80 CLASS PISCES. 



In the lateral line system these sensory patches are modifi- 

 cations of the lining epithelium of a canal, which extends the 

 whole length of the body and on to the head, where it branches 

 in a somewhat complicated manner. The canals lie in the dermis 

 or deeper in the subcutaneous tissue, and their walls contain 

 either stiff connective tissue or cartilage (skates) for the purpose 

 of keeping them permanently open. They communicate at 

 intervals with the exterior by tubules. The trunk section of the 

 canal usually lies at the junction of the dorsal and ventral 

 divisions of the lateral muscles. The sense organs and the 

 tubules seem to be usually metamerically arranged in the trunk, 

 and the sense organs and tubules correspond, but in the head 

 the metameric arrangement is of course out of the question, 

 and the sense organs appear to be more numerous than the 

 tubules. 



That this system has originated from a skin groove is indicated 

 by its development and by the fact that in some Elasmobranchs 

 it has the form of an open groove throughout life. In Chlamy- 

 doselachus it has the form of a groove guarded by overlapping 

 scales. In Chimaera it is also an open groove, though in the 

 head the lips of the groove tend to approximate over the sense 

 organs (Fig. 42.) In Heptartchus it is a groove in the greater 

 part of the trunk, but closes into a canal in front and on the 

 head. The course of the cephalic portion in a typical case is 

 shown in Fig. 41. The lateral 'canal on reaching the head is 

 connected with its fellow of the opposite side by a cross canal 

 the commissural canal which may pass in front of or behind 

 the openings of the otocysts. A short distance in front of this 

 it branches into a canal passing above the eye the supraorbital 

 canal (CSO] and one passing below the eye, the infraorbital 

 canal (CJO). The supraorbital canal extends to the front end 

 of the snout and then passes back to join the infraorbital canaL 

 The infraorbital canal gives off a branch back to the hyoid region, 

 called the hyomandibular canal (Hm), which itself gives off a 

 branch to the mandible. In Chimaera (Fig. 42) the arrangement 

 is very similar. 



In skates the hyomandibular canal is enormously extended backwards 

 in a loop which lies partly on the dorsal and partly on the ventral surface 

 of the pectoral fin, and communicates with the exterior by rather long 

 tubules. In the same animal the lateral line canal near the head gives off 



