No. 2.] VERTEBRATE CEPHALOGENESIS. 239 



the fact that, among the lower vertebrate forms, canal organs are 

 placed in much more favorable position (on the surface of the 

 body) for the reception of stimuli than are the deeply buried ear 

 canals ; besides which, the former lie further from the centre of 

 motion, and would consequently execute greater excursions than 

 the corresponding ear canals during any given motion of the 



body. 



These surface canals occupy positions in the three planes of 

 space just as the ear canals do. 



e. Since from an extended consideration of the experimental 

 evidence, derived from investigations of the function of brain 

 and ear, with special reference to the function of equilibration, 

 we find that the semicircular canals, and their organs, are not 

 more directly connected with the centre of equilibration than 

 many other sense organs, and other parts of the body not sen- 

 sory in function, and since we know, from their history of devel- 

 opment, that they were purely protective structures originally, 

 and have only had the semicircular form and spatial relations 

 impressed upon them, as it seems, by the purely mechanical cir- 

 cumstances of involution, independently of any special function 

 more than they possessed on the surface of the body, we must 

 conclude that the semicircular canals have never, and do not 

 now, possess any peculiar or special relation to the function of 

 equilibration. 



f The system of canal organs is a very ancient, and, so far as 

 the existing vertebrates are concerned, a very primitive, system. 

 Amphioxus apparently has no trace of it, but all other verte- 

 brates show, by the possession of an internal ear with semicir- 

 cular canals, etc., that their ancestors must have possessed a 

 well-developed system of canal organs in the head region. 



From this primitive condition the existing variety of forms 

 of canal organs and other sensory structures of this class have 

 been derived. 



L. The higher sense organs of the Cyclostomata are all paired, 

 since the nose (i.e. the nasal or olfactive epitheliiim) exists in the 

 embryo as well as the adult in the form of two circumscribed areas 

 lying on either side of the median line, each of which receives the 

 entire nerve supply afforded by the olfactory nerve of its side. 



It is evidently not consistent with our morphological ideas, to 



