200 



A HISTORY OF FISHES 



Both the tubes and the shafts of the lateral line are kept filled 

 with mucus, which exudes through the external pores. 



In the Bony Fishes the main tube communicates with the 

 exterior by a series of pores in the skin, or, more generally, by 

 short canals branching off from the main tube, which perforate 

 the overlying scales and end in pores (Figs, ygc-e). In many 

 fishes these branch canals are themselves subdivided into a 

 number of smaller branches, spread out over the surface of the 

 scale, each of which ends in a minute pore: in this way the 

 original single aperture is converted into a number of smaller 

 ones (Fig. 79b). The sense organs of the lateral line are served 

 by fine nerves arising from a special branch of the vagus (tenth) 



Fig. 80. 



Male Rabbit-fish (Chimaera monstrosa), X i ; a. Front view ; b. Upper part of 

 head. Greatly enlarged. 



cranial nerve, running parallel to the hne itself, and conveying 

 the sensory impressions to the brain (Fig. ygd). 



In the head region the canal system is continued as a series 

 of branching tubes, which are, as a general rule, more deep- 

 seated than those of the lateral hne of the trunk region. In 

 the Chimaeras (Holocephali), however, the elaborate system of 

 branching and intercommunicating channels retains a very 

 primitive form, with the sense organs situated in open grooves 

 similar to that along the body (Fig. 80). The majority of 

 Sharks and Rays are provided with a number of separate tubes 

 on the head, running obliquely below the skin, each with its 

 own pore above, and ending below in a swelling or ampulla 

 containing a group of sensory cells (Lorenzini's ampullae). In 

 the Bony Fishes the canal system rarely forms a conspicuous 

 feature on the head, the tubes being sunk well below the skin 



