HABITS AND REACTIONS OF SAGARTIA DAVISI. 209 



a small zone between mouth and tentacles, responds to a mechan- 

 ical stimulation, the greatest irritability being manifested by the 

 tentacles, the tactile organs par excellence. The latter exhibit a 

 very definite adaptive reaction. The preliminary bend of an irri- 

 tated tentacle at and toward the point stimulated makes it possible 

 for the polyp in a sense to pursue its prey actively if, indeed, to 

 but a limited extent. The great advantage of this reaction over 

 the simple inward movement of the tentacle indirectly stimulated 

 is obvious. The latter is also adaptive, however, since it is the 

 most likely movement to clutch food organisms in a polyp whose 

 tentacles are habitually outstretched. Supporting this idea is the 

 fact that the tentacles of hydroids react only in this way, whether 

 stimulated directly or indirectly. It is the simpler, more primi- 

 tive reaction. 



A more efficient adaptive reaction, also indirectly induced, is 

 the extrusion of acontia at the point stimulated, for purposes of 

 defense. Though the reaction is always the same-, the acontia 

 always move in the most desirable direction, which is not always 

 the case with tentacles. 



The passive outward movement of tentacles due to the con- 

 traction of muscles in the oral disk or capitulum is not directed 

 by the position of the stimulating object. It may be toward the 

 latter, but only when the stimulus happens to be applied at a 

 point external to the tentacles, that is, at some point which is less 

 likely to be stimulated by a food organism than points on their 

 inner surface. Wlien a tentacle o'f an outer whorl is moved pas- 

 sively as a result of the stimulation of a tentacle of an inner 

 whorl, the movement is away from, not toward the tentacle 

 stimulated. The reaction in this direction is of no obvious im- 

 portance to the polyp in- this case, and seems to be of no more 

 importance, in the sum of all cases, than a movement in any 

 direction. It appears, therefore, to have no adaptive value 

 whatever. 



By means of its varying sensitiveness to different chemical sub- 

 stances and its ability to discriminate between mechanical and 

 chemical stimuli, 5. davisi is enabled to make certain choices in 

 its quest for food. This capacity, which it possesses in common 

 with other anemones, has been described as olfactory by Romanes 



