EFFECT OF LOW TEMPERATURES ON HYDRA. 79 



Reversal of vital phenomena due to causes other than reduc- 

 tion of temperature has also been described. Loeb, 'oo, in camp- 

 anularian hydroids produced similar effects in the polyps by 

 bringing them in contact with solid bodies. In the same paper 

 he describes like changes brought about by gravity. In Anten- 

 nularia he found that when the branches are placed horizontally 

 the polyps on the lower side are quickly absorbed. 



Miss Thacher, '03, working on Eudendrium, Pennaria, and 

 Campanularia, repeated Loeb's experiments, studying at the same 

 time the histological changes. She found that the absorption as 

 described by Loeb is not due to contact with solid bodies, but that 

 it is a true degeneration caused by unfavorable conditions. At 

 all times whether in contact with foreign bodies or not she found 

 that the polyps of these hydroids are absorbed when brought 

 into the laboratory and that the absorption is always preceded by 

 degeneration. 



Gast and Godlewski, '03, describe a similar degeneration of 

 the polyps of Pennaria when kept in the laboratory. 



The series of experiments described in this paper was begun 

 with a view only of determining the histological changes which 

 take place in Hydra when the temperature is lowered. Greely 

 in his work considered merely the grosser structural changes. 

 It was soon found that Hydra when subjected to low temperatures, 

 often, in fact, usually do not behave as described by Greely. Ex- 

 posure to a temperature of 2° C. for nine days, that is two degrees 

 lower and three days longer than he found necessary to obtain 

 a complete resting stage, if other conditions such as the com- 

 position of the water in which the Hydra are kept are unchanged, 

 produces little or no effect on the structure. There may be no 

 indication whatever of a reversal of vital phenomena. Because 

 of this direct contradiction of results, it was thought advisable to 

 repeat Greely's experiments, using as large a number of Hydra 

 as possible. 



Hydra fusca and Hydra viridis were the forms used. Hydra 

 fusca is more favorable for experimental work than is Hydra 

 viridis, since it is much more resistant to changes in the composi- 

 tion of the water ; and also to sudden changes in the temperature. 

 However with proper care either Hydra may be employed. 



