XIII 



PHYLUM CHORDATA 



177 



tail. In all cases the cells are formed from metamorphosed 

 muscular fibres. 



Luminous organs by the agency of which a phosphorescent 

 light is produced occur on the surface of a few Elasmobranchs. 



Digestive System. Teeth are developed on the palato- 

 quadrate 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 



FIG. 837. A Torpedo - Ray with the electric organs dissected 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. branchiae ; 

 ./', spiracle ; o, eyes ; tr. trigeminal ; tr', its electric branch ; v. vagus ; /, fore-brain ; //, mid- 

 brain ; ///, cerebellum ; IV, electric lobe. (From Gegenbaur.) 



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

 Sharks the teeth are usually large, and may be long, narrow, and 

 pointed, or triangular with serrated 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. 

 VOL. IT M 



