366 FISHES CHAP, xill 



organs are much larger, and extend the whole length of the tail, 

 which is fully four-fifths of the total length of the Fish. The 

 Mormyridae also have their feeble electric organs in the caudal 

 region. In all these Fishes the electric organs are modified 

 portions of the caudal muscles. In the Torpedo, however, these 

 organs are two large oval masses, one on each side of the head, 

 between the gills and the cephalic prolongation of the pectoral 

 fin (Fig. 209). Malopterurus ^ is exceptional in possessing an 

 electric organ derived from the epidermis and not from the 

 muscular system. In this Fish the organ envelops nearly the 

 whole body like a mantle, between the skin and the subjacent 

 muscles of the trunk and tail. An electric organ is composed 

 of an immense number of " electric plates " (modified motor end- 

 plates), abundantly supplied with nerves on one of their sm'faces, 

 and disposed in a series of vertical (Torpedo) or longitudinal 

 {Gymnotus) columns, separated by septa of connective tissue. 

 In the active state of the organ in the Torpedo ^ the ventral 

 surfaces of the plates, on which the nerves are distributed, become 

 negative to the dorsal, and " the effect in all the plates of a 

 column when summed up is, therefore, such that the dorsal end 

 of a column becomes positive to the ventral end." ^ Hence the 

 current in the form of a succession of shocks passes from the 

 ventral to the dorsal surface of the head. In Gymnotics, where 

 the columns are longitudinally arranged, it is the anterior and 

 posterior surfaces which become oppositely electrified, and the 

 current passes from the tail to the head. The shock imparted 

 by an electric discharge is most powerful in Gymnotus^ Malo- 

 pterurus, and Torpedo, in the order named, and relatively weak in 

 the remaining genera. The strength of the shock increases with 

 the number of electric plates included in the circuit. Thus in 

 Gymnotus the maximum shock is given when the body of the 

 Fish is so curved that the head and the tail are in contact with 

 different points on the surface of some other Fish. The discharge 

 may be reflex or voluntary. Eepeated discharges induce fatigue 

 and weaken the shocks. Electric organs are powerful offensive 

 or defensive structures, enabling the Fish to repel the attacks of 

 enemies, or to stun or kill their prey. 



' Ballowitz, Das EUclnschu Organ des Africanischen Zitterwelses, Jena, 1899. 

 ''■ Gotoh, Phil. Trans. 178, 1888, p. 487. ' Id. op. cit. p. 535. " Of. p. 580. 



