136 COMPARATIVE MORPHOLOGY OF VERTEBRATES. 



(electroplaxes) arranged at right angles to the axis of the primitive 

 muscle, each derived, where the history has been traced (Torpedo,' 

 Raid), from a primitive muscle cell. In the typical condition each 

 plate consists of an outer electric layer, differentiated into a nervous side 

 and a so-called nutritive side, with a middle striated layer betwee/i them, 

 the latter in a few cases being weakly developed or absent. Nervous 

 stimulation is always by motor roots leading to the nervous layer, the 

 connexion corresponding to the nerve-end of a muscle cell. Numbers 



FIG. 143. Head of Astroscopes y-grcecum, after Dahlgren and Silvester. The dotted 

 line on right shows extent of electric organ, on the left the eye-muscles, and nerves as forced 

 out of place by the electric organ, ab, abducens; cil, ciliary nerve; e, eye; en, electric nerve; 

 n, naris; olf, olfactory nerve; om, oculomotor; op, optic nerve; re, rif, rint, rs, external, in- 

 ferior, internal and superior rectus muscles; rp, palatine nerve; so, superior oblique muscle; 

 tf, trigeminal-facial nerve. 



of these electroplaxes are included in a connective-tissue compartment 

 with a gelatinous substance between them and all with their nervous 

 layer turned in the same direction. 



In Torpedo the organ apparently is derived from part of the jaw 

 muscles and the prisms of plates are arranged vertically. In Astro- 

 scopus (fig. 143) it is supposed that the tissue comes from one of the eye 

 muscles, while in Gymnotus the ventral trunk muscles are concerned 

 and the columns of electroplaxes are horizontal. In the same fish the 

 discharge is always in the same direction, e.g., in Torpedo* horn below 

 upward. 



