OBSERVATIONS ON HYDRA 613 



it was in line with the main axis of the body, and acted as 

 the true head. The curare thus appears to destroy the power 

 of regeneration in the affected parts : possibly the interstitial 

 cells are killed off. 



Symbiotic Cells. 



Since the differences between the various species of brown 

 Hydra seem less marked than their resemblances, it is 

 of interest to know whether, under any circumstances, a 

 H . V i r i d i s deprived of its green cells would grow to resemble 

 a brown Hydra. 



Goetsch (3) obtained brown Hydras showing pathological 

 features, which, when fed with algae, turned green. As they 

 did so they diminished in size, budding ceased, and under 

 natural conditions they died. Some which were fed with 

 freshly killed Daphnia lived, and j)roduced testes or ovaries. 

 The symbiosis was easily lost, disappearing after four weeks 

 in darkness. Goetsch suggests that this Hydra is a new 

 mutant, capable of receiving the alga, which is a large form 

 of Chlorella. 



Whitney (14) describes a method of ridding H. viridis 

 of its green inhabitants. He kept his specimens in a weak 

 solution of glycerine (0-5 per cent.) for a few weeks. During 

 this period the endodermal cells swelled up and extruded the 

 algae, which were then thrown out by the mouth. Eventually 

 he obtained several colourless Hydra which lived normally 

 in the aquarium for some time without being reinfected. 

 They retained all the features typical of H. viridis except 

 the colour. 



On January 26 I set five Hydras in a jar of 0-5 per cent, 

 glycerine, ^here they were fed as usual. They budded actively- 

 On February 13 they had increased to nine and were removed 

 to 0-75 per cent, glycerine as the' weak solution had had no 

 effect. On March 1 there were twenty-three Hydras, still 

 quite green, and they were removed to 1 per cent, glycerine. 

 One was fixed and sectioned. The endoderm appeared to be 

 quite normal in size, and the green algae were arranged as 



