SENSORY CANALS OF L^EMARGUS. 73 



the vertical parts of the Fallopian canals ; and its apparent altered position is due to 

 these canals being continued some distance backwards under the skin, before opening to 

 the exterior. In Heptanchus, the aural portions of the lateral canals do not unite to 

 form a commissure ; while in Acanthias, the commissure, instead of running right across, 

 bends forwards a short distance between the auditory pores^ 



The main part of the lateral canal (L., fig. 1) begins on a level with the spiracle (sp., 

 fig. 1), and runs backwards in a nearly straight line until on a level with the posterior 

 margin of the lower lobe of the caudal fin, when it bends downwards to reach the lower 

 edge of the caudal muscles. It then runs obliquely upwards along the edge of the 

 muscles to terminate on a level with the last seo-ment of the vertebral column. Through- 

 out its whole extent, the lateral canal is tubular, and thus differs from the arrangement in 

 Chlamydoselachus and Heptanchus, in which the lateral eanal is represented throughout 

 the whole, or nearly the whole, length by a groove or furrow. Along its entire length, 

 the lateral canal lies immediately beneath the skin, or partly embedded in its substance. 

 The first part of the lateral canal behind the commissure resembles the supra- and infra- 

 orbital canals ; it has nearly the same form and diameter as the part of the canal in front 

 of the commissure. But about 8 cm. beyond the commissure, the canal is reduced in size 

 (in the greater part of the trunk to, on an average, 2*5 mm., and in the caudal region to 

 about 2 mm. in breadth) ; and throughout it presents a somewhat flattened appearance. 

 The tubules are more slender in the trunk than in the head, and the apertures are slightly 

 smaller, and consequently less evident. The anterior tubules, owing to their running 

 obliquely outwards through the skin, are nearly one centimetre in length ; the remainder 

 are slightly shorter — the length, in all cases, depending chiefly on the angle at which they 

 traverse the skin. As in the cranial canals, the tubules are all quite simple. 



Innervation of the Lateral Canal. — In the embryo Selachian, five sense organs are 

 said to lie above the five branchial clefts ; one above the cleft of the glossopharyngeal 

 nerve, and four above the clefts of the four branchial divisions of the vagus. Hence, in 

 a typical condition, we might expect to find, extending from the auditory region back- 

 wards, a canal innervated by five dorsal (supra-branchial) branches ; one branch from 

 the glossopharyngeal, and four branches from the vagus. If, however, we consider the 

 condition in the adult, we find, e.g., in the case of Amia, that while the glossopharyngeal 

 nerve supplies one of the cranial sense organs, all the other (post-auditory) sense organs 

 are innervated by the nervus lateralis of the vagus complex. In Lsemargus, the 

 glossopharyngeal nerve has the typical branches, and a well-marked ganglion; but I 

 have failed to trace any of its fibres to the sense organs of the lateral canal. The four 

 branchial divisions of the vagus are all well developed ; they have in connection with 

 them an extremely large ganglion ; nevertheless, they take no part, as far as I can dis- 

 cover, in innervating any portion of the lateral canal. 



In all the specimens of Lsemargus examined, the lateralis division of the vagus has: 

 been the only nerve found passing to the lateral canal— the commissural, and pre-com-; 

 missural portions included. .'..„.. : ■ . . :l 



