262 THE ELASMOBRANCH FISHES 



opposite side, and then passes outward and forward to the tip of the rostrum, 

 where it^rfds blindly (fig. 227b). A mandibular groove {mg., fig. 227b) runs 

 along the lower jaw parallel with the membranous lower lip. 



In the groove on the body and in the canals on the head are located sense 

 organs, the neuromasts, of the sensory canal system. In Hepfanchus, as in 

 other Elasmobranchs, the sense cells joining these organs are specializations 

 of the cells in the wall of the canal or groove. The sense organs along the 

 lateral line are supplied largely by branches of the vagus nerve. The glosso- 

 pharyngeal and the otic division of the facial supply a few of the anterior 

 neuromasts of the lateral line canal. The canals of the head are innervated 



Fig. 229. Pit organs, Heptanchvs macuJahis. (Madeline Marlowe, orig.) 

 A., anterior; II., lateral line; p.o., pit organs; P., posterior. 



wholly by the facial nerve. The superficial ophthalmic division supplies the 

 supraorbital; the buccal branch, the infraorbital; and branches of the hyoman- 

 dibular nerve supply the hyomandibular canal and the mandibular groove. 



In addition to the sense organs characteristic of the sensory canal system in 

 the head are other integumentary sense organs of a tubular nature. The first 

 of these tubular organs are the ampullae of Lorenzini, which take the same 

 relative position as do the canals of the head, and like them are similarly 

 named. The ampullae, however, are grouped together, five or six such groups 

 in Heptanchus maculatus being shown in figures 227 and 228. These are the 

 supraophthalmic (soa.) ampullae above the supraorbital canal and in front 

 of the eye; outer buccal {oba.'^), posterior outer buccal (aha-), and inner 

 buccal (iha.) groups located respectively above and below the infraorliital 

 canal and a part of the hyomandibular canal. A small bundle of ampullae lies 

 ventral to the spiracle (isa., fig. 228). Each sense organ of an ampulla of 

 Lorenzini sinks into the integument and is put into connection with the out- 

 side ])y means of a longer or shorter canal (or canals) opening by a mucous 

 pore (see p. 280, fig. 246). Connecting with the base of such an organ is a 

 nerve twig, the twigs being furnished by the branches of the facial nerves 

 which also supply the canals of the head region. 



A second kind of sense organ is the pit organ (figs. 227b, mpo., and espe- 

 cially 229) . These pit organs are particularly interesting in Heptanchus macu- 

 latus in that they have a wide distribution and are distinctly segmental in 



