172 



ZOOLOGY 



SECT. 



at. 



light is produced occur on the surface of a few pelagic Elasmo- 

 branchs. 



Digestive System. Teeth are developed on the palatoquadrate 

 and on Meckel's cartilage. They are arranged in several parallel 

 rows, and are developed from a groove within the margin of the 

 jaw, successive rows coming to the front, and, as they are worn 

 out, falling off and being replaced by others. In the Sharks the 



teeth are usually large, 

 and may be long, nar- 

 row, and pointed, or 

 triangular with ser- 

 rated edges, or made 

 up of several sharp 

 cusps ; in the Rays, 

 however, the teeth are 

 more or less obtuse, 

 sometimes, as in the 

 Eagle-Rays, forming a 

 continuous pavement 

 of smooth plates 

 covered with enamel, 

 adapted to crushing 

 food consisting of such 

 objects as Shell-fish 

 and the like. The 

 Sharks have a promi- 

 nent tongue supported 

 by the median basi- 

 hyal ; this is entirely 

 or almost entirely ab- 

 sent in the Rays. The 

 various divisions of the 

 enteric canal are simi- 

 lar in all the members 

 of the class to what 

 has already been de- 

 scribed in the case of 

 the Dog-fish. A spiral 

 valve is always pre- 

 sent in the large intes- 

 tine, though its arrangement varies considerably in the different 

 families. In some cases (e.g. Carcharias), the fold is not a spiral 

 one, but, attached by one edge in a nearly longitudinal line to the 

 intestinal wall, is rolled up in the shape of a scroll. A pair of 

 pyloric caeca occur in Lcemargus. Appended dorsally to the 

 rectum is a median glandular caecum, the rectal gland. The rectum 

 always terminates in a cloaca, into which the urinary and genital 



FIG. 848. A Torpedo-Ray with the electric: organs dis- 

 sected out. On the right the surface only of the electric 

 organ (O.E.) is shown, on the left the nerves passing 

 to the organ are shown. The roof of the skull is removed 

 to bring the brain into view. br. gills ; /. spiracle ; 

 o. eyes ; tr. trigeminal ; tr'. its electric branch ; v. vagus ; 

 /, fore-brain ; //, mid brain ; ///, cerebellum ; IV, 

 electric lobe. (FromGegenbaur.) 



