ELECTRIC ORGANS.—Electric organs capable of generating more or less powerful electric discharges are present in certain Fishes, both marine and freshwater. They occur in a few Elasmobranchs (species of _Raia_, _Torpedo_, and _Hypnos_), in such Teleosts as the African Silurid _Malopterurus_ the "Electric Eel" (_Gymnotus_), and in species of Mormyridae (e.g. _Mormyrus_). With one exception electric organs are composed of metamorphosed muscular fibres, and their nerve-endings or motor end-plates. The species of _Raia_ have two small electric organs, one on each side of the terminal portion of the tail.[432] In _Gymnotus_[433] the {366}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_[434] 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 (_Torpedo_) or longitudinal (_Gymnotus_) columns, separated by septa of connective tissue. In the active state of the organ in the Torpedo[435] 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."[436] 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_,[437] _Malopterurus_, 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. {367}CHAPTER XIV