402 Harry Beal Torrey. 



Thermal Stimuli. A rapid rise in temperature of several de- 

 grees, caused by flooding the hydroid gently with warm water from 

 a pipette, produced a general contraction of the same character as 

 the response to a strongtactuai stimulus.^ Gradual changes in tem- 

 perature aflPect both irritability and the rate of growth, increase of 

 temperature resulting in increased irritability and more rapid 

 growth, and vice versa; the limits, however, were not determined. 



The reactions of Corymorpha to the various stimuli considered 

 above may be summarized as follows: All parts of the hydroid 

 are very sensitive to mechanical stimuli, irritating chemicals and 

 abrupt increases of temperature. Proximity to odorous sub- 

 stances, especially flesh, which might serve as food, awakens no 

 appreciable response until the substances are actually touched. 

 Food organisms, therefore, are probably detected only when they 

 strike the hydroid. The mechanism for capturing them is inter- 

 esting on account of the definite but dissimilar responses of the 

 two sorts of tentacles and the coordination exhibited in the activi- 

 ties of all the parts. The proximal tentacles with their great 

 spread (which sometimes almost equals the length of the stem) 

 serve as the chief means of advertising the presence of food and 

 carrying it to the mouth. These functions are sufl&ciently well 

 discharged b)^ a movement in one direction only — toward the 

 mouth; but the absence of the preliminary movement in the direc- 

 tion of the stimulus, which has been noted among the anemones, 

 entails a certain loss of efficiency. This loss of efl&ciency is com- 

 pensated for to some extent by the movements of the distal ten- 

 tacles and the proboscis. The stimulus which causes the move- 

 ments is in the great majority of cases liable to be applied to the 

 proximal tentacles, on account of their relatively much greater 

 spread. And apparently because of this, whether acquired by 

 habit or selection, the first movements of the distal tentacles in 

 response to direct or indirect stimulation are downward and out- 

 ward, toward the proximal tentacles; that is, toward the usual 



^This contraction is the result of muscular activity, does not concern the axial 

 endoderm (to be especially considered later under Geotropism) and is not to be com- 

 pared, therefore, with such growth processes as were shown by True ('95) to follow 

 in radicles of seedlings, transference from water at 0°C. to water at 18°-21° C. 



