77 



means of short tubes which open on the surface of the skin ; the supra-temporal and 

 sub-ocular dermal tubes where they exist, are also connected by short tubes with 

 superficial pores, so that aU the tubes of the system are related to the skin in almost 

 the same way as the tube of the lateral line. The cephalic tubes also in all probability 

 contain sense-organs similar to those of the lateral tube. The tubes of the head are 

 innervated by branches of the cranial nerves. The supra-ocular part of the lateral 

 cephalic tube receives branches from the orbito-nasal nerve, the sub-ocular tube is 

 innervated by branches from tlie maxillary and palato-nasal nerves, the pre -opercular 

 tube is supplied from the seventh or facial nerve. There is little doubt that the 

 lateral and pre-operoular cephalic tubes, now enclosed in solid bone, were, like the rest, 

 originally dermal tubes enclosed by tubular scales. In fact, the bones through which 

 they pass represent dermal scales fused together. The tubes and the bones thus 

 produced have sunk beneath the skin, and the bones have become part of the skull. 



Pig. B. A diagram to illustrate the distribution of the dermal tubes in the head of the cod. Letter.^ 

 as in the diagram of the sole. 



But all these dermal tubes at a still earlier period were part of the external surface of 

 the skin. The sense-organs of the tubes and their epithelium are really derived from 

 the epidermis. In some cases the organs of the lateral line are superficial throughout 

 life, for example, in the gobies and the mullet, and in all cases they are developed in 

 the embryo or young fish in the external epidermis. The tubes are formed both in 

 evolution and development by the deepening and ultimate enclosure of grooves on the 

 surface which include the sense-organs. 



In the cod, which may be taken as exhibiting pretty nearly the original condition 

 of the cephalic tubes of the sole and other flat-fishes, there are no superficial sense- 

 organs on the head, and the tube.s of the two sides are symmetrically arranged. 



