176 



THE ORIGIN AND EVOLUTION OF LIFE 



producing organs in the fishes themselves and in other animals 

 on which they prey. 



Another extreme of chemical evolution among the fishes is 

 the production of electricity as a protective function, which is 



even more effective than bony arma- 

 ture because it does not interfere with 

 rapid locomotion. In only a few of 

 the fishes is electricity generated in 

 sufficient amounts to thoroughly pro- 

 tect the organism. It develops through 

 modified body tissues in the form of 

 superimposed plates (electroplaxes) se- 

 parated equally from one another by 

 layers of a peculiar jelly-like connec- 

 tive tissue, all lying parallel to each 

 other and at right angles to the direc- 

 tion of discharge.^ The electric organ 

 is formed from modified muscle and 

 connective tissue and is innervated by 

 motor nerves. The physical principle 

 involved is that of the concentration 

 cell, and the electrolyte used in the 

 process is probably sodium chloride. 

 The theory is that at the moment of 

 discharge a membrane is formed on one 

 surface of the electroplax which prevents the negative ions 

 from passing through while the positive ions do pass through 

 and form the current. The strength of the current varies 

 from four volts in Mormyriis up to as much as 250 or more 

 in Gymnotus, the electric eel, and consists of a series of shocks 

 discharged 3/1000 of a second apart. 



1 Dahlgren, Ulric, iqo6, pp. 389-398; 1910, p. 200. 



Fig. 57. The Earliest 

 Known Limbed Animal. 

 Footprint of Tliinopus anli- 

 qiius Marsh, an amphibian 

 from the Upper Devonian of 

 Pennsylvania. Type in the 

 Peabody Museum of Yale 

 University. Photograph of 

 cast presented to the Ameri- 

 can Museum of Natural His- 

 tory by the Peabody 

 Museum. 



