396 



THE BIOLOGY OF MARINE ANIMALS 



fins and are made up of plates arranged in vertical columns (Fig. 9.17). 

 Each organ contains 400-500 columns and in each column there are about 

 400 plates {Torpedo torpedo). Their importance to the animal is attested 

 by the fact that electric tissue forms about one-sixth of body volume in 

 Torpedo. In the electric ray (Raja) the organs extend along the sides of the 



Fig. 9.17. Electric Organs of Torpedo torpedo 

 (a) Dissection of upper surface showing two electric organs (O). (b) vertical transverse 

 section through the fish in the plane indicated by the transverse line in (a). (After 

 Fritsch and Dahlgren.) 



tail and consist of a series of prisms in parallel, each prism made up of a 

 large number of plates. Some 13,000 discs or plates occur in each electric 

 organ of Raja pulchra. 



The electroplates are derived in the embryo from electroblasts. These 

 resemble sarcoblasts, the precursors of muscle, but they have lost the 

 ability to contract, and develop greatly expanded end-plate surfaces. 



The electric organs are under nervous control and are richly innervated. 



