CRANIAL NERVES AND SENSE ORGANS OP FISHES. 247 



a tube of variable length in connection with this pore — 

 thus giving rise to a dermal tubule. One sense organ, 

 therefore, should be placed between adjacent tubules. I 

 have discussed elsewhere the metamerism of this system 

 of sense organs, and have, I think, shown that the facts 

 are entirely against such a view. The only evidence of 

 metamerism is found in the body canal, where in many, 

 if not in most, forms the sense organs occur metamerically. 

 This, however, I have shown to be a secondary develop- 

 ment. On the head the lateral sense organs are certainly 

 not metameric. With regard to the auditory organ, few can 

 now doubt that it is a differentiated portion of the lateral 

 line system. Concerning its mode of development, the 

 facts are not so conclusive. Ayers ably contends that the 

 semicircular canals represent portions of lateral canals, 

 and have therefore always been present during the 

 phylogeny of the ear. I have, in the work previously 

 alluded to, ventured to join issue with him on this point, 

 and have expressed the belief that the auditory organ was 

 primitively a sac from which the semicircular canals were 

 subsequently differentiated. 



