THE GYMNOTID EELS OF TROPICAL AMERICA. 159 
other Gymnotids and was unable to say whether the method described for EF. 
electrophorus was common to all. 
In his phylogenetic-ethologie study of the Gymnotidw, Schlesinger (1910) 
states that the use of the anal fin described by Sachs must be general throughout 
the family. He bases his conclusion entirely on a morphologic comparison between 
the Mormyrid Gymnarchus and the Gymnotids Electrophorus and Gymnotus. Ina 
footnote he adds that Dr. Franz Steindachner told him that he had seen living 
Gymnotide in Brazil swimming in the method described. 
While in British Guiana in the summer of 1910 I had opportunity to study the 
motion of a number of species in their normal environment. Three species in 
particular were examined, Higenmannia virescens and Sternopygus macrurus, which 
are abundant in the trenches in and about Georgetown, and Gymnotus carapo in 
Hubabu Creek. Several other species were also seen alive. In every case two 
methods of swimming were observed, (1) the use of the anal fin alone, and (2) the 
use of the anal aided by the body proper or the pectorals or both. 
When at rest the Gymnotids face the current of the stream, the entire body 
and caudal appendage being in a straight line and the pectorals laid back against 
the body. The anal fin was kept moving just enough to counteract the motion 
of the stream, and the pectorals gave an occasional stroke. The movements of 
the anal fin were similar to those described by Sachs for the electric eel. From the 
cephalad end of the anal fin a series of undulating waves passed caudad so that a 
longitudinal section of the entire anal fin in motion resembles a fairly regular sine 
curve. There were usually six of these waves traversing the fin at any one time, 
rarely five or seven. The speed of the wave varies with the speed of the current 
of the stream—always being just sufficient to maintain the position of the fish. 
If the current varied in direction the fish responded at once with a stroke of one or 
both of the paddle-shaped pectorals, which kept the long axis of the fish parallel 
to the direction of the current. Otherwise the pectorals were not used. During 
these resting periods the caudal appendage streams out behind the fish. 
If a resting fish were slightly disturbed it merely increased the speed of the 
waves traversing the anal and moved away. If frightened (all of the Gymnotide 
were very easily frightened) it swam rapidly away by the same motion of the anal 
fin, the use of the pectorals being more frequent in guiding the fish. If it became 
necessary for the fish to make a sudden turn, the entire body was slightly curved 
in the desired direction. This curving of the body together with the rapid use of 
the pectorals enables these long fishes to make quite abrupt turns. 
The second method of swimming involved the use of the entire body as well 
as the fins. When the fish was being pursued, the anal fin moved, as before, in a 
series of rapid waves, but in addition the entire body was at the same time moved 
in a serpentine fashion. In this way it was able to swim very rapidly. An indi- 
vidual would move the anal fin rapidly in the peculiar manner of swimming when 
held in the air. 
