246 Journal of Comparative Neurology and Psychology. 



cirri and velar tentacles. The region in which the olfactory 

 epithelium is formed in vertebrates corresponds closely in posi- 

 tion to Hatschek's pit in Amphioxus. This is based upon the 

 description of v. Wijhe (120) and Legros (83). This pit lies 

 in the roof of the invagination which corresponds to the hypo- 

 physis of vertebrates. It follows that Hatschek's pit of 

 Amphioxus and the olfactory organ of vertebrates lie in the roof 

 of the ancestral mouth and in this position correspond to the 

 prostomial area of invertebrates in which sense organs are espe- 

 cially numerous. The hypothesis is suggested, therefore, that the 

 olfactory organ represents a collection of sense organs of the 

 type found in segmented worms which at first was related to 

 the vertebrate paleostoroa as the special collection of sense or- 

 gans is related to the annelid mouth. These organs probably 

 had an undifferentiate chemical sensibility and served the worm 

 and the primitive vertebrate in seeking food. It is not neces- 

 sary to this hypothesis that Hatschek's pit should prove to be 

 a sense organ in Amphioxus. The reference to it is made only 

 for the sake of indicating as clearly as possible the point at 

 which the sense cells are supposed to have collected. 



The question of the segmental value of the olfactory nerve 

 was discussed in a previous paper with the result of denying it 

 primary segmental value. This was based on the description 

 of its nerve centers and the specific conclusion reached was that 

 the olfactory nerve and its centers cannot serve to define the 

 segments in which they lie because they have no resemblance 

 to the typical nerves. I see no way to minimize that argument 

 but it may be said nevertheless that we have in the first seg- 

 ment of the head in addition to a general cutaneous nerve a 

 nerve whose functions are visceral sensory. And now I may 

 go further and suggest how the very differences between the 

 olfactory apparatus and the other cranial nerves may be taken 

 to indicate the morphological status of the region of the head in 

 which it is. If the comparison of the olfactory epithelium with 

 the special collection of sense cells in the prostomium of annelids 

 is accepted, it implies that the olfactory segment is equivalent 

 to the invertebrate prostomium. So far, then, the writer is in- 



