THE SENSORY CANAL SYSTEM OF FISHES. 525 
known which show the gradual specialisation of a dermal 
canal to one enclosed in a bony tube and then in bony plates. 
The system of pores, sensory organs, &c.,is one showing the 
modifications and evolution of a sense-organ. 
The form, number, and branching of the cranial nerves has 
been very largely modified by the presence of an innumerable 
series of sensory organs (cluster and primitive pores, &c.) dis- 
tributed over the cephalic region. 
The Ganoids as a group have very rightly been divided by 
Bridge (16) into Selachoidei and Teleosteoidei. For the reten- 
tion of the group there seems to be sufficient morphological 
evidence. Nota few zoologists would join the Ganoids with the 
Teleosts. Possibly there are forms included in the Ganoids 
whose affinities are undoubtedly with the Teleosts, but the 
division termed by Bridge Selachoidei is much more closely 
allied to the Elasmobranchs. Indeed the position, origin, 
number, and course of the cranial nerves in Polyodon suggests 
a much closer relationship to this latter group than even Bridge 
supposed, who was the first, I believe, to institute a comparison 
between them. 
The conclusions of Ewart (26) are very largely confirmed 
and augmented by this investigation. 
From the variability of the sensory canal system in the 
Ganoids but little importance can be attached to it for pur- 
poses of classification, other than of the most generalised 
nature. 
xii. COMPARISON OF PoLYODON WITH ACIPENSER. 
As we should naturally expect, there is a great difference in 
the sensory canal system of the two forms. Polyodon, on the 
one hand, may be regarded as a form on the very border-line 
between the Elasmobranchii and the Selacheoid group of 
Ganoids, whilst Acipenser, on the other hand, may be 
placed on the border-line between the latter group and the 
Teleosteoid Ganoids. 
Although widely separated from one another by very many 
important differences, the two species show at the same time a 
