THE GYMNOTID EELS OF TROPICAL AMERICA. 189 
the body are not equally exposed to injury. These fishes frequent the weeds of 
the trenches and small streams, for the most part feeding upon insects and small 
Fic. 31. Air-Bladders of Higenmannia virescens (Val.). Side-view. 
crustacea. They are easily frightened, and, being very swift swimmers, seek 
safety in flight. This habit naturally exposes the caudal region more than any 
other to the attack of the pursuing enemy. Because of the tapering shape and 
the straight dorsal profile, the sloping ventral quarter is also more or less exposed to 
attack from the rear. 
Fig. 32. Air Bladders of Hypopomus brevirostris (Steindachner). 
That injury to some regions of the fish would be more apt to be fatal is easily 
seen from the anatomy of these fishes. The anterior third of the body of the two 
species of Higenmannia contains all of the viscera, in addition to the most important 
parts of the circulatory and nervous systems. S. macrurus does not differ materially 
from the two species just described, except in the size and shape of the air-bladder, 
previously mentioned. Its posterior air-bladder is conical and terminates about 
the length of the head caudad of the viscera proper. It is interesting to note in 
this connection that no individuals of Sternopygus showed injuries extending en- 
tirely across the body as far cephalad as several of those which H. virescens was 
regenerating. (See figures 30 and 32.) 
With the distribution of injuries just discussed in mind, a number of experi- 
ments were made in order to ascertain the effect of various injuries and their 
relative severity. 
Part III. EXPERiMENTS. 
Forty-three specimens of S. macrurus, having an average length of two hundred 
and fifty millimeters, were collected from a large trench near the Botanic Garden, 
Georgetown, at about 8 A. M., Sept. 27, 1910. As these fishes were seined, they 
were put into buckets of the trench-water from which they had been taken and 
were operated upon within ten or fifteen minutes afterwards. All injuries were 
made with a razor, and the fishes, immediately after being operated upon, were 
