HYDKOIDS 183 



such a tentacle shortens a little, perhaps a quarter of its 

 total length, and bends vigorously inward toward the 

 mouth, its free end meanwhile often curling in spiral 

 fashion through one or two turns. The tentacle then 

 gradually relaxes and slowly returns to its original shape 

 and position. 



The axis of the tentacle is filled with vacuolated cells 

 not unlike those occurring in the axis of the stalk, and 

 the action of the tentacle can be explained entirely on 

 the assumption that the longitudinal muscle works against 

 the axial cells, whose elasticity returns the tentacle to its 

 original position. In an anesthetized polyp the tentacles 

 are always expanded in what has been assumed to be the 

 resting position and no response can be obtained from 

 them whatever. It is, therefore, probable on physiologi- 

 cal grounds at least that there are no circular muscles in 

 the tentacles, but that longitudinal muscles under the 

 control of a nervous system act upon an elastic skeleton 

 of vacuolated cells. The fact that the proximal tentacles 

 on stimulation always bend toward the mouth would lead 

 one to expect that the longitudinal muscle would be more 

 fully developed on the oral than on the aboral side of the 

 tentacle, but of this no direct evidence has been obtained. 



The distal tentacles, which are much more incessantly 

 active than the proximal ones (Torrey, 1904 6) in mo- 

 ments of rest form a cluster more or less surrounding the 

 mouth. On stimulation they quickly jerk backward away 

 from the mouth and point their tips toward the aboral 

 portion of the hydranth, after which they more slowly 

 return to their original position. On applying an anes- 

 thetic, magnesium sulphate of chloretone, to a whole hy- 

 dranth or to a single isolated distal tentacle, all these 

 reactions disappear in from one to three minutes, to re- 



