SCYLLIUM 



39 



portion goes forwards over the eye as the supraorbital canal, 

 the other beneath it as the infraorbital canal. In addition there 

 is typically a canal which runs down behind the spiracle 

 (hyomandibular canal) to the lower jaw (mandibular canal) ; 

 but these two are more or less interrupted in the dogfish. 



Close to the lateral-line canals in various places there are 

 little pits leading from pores on the surface down narrow tubes 

 to ampullar at the bottom , where there are sense-organs . These 



Fig. 20. — Scyllium : section through the skin showing the ampullar of 

 Lorenzini (aL), denticles (d), nerve-fibres (w), opening of a lateral- 

 line canal (oe), and a sense-organ (so) in the canal. 



are the pit-organs, or ampullae of Lorenzini. Together with 

 the lateral-line system they constitute the neuromast, or 

 acustico-lateralis organs, to which the ear also belongs. These 

 organs are also present in Petromyzon, but not so definitely 

 arranged. 



Ear. — The ear consists of a pit sunk in from the skin and 

 forming a sac, which communicates with the exterior by a long 

 tube and a fine pore ; the ductus endolymphaticus. The sac 

 is divided into a more dorsal utricle, and a more ventral saccule. 



