52( 



THE POPULAR SCIENCE MONTHLY. 



The rays have the mouth, nostrils, and gill-openings, on the under- 

 side. Like the sharks, they are without gill-covers, and have the gills 

 iixed by both margins. Their teeth differ from those of all ordinary 



FiQ. 2. Sting-Eat {Trygon hastata, Storer). 



fishes, being of a definite form, bounded by planes symmetrically ar- 

 ranged, the whole forming a beautiful mosaic (Fig. 3). 



Remarkable as are the form and general structure of the rays, as 

 indicated above, a still more remarkable structure is exhibited in some 



Fig. 3. Teeth of a Eat. 



of them ; for those known as torpedoes (Fig. 4) are so constructed 

 that they are a powerful galvanic battery. These have the space be- 

 tween the pectoral fins, the head, and the gills, on each side, filled 

 with membranous tubes which are divided by horizontal partitions 

 into small cells filled Avith a sort of mucus and traversed by nerves; 



