithelium and its relations to the mouth 

 are very similar in the early stages to the condition seen in the 

 Selachians, but that the extension and rotation of the upper lip 

 brings the opening of the pit to the dorsal surface of the head. 

 This displacement and removal of the opening from the neigh- 

 borhood of the mouth necessitates the enlargement of the 

 naso-palatal canal to supply some ready means of providing 

 the olfactory organ with a current of water. This was long ago 

 pointed out by Johannes Miiller (24): "Die Cyclostomcn 

 bedienen sich aber entweder gar nicht des Mundes zum Einath- 

 men oder wengistens nicht beim Ansaugen, vielmehr muss dann 

 das Einathmen und Ausathmen durch dieselben Oefifnungen 

 der Kiemen geschehen. Da nun letztere zugleich weiter als bei 

 den iibrigen Fischcn zurlickweichen, bei den Myxinoiden sogar 

 durch einen sehr grossen Raum vom Kopfe getrennt sind, so folgt 

 dass das Athmen der Cyclostomcn nur geringen oder gar keinen 

 Einfluss auf die Erneuerung des Wassers an ihren Geruchsor- 

 ganen haben konne, und daraus folgt die Nothwendigkeit eines 



