78 



The diagram, Fig. B, shows the position and the relations of the cephalic tubes In 

 that species. The supra-temporal tubes are short, and are surrounded by thin tubular 

 scale-like bones. The sub-ocular tube is also enclosed in a series of tubular bony 

 scales, the sub-orbital bones. Both these and the supra-temporal bones, though 

 homologous with the ordinary dermal scales, are not similar to them, having a much 

 larger size and deeper position. 



In the sole the changes which have taken place in the course of evolution are as 

 follows : The supra-temporal tubes have become much elongated, having grown 

 forwards with the anterior extension of the dorsal fin till they reach the extremity of 

 the snout. On the right side the tube is no longer enclosed by peculiar scale-like 

 bones, but by tubular dermal scales, closely resembling the ordinary scales of the skin. 

 On the left side of the head, the scales having disappeared anteriorly, the supra- 

 temporal tube has opened out on to the surface, and the sense-organs which belong to 

 it have become superficial. The two supra-temporal tubes are symmetrical in relation 

 to the two sides of the dorsal fin, but entirely asymmetrical in relation to other parts 

 of the head ; for in consequence of their relation to the fin they both lie morpho- 

 logically ventral to the left eye, between the left eye and the mouth. 



The lateral cephalic tube of the left side has been distorted by the change of 

 position of the left orbit. Its posterior part retains its original position, but its 

 anterior or supra-ocular part lying in the inter-orbital process of the left frontal bone, 

 bends round towards the right side and runs close beside the right supra-ocular tube 

 between the two eyes. 



The sub-ocular tube of the right side has disappeared ; that of the left side is 

 represented by the numerous superficial sense-organs which lie above the mouth on 

 the lower side of the head. 



The two pre-opercular tubes remain in their original position, but on the left side a 

 number of superficial sense-organs have been developed over the region of the 

 pre-opercular tube. This is, in some respects, one of the most remarkable of the 

 peculiar developments in the head of the sole. It is intelligible that a dermal tube 

 originally derived from a superficial groove should again become superficial ; but it is 

 surprising to find surperficial organs developing anew, outside a tube which contains 

 similar organs originally derived from the surface. 



The supra-temporal tubes are innervated by a long branch from the lateral nerve 

 belonging to the vagus. On the left side a similar branch of the vagus passes 

 forwards towards the sub-ocular sense-organs. Along the proximal part of the course 

 of this nerve is a single series of superficial sense-organs, which lies over the proximal 

 part of the lateral cephalic tubes. I have not been able to decide whether this series 

 really belongs to the sub-ocular series of the left side, or bears the same relation to the 

 proximal part of the lateral cephalic tube as the pre-opercular superficial organs to 

 the pre-opercular tube. In studying the relations of the mucous tubes and epidermic 

 sense-organs in the head of the sole, I have been very greatly assisted by a paper by 



