THE ACTIVITIES OF CORYMORPHA 311 



The time required for a single constriction to pass from one 

 end of the proboscis to the other is from half a minute to a 

 minute. Usually not more than one constriction is present on 

 the proboscis at a time. The constrictions are quite obviously 

 due to local contractions in the circular muscle. They must be 

 very effective means of mixing the contents of the gastrovas- 

 cular cavity and probably are concerned with driving some of 

 the digestive products of the hydranth downward into the stalk. 

 A similar movement has been known for some time in actinians 

 and has been recently pointed out in Metridium (Parker '16, p. 

 478). Here, as there, it is probably indicative of nervous sup- 

 ervision of a muscle otherwise independent, but on this point no 

 positive evidence has been obtained. 



The proximal - tentacles in a resting expanded Corymorpha 

 radiate more or less horizontally from the basal disc to which 

 they are attached. Each tentacle curves a little downward away 

 from the mouth, but at its free end it turns in the opposite direc- 

 tion and comes eventually to point nearly outward. When 

 stimulated mechanically such a tentacle shortens a little, per- 

 haps a quarter of its total length, and bends vigorously inward 

 toward the mouth (Torrey, '05, p. 333), 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 

 we have assumed to be the resting position and no response can 

 be obtained from them whatever. I, therefore, conclude on 

 physiological as well as on anatomical grounds that there are 

 no circular muscles in the tentacles, but that longitudinal mus- 

 cles under the control of a nervous system act upon an elastic 

 skeleton of vacuolated cells. The fact that the proximal ten- 

 tacles on stimulation always bend toward the mouth would lead 



