LATERAL CANAL SYSTEM OF SELACHIANS 21 



posteriorly. Innervation is from the supratemporal ramus of 

 the lateral nerve. 



Sm-face organs. In addition to the organs which are enclosed 

 m canals, three lines of pit-organs deserve mention: the dorsal 

 and mandibular series of the adult, and a short line of tempo- 

 rary pit-organs which are found in the pup stages on each side of 

 the scar left by the yolk-stalk. 



The dorsal series consists of approximately 80 pit-organs 

 running dorsal to the pores of the lateral line in a somewhat 

 kregailar com'se. In iNIustelus (fig. 1) this set or organs extends 

 from a point a little posterior to the eye throughout the length 

 of the body. In Squalus the corresponding set of pit-organs 

 extends only to the region of the first dorsal fin. The nerve 

 supply of these organs comes from a slender nerve which arises 

 from the lateraUs ganghon (figs. 45 and 50) and runs nearly paral- 

 lel to the lateralis nerve. 



The mandibular series of the adult contains approximately 

 45 sense organs. They begin at the posterior margin of the 

 spiracle and then curve backward around the end of the hyo- 

 mandibular canal and forward reaching nearly to the median 

 plane (fig. 3). The innervation of this line of organs has not 

 been determined. 



The two short lines of surface organs situated on opposite 

 sides of the yolk-stalk appear to be very poorly developed (fig. 

 3). Each line contains 9 or 10 organs. These organs have not 

 been observed in the adult and their innervation is unknown. 



2. Structure of the sensory canals 



a. General anatoiny. The dehcate tubes of the sensory canals 

 in Mustelus have walls consisting of two layers of epithehal 

 cells, except on the dorsal side, where the canal wall is greatly 

 thickened into a ridge of sensory epithelium (figs. 4, 6, 7). 

 The diameter of the canals varies in different parts of the body 

 as well as in specimens of different size. The average diameter 

 of the head canals is about 0.3 mm. in a specimen two feet long, 

 while in the same specimen the trunk canal varies from about 

 0.2 mm. at its anterior end to 0.08 nnn. or less posterior to the 



