
MOLLUSCS, VERMES AND HYDROZOA 

has greater power of extending its tentacles, often to a stretch of 2 to 5 
inches. Otherwise it resembles the foregoing. 
Hydra are destructive to very young fishes. The observations of 
Mr. A. E. Beardsly, in the investigation of the deaths of small trout, at 
the hatchery of Leadville, Col., will illustrate this fact. The sediment 
from the hatching troughs was found to contain innumerable hydra about 
1 to 2 centimeters in length. Some of this sediment was removed to a 
number of vessels into each of which five newly hatched trout in good health 
were placed, and in one, as a control, the clear water of the main supply. 
In this experiment 25% of the trout were killed in less than 30 minutes, 
60% in 45 minutes, 80% in 60 minutes and 100% in 75 minutes; while 
those in the control jar were all alive and in good health the following 
day. He states that “the hydra were seen with their mouths closely ap- 
plied to the surface of the fish, particularly on the yolksac. In some cases 
more than a dozen hydras were attached to a single fish.” This has also 
been the experience of a number of goldfish breeders. 
CorpyLopHora. These polyps consist of single individuals or of a 
number joined by a common stalk, each developing gonosphores or ger- 
minating buds similar to those of the hydra. One species, Cordylophora 
lacustra, Fig. 194, frequently occurs in fresh- 
water attached to stones, shells or other firm 
objects. Its waving branches somewhat re- 
semble some of the aquatic mosses but when 
touched the immediate change in form betrays 
its animal nature. It is not an active enemy 
to young fishes but should not be admitted 
to the hatching troughs. 
Hydra and Cordylophera can be 
removed from the aquarium by the introduc- 
tion of Sticklebacks, and Paradise fishes, or 
by its complete rearrangement. ‘The plants 
should then be placed in a strong solution 
of Phenol-sodique, or in a bichloride of mercury solution of one tablet 
the pint of water, and to then carefully washed in clean water before 
they are returned to the aquarium; or the bichloride used directly in 
the aquarium after removing the animals, and the water afterwards 
removed and changed a number of times. The former is the preferred 
method, but unless the pests are very numerous, a few Paradise fishes, if not 
otherwise fed, will “clean them up” in a few days. Hydra are principally 
destructive when they are introduced into the spawning and hatching tanks. 

FIG. 194. Freshwater polyp, Cordylophora 
lacustra. Enlarged. 
248 
