C. M. CHILD AND L. H. HYMAN. 



The stalk, with the exception of the foot region, is under nor- 

 mal conditions less sensitive to direct mechanical stimulation 

 than the body. It is often possible to puncture the stalk re- 

 peatedly with a needle, without producing any effect beyond a 

 slight local contraction, or at the most some contraction of the 

 whole stalk, but when the stimulus is adequate to produce more 

 than a local effect, contraction of the whole animal usually follows. 

 It appears then that because of the greater sensitiveness of more 

 apical levels and probably also because of greater conductivity 

 in the basipetal direction the physiological state at any level, so 

 far as it is dependent upon excitation, is determined to a greater 

 degree by the conditions in levels apical to it than by conditions 

 at more basal levels. In other words the more apical levels are 

 relatively dominant. 



It should be noted, however, that the foot of the attached ani- 

 mal is rather sensitive to direct mechanical stimulation which de- 

 forms or stretches it, and the stalk likewise is sensitive to tension, 

 whether produced by currents of water or through the tentacles, 

 and after such stimulation of the foot or stalk, contraction of 

 the. whole animal usually occurs. Moreover, the stalk and foot 

 of animals which have been forcibly detached are evidently in a 

 different physiological condition from those of attached individ- 

 uals, for the stalk of such animals performs searching movements 

 to a greater or less extent, and the foot responds to contact by 

 attachment except in conditions of extreme irritation. The stalk 

 and foot apparently constitute a more or less specialized sensory 

 and motor apparatus which under the usual conditions, i. e., in 

 attached individuals, is to some extent under the control of more 

 apical regions, but which under certain conditions may respond 

 independently or initiate the response of other parts. 



THE AXIAL SUSCEPTIBILITY RELATIONS UNDER VARIOUS 



CONDITIONS. 



It is impossible to proceed far in the analysis of the suscepti- 

 bility relations of hydra without becoming convinced that the 

 contractile activity of any particular region is a factor in deter- 

 mining its susceptibility. The very direct and considerable in- 

 fluence of muscular contraction upon susceptibility is undoubt- 



