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 

 I 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. 



CoRDYLOPHORA. Thcsc 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 ^^^' '94- Freshwater polyp, cordyhphora 



h, J , , , '■ lacustia. Enkrged. 



ould then be placed m 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. 



248 



