70 SYDNEY EVANS JOHNSON 



In both Squalus and Mustelus the sensory epithelium con- 

 stitutes a continuous column of cells along the superior or 

 supero-medial wall of the sensory canals. In the sensory ridge, 

 however, the hair-cells (sense end-organs) are arranged in little 

 groups or clusters which are separated and surrounded by the 

 supporting cells. 



In Squalus the sensory epithehum is less extensive than it is 

 in Mustelus. There are approximately three to five hair-cells' 

 in each group and about eight groups between adjacent tubules 

 and ramuh of the lateral nerve. In Mustelus there are several 

 times as many hau'-cells in each group and approximately twice 

 as many groups. 



In Squalus the sensory canals are somewhat smaller in 

 diameter, they are more nearly round in transverse section, 

 and the sensory epithelium does not occupy as wide a part of 

 the canal wall as it does in Mustelus. The pecuhar cells found 

 at each side of the sensory ridge in Mustelus have not been ob- 

 served in Squalus, nor do the blood capillaries occupy as prom- 

 inent a position in the latter. 



The nerve supply of the sensory canals is practically the same 

 in the two forms considered. The innervation of the lateral 

 canal, which has received more careful attention than that of 

 the other canals, is by a large cranial nerve, the N. lateralis. 

 This is unique among the cranial nerves, extending from the 

 medulla to the caudal extremity of the body. 



The cells of origin of the lateral nerve are located in a greatly 

 elongated central ganglion which is partly enclosed in a common 

 sheath with the vagus roots and gangha, but is, however, in- 

 dependent. From this ganglion the nerve passes posteriorly 

 between the muscles of the lateral body- wall at a considerable 

 depth from the surface. At intervals corresponding to the 

 surface tubules bundles of fibers (ramuli) pass to the base of the 

 sensory column and then branch forward and backward, form- 

 ing a continuous longitudinal fiber zone from which distribution 

 to the sensory epithelium takes place. 



In Mustelus it has been showTi that the peripheral termina- 

 tions of the lateral nerve are in the nature of free ramifications 



