Hydrozoa 141 



enough to be ingested by the green hydra. Cultures of the green hydra 

 should be exposed to the light, but for other species a moderate light 

 is best. 



The first prerequisite of laboratory cultivation of hydras is suitable 

 water. Hydras should never be put into freshly drawn tap water and 

 in general only natural pond or river water should be used for their 

 culture. If tap water must be used, it must stand for several weeks 

 with growing algae or other aquatic plants in it so that it may become 

 conditioned. The suitability of water for hydras should be tested by 

 placing a few specimens in it. If these expand fully, with tentacles 

 extended to their maximum extent, the water is suitable. In unsuitable 

 water, the column remains contracted and the tentacles fail to expand. 

 The presence of plants is not necessary in a hydra culture except in the 

 case of Hydra Uttoralis, as stated above. 



When a suitable water has been found, the hydras are placed in it, and 

 fed daily. At intervals, the accumulation of bottom debris should be 

 removed and small amounts of suitable water may be added from time 

 to time. In general it is desirable that the cultures be covered. 



In general hydra cultures succeed better if the temperatures are not 

 too high; 20 C. is a very suitable temperature. The brown hydra, 

 P. oligactis, is more susceptible to rise of temperature than any other 

 species and usually dies at 25 C. 



The great difficulty in the continuous culture of hydra is the occur- 

 rence of the phenomenon of depression. In spite of every care, hydra 

 cultures will pass into this state at intervals and, unless prompt measures 

 are taken, will die out. In depression, column and tentacles fail to 

 expand, the animal ceases to feed, shortens to a stumpy appearance, and 

 finally gradually disintegrates from the tips of the tentacles aborally. 

 Depression is caused by over-feeding, fouling of the water, too high 

 temperatures, and general aging of the culture with accumulation of 

 waste products. The most successful method of reviving the animals 

 from the depressed state is to transfer them to a fresh jar of suitable 

 water. Lowering the temperature is also of assistance. 



Sex organs when wanted for class display may be induced in most 

 species of hydra, notably in the brown hydra, by placing the culture in a 

 refrigerator for two or three weeks at a temperature between 10 and 15 C. 

 Such cultures should be fed regularly. In nature most species are found 

 with sex organs in the late fall but the green hydra is said to be sexual 

 in the summer. 



Food. Naturally for the continued maintenance of hydra in the 

 laboratory it is necessary to have a food source that is easily cultured. 

 The most suitable animal for this purpose is Daphnia because of its 

 slow movements, weak resistance, and habit of moving about continu- 



