514 journal of Comparative Neurology and Psychology. 



rhythm was observable in actinians taken from pools where the tidal changes were 

 less important. It was masked by the tidal rhythm in specimens from high rocks, 

 but might be observed in these when the tidal rhythm had disappeared. The 

 effect of habitat was further shown by the fact that actinians collected from sunny 

 places showed a reverse day and night periodicity, opening by day and closing 

 at night, while those taken from dark places seemed to suffer under the influence 

 of light. The effect of light is exhibited by the fact that actinians display more 

 activity, the more light they have been subjected to in the past. Impurity of the 

 water increases the effect of light. Mechanical agitation seems to destroy a state 

 of inertia in the animals, and may reveal a tidal rhythm. Anthea cereu« and 

 Actinoloba dianthus show a more marked response to light than Actinia equina, 

 the former probably because it contains chlorophyll-bearing alg^e. A. cereus 

 converges its tentacles towards the light; A. dianthus orients its column in the same 

 direction, but the orientation is reached only after a series of oscillations which 

 suggest trials and errors, but in Bohn's opinion are the absolutely determined 

 effect of past causes, combined with "sensibility to difference" in Loeb's sense. 

 There is a general tendency for actinians to expand under a thin layer of water; 

 this is doubtless connected with the fact that the food supply is best near the sur- 

 face. Tealia crassicornis shows the same general features of behavior as the 

 other actinians mentioned. 



19. The most important point in this paper is as follows. The leech observed 

 is positively phototropic; at the outset of its movements toward the light, its 

 orientation is precise, but the further it advances, the more it waves its body from 

 side to side. If these deviations were "trials," they ought, BoHN contends, to 

 diminish rather than increase in number. They are, rather, the effect of the 

 progressive weakening of the light's attraction. 



20 and 21. These communications may be summarized in the author's own 

 words. "An animal which has just been immersed in still water or undergone 

 mechanical excitation, if it moves in a restricted area of a constant luminous 

 field, shows in general a progressive and more or less rapid weakening of the effects 

 of phototropism and of sensibility to difference. This weakening is connected 

 with the progressive return of the animal to a state of rest. We should see in this 

 return, not the consequence of " fatigue or an alteration in external circumstances, 

 but "the progressive exhaustion of the nervous effects of the initial mechanical 

 excitation, which momentarily overcame the inertia of the animal." "In a lumin- 

 ous field, phototropic animals follow fatally, in a given direction, certain lines. 

 But during a change of illumination, the animals tend to turn about upon the lines 

 and follow them momentarily in the opposite direction, hence they may deviate for 

 a time. Many of the supposed "trials" of Jennings might be explained by apply- 

 ing this law." 



22. This is a summary of recent work in America and elsewhere. 



23, 24, 25, 27. The echinoderms studied by BoHN were the following: Asterias 

 rubens, Asteriscus verruculatus, Astropecten irregularis, Ophiolepsis ciliata, O. 

 albida, Ophiothrix fragilis, Ophiocnida brachiata. The salient results may be 

 grouped under three heads, {a) Sensibility to difference is shown by the tendency 

 of sudden changes of illumination or of slope to change the sign of phototropism 

 and geotropism, producing oscillations which are not properly "trials," but are 

 as fatally determined as the tropisms themselves. Further evidence that these 



