RAYS. 



109 



shock voluntarily, when, it is excited to do so in self-defence or [Case 4: 

 intends to stun or kill its prey ; but to receive the shock the 

 object must complete the galvanic circuit by communicating with 

 the fish at two distinct points, either directly or through the 

 medium of some conducting body. It is said that a painful 



Fig. 96. 



Electric Ray (Torpedo mannornta). (From the Mediterranean.) 



sensation may be produced by a discharge conveyed through 

 the medium of a stream of water. The electric currents created 

 in these fishes exercise all the other known powers of electricity : 

 they render the needle magnetic, decompose chemical com- 

 pounds, and emit the spark. The dorsal surface of the electric 

 organ is positive, the ventral negative. This family contains 

 several genera, of which Torpedo is the best known. Of the latter 

 six species are known, distributed over the Atlantic and Indian 



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