THE ELASMOBRANCH FISHES 



275 



canal {lime.) passes backward from the infraorbital to the region of the hyoid 

 arch; and a fourth, the mandibular (wic), traverses the lower jaw. Various 

 modifications of these and their accessory parts will be noted later. 



The formation of the lateral and cephalic canals may first be briefly de- 

 scribed. The earliest rudiment of the lateral line appears as a flattened plate 



Fig. 241. Sensory canals and ampullae of Lorenzini, Squalus suclclii. (Olive Swezy, orig.) 

 A. Dorsal view. B. Ventral view. 



CO., commissural or supratemporal canal ; e.d., endolymphatic duct ; hmc, hyomandibular 

 canal; iba., inner buccal ampullae; ioc, infraorbital canal; isa., infraspiraeular (hyoid) 

 ampuhae ; II., lateral line ; vie, mandibular canal ; mpo., mandibular ampullae ; n.ap., nasal 

 aperture; oba., outer buccal ampullae; soa., supraophthalmic ampullae; soc, supraorbital 

 canal ; sp., spiracle. 



of ectoderm continuous with, and like that of, the placode for the ear, but this 

 extends both forward and backward. Forward it gives rise to the long canals 

 of the head region. Backward it becomes the lateral line organ. Either way, 

 in order to become a canal rather than an open groove the sensory cord or 

 plate of ectoderm, usually as a pocket, pushes deep into the underlying corium. 

 It may be mentioned here that at regular intervals throughout its course the 

 sensory cord differentiates into patches of ectoderm, the end organs or neuro- 

 masts characteristic of these canals. 



In the adult Elasmobranch the lateral line canals (figs. 240-242) are simi- 

 lar in distribution. They extend in or under the skin from the tip of the tail 

 to the segment of the ear. The lateral line canal in Chlamydoselachns is an 



