LATERAL CANAL SYSTEM OF SELACHIANS 23 



a specimen 300 nmi. long had a hundred and thirty-eight. In 

 the adult the tubules do not lead in a direct path to the surface, 

 but pass postero-ventrally for the distance of 1 mm. or less and 

 then bend laterally, passing through the epidermis and opening 

 to the exterior near the base of a slightly modified scale (fig. 6). 

 In the posterior region of the body the tubules are much shorter 

 and take a more direct course to the surface. 



The sensory canals may be said to occupy 'dermal tunnels' 

 which, on the trunk, lie within the dermis, but on the head most 

 of the canals sink to a somewhat deeper level. 



h. Histology of the sensory epithelnmi. The sensory epithe- 

 lium of the lateral canal is limited to a narrow ridge or column 

 which forms the greater part of the dorsal wall of the canal. As 

 mentioned before, this column of sensory epithelium is prac- 

 tically unbroken throughout the length of the canal except for 

 a great many minute transverse ridges. These ridges are usu- 

 ally a little less than 0.1 mm. apart (in a 2 foot specimen) 

 and mark the spaces between the gToups of hair-cells. The 

 sensory column varies considerably in thickness from the anterior 

 to the posterior end of the canal, and usually presents a marked 

 depression between consecutive ranmli of the lateral nerve. 

 This is not invariably the condition, howe\'er, for in many 

 preparations the sensory epithelium is as thick between the 

 ramuli as it is elsewhere (fig. 5). 



Four different kinds of cells have iDeen distinguished in the 

 sensory column: (1) hair-cells; (2) basal cells; (3) spindle-shaped 

 supporting cells; (4) columnar supporting cells. 



The haii'-cells are arranged in clusters and are considerably 

 larger than the other cells. They are somewhat club-shaped, 

 their proximal ends being greater in diameter than their distal 

 ends. The nuclei are large, round, and centrally located as a 

 rule. In suitable preparations hair-like processes always appear 

 at the distal ends of these cells (figs. 7, 8). Whether there is a 

 single process on each cell, or whether there are several clumped 

 together by reagents so that they appear as one could not be 

 determined, but the evidence is in favor of the ^dew that there 

 is a single 'hair' on each cell (figs. 7, 8, 9, 11). The hair-cells 



