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 



Fig. 31. Mr-BleLddeTsoiEigenmanniavirescens(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 Eigenmannia 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 E. 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 



