114 



THE ELASMOBRANCH PISHES 



The electric discs (ds.), although directed more at right angles to their con- 

 nective tissue septa, have otherwise the same general relations as the fibers of 

 the muscle. Like them they are smaller and less regular at the tip and base of 



the cone. They are, however, much larger 

 than the muscle fibers. As individual units 

 they are more or less quadrangular in 

 shape, their walls being formed of connec- 

 tive tissue. A glance will show that there 

 are a great number in a single organ. It has 

 been estimated that as many as 20,000 discs 

 are present in an adidt Rata hatis. 



One of the most interesting things about 

 the electric organ is the fact that, whether 

 it be in Raja or in Torpedo, it is formed as 

 a series of metamorphosed muscle fibers. 

 The organ as described by Ewart in Raia 

 hatis first appears when the emlu-yo is 

 about an inch long. Here it is confined to 

 the tail and only those muscle fibers are af- 

 fected Mhich belong to the lateral bundles, 

 as described for Lamna (fig. 101). These 

 fibers undergo complete change of form 

 and assume an entirely secondary func- 

 tional role. 



A group of such fibers (fig. 116a) shows 

 the beginning stages in this metamorphosis. 

 The anterior fibers, near their attachment 

 to the nn'oseptum, are beginning to enlarge 

 into club-shaped structures {el.c), while 

 the posterior fiber (inf.) is still of the mus- 

 cle type. By further growth and difi^erentiation each incipient electric club 

 comes to assume the form of a cone, in the enlarged end of which in time a con- 

 cavity forms. A nerve entering this concavity breaks up into many branches 

 {n., fig. 116b). At this stage, striation characteristic of the muscle fiber has 

 decreased on the cups, but striations are still present on the body of the cone. 

 In a later stage (fig. 116c) the organ has acquired essentially the adult char- 

 acteristics. In this stage it is seen that the disc (ds.) has greatly enlarged and 

 that the terminal part of the cone has lost something of its muscle-like appear- 

 ance. From a somewhat more mature organ we may study the detail of its 

 finer structure. 



FIXER AXATOMY OF ELECTRIC ORGAN 



Fig. 117. Finer structure of electric 

 disc. (From Ewart.) 



av., alveolar layer; o.e., outer elec- 

 tric layer; str., striate layers. 



The disc may be divided into three well defined layers (fig. 117), an outer 

 electric layer (o.e.), a middle striated layer (str.), an inner alveolar layer 

 (av.). The outer layer is in fact composed of two layers, the more superficial 

 of which is made up of a net of nerves, and the next layer is characterized by 



