158 
BULLETIN OF THE UNITED STATES FISH COMMISSION. 
hydras were found quite equally distributed through the first division of all troughs 
supplied directly from the main pipes. A careful count of the number on several 
square inches in different troughs gave an average of 131 hydras per square inch 
(20+ per square centimeter). Comparatively few were found in the middle division 
of the troughs, most of them having fixed themselves before reaching the first screen. 
Very little animal life other than hydra was found in the sediment of the troughs. 
Since no other cause for the mortality of the young fishes could be discovered, 
and as the hydras were exceedingly abundant and are well known to be armed with 
great numbers of dart cells or nettling cells which secrete a fluid that quickly causes 
paralysis in small crustaceans and other minute forms of animal life, it appeared 
that the injury was probably due to the hydras. In so far as the writer is aware, no 
injury to fishes by hydra has heretofore been known. The following experiment 
was therefore instituted to determine what injury, if any, was to be attributed to 
this cause: 
Five beakers, each of 250 cubic centimeters capacity, were filled with water from 
the supply pipes; in each of the first four of these was placed the sediment from 21 
square centimeters of the bottom of the first division of one of the hatching-troughs, 
containing about 430 hydras; the fifth beaker was intended as a control, and contained 
water only. Five trout newly hatched and apparently in good health were taken 
from the hatching-trays and placed in each beaker. Nos. 1 and 2 were filled with 
water from the spring and were placed in running water, so that the temperature was 
nearly constant; Nos. 3, 4, and 5 were filled from the main supply pipes, No. 4 having- 
been kept over night in the office, and all three were set on a shelf in the hatching- 
room. At the end of the experiment, Nos. 1 and 2 were at nearly the same tem- 
perature as at the beginning, while Nos. 3, 4, and 5 had acquired the temperature of 
the hatching-room. 
The following table shows the result of this experiment: 
1. 
2. 
3. 
4. 
5. 
Temperature at beginning of experi- 
ment. 
Hour of beginning 
43° F 
43° F 
48° F 
5S° F 
48° F. 
9.23 a. m. 
0 dead. 
0 dead. 
0 dead. 
0 dead. 
55° F. 
9.13 a. m 
9.16 a. m 
1 dead 
9.20 a. m 
2 nearly dead. 
3 dead a 
1 dead 
4 dead 
4 dead 
4 dead 
4 dead 
5 dead 
5 dead 
Temperature at end of experiment. . 
44° F 
44 c F 
55° F 
55° F 
a One of these had burst the yolk-sac in its struggles. 
In this experiment 25 per cent of the trout were killed by hydras in less than 30 
minutes, 60 per cent in 45 -minutes, 80 per cent in 60 minutes, and 100 per cent in 75 
minutes; those trout which were least active in the beginning of the experiment were 
the ones that survived longest, probably because they came in contact with a smaller 
number of stinging ceils of the hydra. With the aid of a lens, the hydra| could be 
seen with their mouths closely applied to the surface of the fish, particularly on the 
yolk-sac; in some cases more than a dozen hydras were seen attached to a single fish. 
Soon after the fishes were placed in the beakers most of them were seen to struggle 
violently, one of them bursting its yolk-sac in its struggles and dying immediately; 
these struggles recurred at intervals, but with diminishing intensity, until death 
