366 FISHES CHAP. XIII 
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). Mualopterurus’ 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 surfaces, 
and disposed in a series of vertical (Zorpedo) 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 Gymnotus, 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. Repeated 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. 
1 Ballowitz, Das Electrische Organ des Africanischen Zitterwelses, Jena, 1899. 
2 Gotch, Phil. Trans. 178, 1888, p. 487.  ° Id. op. cit. p. 585. 4 Cf p. 580. 
