THE AXIAL GRADIENTS IN HYDROZOA. 189 



H. oligactis indicate that the tentacle represents to some degree a 

 gradient in physiological condition. While the reactivity of the 

 tentacle varies and exact control of intensity of stimulation is not 

 possible, the sensitiveness of the tentacle seems to decrease some- 

 what basipetally. Moreover, adequate stimuli near the tip 

 usually induce contraction over some considerable portion or 

 even the whole length of the tentacle, the contraction progress- 

 ing basipetally, while similar stimuli near the base commonly 

 induce merely a local contraction or bending of the tentacle at 

 the region of contact with the concavity toward the exciting ob- 

 ject. A more intense stimulation of the basal region may of 

 course bring about contraction of the whole tentacle, but con- 

 duction of the excitation apparently occurs more readily basi- 

 petally than in the opposite direction. 



Stimulation of one tentacle, if slight, may induce partial or total 

 contraction of that tentacle only, if more intense, of all tentacles 

 and contraction of all tentacles is usually followed by more or 

 less contraction of the column. 



As regards the column, contraction of the stalk usually occurs 

 under normal conditions in response to excitation conducted from 

 the tentacles or from some more apical level of the body. Cer- 

 tain degrees of stimulation produce contraction of only the more 

 apical regions of the body or of the body and not the stalk, while 

 more intense stimulation brings about contraction of both body 

 and stalk. In animals partially anesthetized by alcohol, ether 

 and other agents the decrement in conduction apparently in- 

 creases, even when the body contracts vigorously, and the con- 

 traction can be seen to die out before reaching the stalk or in the 

 anterior portion of the stalk. In such cases the stalk may become 

 inactive before the body, not because it has lost its contractility, 

 but because the excitation fails to reach it. 



Both body and stalk are much less sensitive than the tentacles 

 to direct contact stimulation. Contraction of the whole column 

 can be induced by adequate stimulation at any point, as in the 

 case of the tentacle, but stimulation of the more apical levels of 

 the body induces, according to its intensity, contraction of only 

 the more apical body region, of the whole body or of body and 

 stalk. Here, as in the tentacle, a decrement in the transmitted 

 excitation evidently exists. 



