222 G. H. PARKER 



of the modified response in the influences that had emanated 

 from the original response and thus brought that response into 

 historical relation with the second and modified one. This form 

 of explanation emphasizes the effect of the history of the animal 

 on its immediate state. Both forms of explanation have their 

 places, but they are sufficiently diverse to require separation, a 

 condition not always observed in discussions of this kind. 



The opinion that the past histoiy of an individual actinian is 

 a potent factor in understanding its behavior has been expressed 

 not only by Jennings ('05) but also by Pieron ('06 c, p. 15), 

 who declared that the responses of actinians could not be looked 

 upon as purely mechanical operations, but included traces of 

 those activities characteristic of the central nervous organs of 

 higher animals. But very little work has been done on actin- 

 ians to ascertain the extent to which such central activities as 

 those just indicated may extend. The limited range of response 

 in these animals restricts such experimentation considerably. 

 Heretofore associative processes have never been directly identi- 

 fied in actinians and my own efforts in this direction have always 

 yielded negative results. 



One of these attempts may be briefly described. When Me- 

 tridium is shghtly stimulated mechanically on the pedal edge 

 of its column, it responds by a slight initial retraction. Wlien 

 food is put on the tentacles, the first response is an irregular but 

 very characteristic waving of these organs in the immediate 

 "vdcinity of the food. An attempt was made to associate the two 

 stimuli mentioned so that the tentacle response might be called 

 forth by a mechanical stimulation of the pedal edge of the col- 

 umn. A specimen of Metridium attached to a stone was placed 

 in an aquarium of running seawater in a dimly lighted situation. 

 After it was expanded it was mechanically stimulated and at 

 the same time fed. These two stimuli were applied at the same 

 time at intervals of half an hour from 9.30 in the morning till 

 4.30 in the afternoon. At each application the slight retraction 

 and the initial waving of the tentacles were observed. After 

 fifteen such trials the mechanical stimulus alone was applied 

 but, though retraction was evident, no waving of the tentacles 



