454 FISHES 



of fiat plates lying one over the other, and separated by parti- 

 tions of fibrous tissue. Thus the organ is divided by the fibrous 

 tissue into a number of fiat compartments, and each compart- 

 ment contains an electric plate. The latter is a muscular fibre, 

 which has undergone a transformation, and consists of a special 

 granular substance containing numerous nuclei, while on one 

 side of it is a network of nerve-fibrils connected with one of the 

 nerve-fibres derived from the nerves supplying the organ. In 

 Torpedo the nervous expansion, which corresponds to the plate 

 forming the nerve-termination on a muscle-fibre, is on the lower 

 or ventral side of the electric plate, in Gymnotus it is on the 

 posterior side. In each case the nervous side of the electric 

 plate is negative to the other side, and the current of electricity 

 outside the fish passes from the positive end of the organ to 

 the negative. In Torpedo and various species of Skate the 

 actual development of the electric plates has been traced out in 

 the embryo, or young stages of the fish, and in the latter the 

 characters of the modified fibres are not entirely lost in the 

 fully developed organ. In the Skate it is seen that the muscle- 

 fibres are flattened to form the plates from end to end, not 

 sideways, and in fact in many cases the flattening is confined 

 to one end of the fibre, that to which the nerve-ending is 

 attached, while the other end is still elongated. In Torpedo 

 the transformation of the fibres begins while they are still in a 

 very embryonic condition, and proceeds so far that the adult 

 structure bears no resemblance to muscle-fibres. There can be 

 no doubt that in Mormyrus and Gymnotus, the electric plates 

 are developed in the same way, though their condition in the 

 embryo or young fish has not been investigated. It is not 

 always possible to decide which particular muscles have been 

 transformed into electric organs, but in Torpedo it is known 

 that the organs are derived from the muscles of the outer parts 

 of the branchial arches, and in accordance with this fact the 

 nerves which supply the organs are branches of the cranial 

 nerves. There are four of these nerves, of which the first is a 

 branch of the fifth cranial nerve, the others, branches of the 

 vagus. These nerves are enormously enlarged, and proceed 

 from a special outgrowth of the brain, called the electric lobe, 

 situated on each side, immediately below the cerebellum. In 

 Gymnotus and Mormynis the electric organs are derived from 



