cox AND OTHERS: ELECTRIC TISSUE 489 



The main organs are of nearly uniform cross-section for some dis- 

 tance from their anterior end, but they taper toward the tail, conform- 

 ing to the ventral surface of the body and, in the posterior portion, to 

 the under surface of the organs of Sachs. Although this tapering pre- 

 vents the arrangement of electroplaxes in uniform columns, the series- 

 parallel array already noted in the rays is, nevertheless, clearly dis- 

 cernible. In Electrophorus, the axis of polarity is along the length of 

 the fish. Thus, the organs, in comparison with those of the rays, are 

 very much elongated along the line of series connection of the electro- 

 plaxes. 



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Drawing: by Ralph Graeter 



Figure 2. Electrophorus, with skin removed to show the electric organs. 

 A, main organs; B, organs of Sachs, overlapping the main organs; C, organs of Hunter. 

 (From Bull. N. Y. Zool. Soc.) 



This contrast between the most highly specialized electric fishes of 

 the fresh-water and marine groups has a significance which seems to 

 have been noticed first by du Bois-Reymond.^ The combination of a 

 fixed number of electromotive elements to supply power to an external 

 circuit of given resistance is a well-known problem in the theory of 

 electric networks. The solution of the problem shows that maximum 

 power will be delivered to the external circuit by the combination of 

 the elements in a series-parallel array, such that the resistance of the 

 combination is equal to that of the external circuit. Thus, if the 

 external resistance is high, more electromotive elements will be joined 

 in series; if it is low, more will be joined in parallel. Fresh water has 

 a much higher specific resistance than sea water. Consequently, if the 

 condition for maximum external power is equally approximated in the 

 different genera, the organs of the fresh-water fishes will be elongated, 

 and those of the marine fishes will be flattened, along the axis of polar- 

 ity. Most of the varieties confirm such a generalization. The electric 

 skates, which are marine fishes with weak electric organs elongated in 

 the direction of the axis of polarity, make a rather puzzling exception. 



The arrangement of the electroplaxes, by means of which the main 

 organs of Electrophorus are accommodated to the tapering body of the 

 fish, is simple and rather interesting.^' * It is best described in terms 

 of a transverse slice just thick enough to contain a single layer of elec- 

 troplaxes. Near the anterior end of the organs of a fish about 1 meter 



