LATERAL CANAL SYSTEM OF SELACHIANS 19 



terns of branched tubules (dendritic systems) found in some of 

 the more recent fishes. The former assumption seems improb- 

 able in view of the fact that the anlage of the canal is a single 

 cord of thickened extoderm. 



The supraorbital canal is innervated by the N. ophthalmicus 

 superficialis VII. This division of the VII nerve, as well as 

 the other nerves which innervate sensory canals, has an in- 

 dividual ganglion near its root and, like these other nerves, it 

 terminates in the same internal nucleus in the tuberculum acus- 

 ticum (Herrick), or better the area acustico-lateralis. 



The infraorbital canal. For descriptive purposes, and on 

 account of its mode of development in Squalus (fig. 53), we 

 may consider the infraorl^ital as beginning where it unites with 

 the supraorbital. Allis says that in embryos of Mustelus the 

 nerve distribution of buccalis and otic fibers goes back to the 

 supratemporal commissure and on that basis the infraorbital 

 would extend to the supratemporal commissure. Commencing 

 ^\ith its point of union with the lateral and vsupraorbital canals 

 the infraorbital passes ventralwards between the eye and the 

 spiracle, crosses below the eye, and passes forward as far as the 

 anterior margin of the orbit. Here it bends ventrad and slightly 

 caudad, anastomosing with the inferior limb of the supraorbital 

 immediately posterior to the nostril. Slightly anterior to this 

 anastomosis a prebuccal limb of the canal passes antero-medially 

 to the median line on the ventral surface of the head, where it 

 fuses with its fellow of the opposite side. The median canal 

 represented by this miion passes forward a short distance and 

 forks into two canals which extend forward and join the supra- 

 orbital near the tip of the snout, as mentioned above. 



Between the eye and the spiracle the infraorbital canal lies 

 deeper than any of the other canals, having separated completely 

 from the inner surface of the dermis. 



The tubules of the infraorbital canal were not all counted, but 

 in number there are considerably over one hundred. On the 

 lateral region of the head the tubules are simple and lead directly 

 to the surface. On the ventral surface of the head, however, 

 most of the tubules are branched or double and open on both 



