I 10 



Comparative Morphology of Chordates 



Btto 



discharge its powerful electric batteries. The fish is the ''electric eel," 

 Electrophorus (or Gymnotus), a teleost. The electric organs are 

 bulky paired masses of peculiar tissue occupying most of the ventral 

 region of the very long tail (Fig. 339). The corresponding region 

 in other fishes is occupied by the hypaxial divisions of the caudal 

 myomeres. 



Another formidable fish is the electric ray, Torpedo, an elasmo- 

 branch commonly found in the tropical and temperate waters of the 

 Atlantic and occasionally elsewhere (Fig. 339). It is 

 a large fish, in extreme cases attaining a length of 5 

 feet and a weight of 200 pounds. The electric organs 

 consist of a pair of large masses of electric tissue, 

 each situated between the gill-chambers and the 

 musculature of the pectoral fin of one side, and filling 

 the entire space between dorsal skin and ventral skin. 

 Malapterurus, a fresh-water catfish (Family 

 Siluridae) of tropical Africa, is another electrically 

 potent teleost. Its electric tissue is disposed as a sub- 

 cutaneous layer (i.e., between skin and body-muscle) 

 investing the greater part of the body but thickest 

 in the middle region of the trunk. 



In several other groups of fishes, relatively weak 

 electric organs are present. The "stargazer" (Astro- 

 scopus), an Atlantic teleost having eyes directed up- 

 ward, possesses a pair of electric organs situated 

 dorsally, each close behind an eye. Members of the 

 teleostean Family Mormyridae, African fresh-water 

 fishes, have electric organs in the tail. Among 

 elasmobranchs, some of the common skates have 

 weak electric organs in the anterior region of the tail. 

 The structural unit of an electric organ, wher- 

 ever situated, is an exceedingly thin plate of proto- 

 plasm containing numerous nuclei, but not subdivided 

 by cell-walls. The internal structure of this electroplax 

 is too complex to admit of brief description (Fig. 340). Large numbers 

 of these plates, usually placed together flatwise, form rows or columns 

 of electric tissue. An electric organ includes numerous such columns of 

 plates. 



Except in Malapterurus, the electric organs occupy space corre- 

 sponding to that which, in nonelectric fishes, is occupied by muscles. 

 Consistent with this fact, it has been found (in such cases as have been 

 investigated) that the electric tissue is produced by mesoderm cor- 

 responding to that which, in other fishes, produces striated muscles. 



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Fig. 340. 

 Diagram of 

 electric organ 

 of Torpedo 

 (e) Electroplax; 

 (g) gelatinous 

 layer; (n) nerve. 

 ( After Biitschli. 

 Courtesy, 

 K i n g s 1 e y : 

 " Comparative 

 Anatomy of 

 Vertebrates," 

 Philadelphia, 

 The Blakiston 

 Company.) 



