166 ^Payera fruin the Deyariinent of Murine Biology. 



not lost iill relation to the original muscle-cell. Also, in the Mor- 

 myridsD the electric organs of some of the forms are connected to the 

 muscle in the adult instead of being completely free as usual. In 

 Astroscopns the striations are as definite as in the weakest fishes and so 

 numerous as to be seen up into the electric layer. In Torpedo and 

 Gynmotus, on the other hand, the electroplaxes have no structures to 

 remind one of their muscular origin. In Torpedo they exist in the 

 embrj'o only and are mere fibrillations, which in the case of Gymnotus 

 are so reduced as to be recognized as striations only from a compara- 

 tive study of the other forms. Are we here witnessing one i)hase in the 

 evolution of electric organs, and are we justified in making these com- 

 parisons between such widely unrelated forms? We can not, of course, 

 consider the transition from a mormyrid to one of the Rajidse a case of 

 natural selection, but if selection is taking place separately in each of 

 the groups we may say that in the Mormyridae the evolution of the 

 electric organs has advanced to a much less degree than in Torpedo or in 

 Gymnotus, and the lack of transitional stages in the groups themselves 

 may not mean that they do not or have not existed. In the Rajidse a 

 very definite series exists, from forms with wholly undifTerentiated 

 electric tissue to those with definite electroplaxes, and not all toipe- 

 does have shocks of the same intensity. It is probable, therefore, 

 that the appearance of electric organs in the separate groups is a case of 

 parallelism, e^'olution having taken place independently in each indi- 

 vidual group. This }:)arallelism may be accounted for by assuming that 

 the evolutionary processes are continually working on the minute elec- 

 trical discharge known to accompany the activities of muscle-cells, and 

 that under proper environmental stimulus any fish might develop simi- 

 lar organs. What external factors initiate this development are not 

 apparent, since electric organs appear to have developed simultane- 

 ously in fishes which lie buried in the sand on the bottom of the ocean 

 and in fishes which swim freely about in both fresh and salt waters. 

 The position of the organ in the body of the fish is individually adapted 

 to the mode of life, those fishes which are commonly attacked from 

 above developing electric organs in a dorso-ventral position, while 

 those which swim freely in the water have antero-posterior electric 

 organs. The primary use of the organ is probably that of protection, 

 the capturing of food being a secondary adaptation. 



SUMMARY. 



1 . Each of the two electric organs of Astroscopus guttatus is originally 

 composed of four separate elements, each derived from one of the six 

 eye-muscles. The rectus inferior and obliquus inferior muscles are the 

 nearest to the \'entral surfaces of the animal and are the only ones which 

 do not contribute anv electric tissue to the formation of the organ. 



