HABITS AND REACTIONS OF SAGARTIA DAVISI. 205 



esting from the fact that the major axis of the animal is not par- 

 allel with the direction of locomotion, a peculiarity which dis- 

 tinguishes it from the reactions of the majority of animals to 

 directive stimuli. The major axis is the axis of geotropic orien- 

 tation, but it can only be the axis of locomotion in swimming 

 forms ' and those which lack a foot disk and creep on the column 

 (e. g., Pcachia]. 



It is possible to reconcile these different cases if we think of 

 the foot disk merely as a differentiated portion of the body wall. 

 Edivardsia has no well defined foot, though its aboral end is 

 rounded and adhesive. The hydroid, Coryuiorpha, again, has no 

 foot such as is possessed by Hydra, its aboral end coming to a 

 point ; yet the sides of this tapering extremity are adherent, and 

 through their amoeboid cells the hydroid, orienting negatively to 

 gravity, tends also to move vertically upward. It adheres in this 

 case by a portion of what may truly be called its lateral wall ; in 

 consequence of which the axes of locomotion and geotropic orien- 

 tation coincide. 6". davisi is an extreme case in the other direction. 

 Having a large and well defined foot, it can hardly be said to 

 cling obviously by a portion of the lateral (i. e., column) wall. 

 At the same time, when on a vertical surface, its axes of locomo- 

 tion and orientation are as nearly parallel as the differentiation of 

 foot and column will allow. This, however, is not equivalent to 

 saying that the direction of locomotion in response to a directive 

 stimulus is determined by the orientation of the major axis of the 

 polyp, for the elements of the foot may be directly affected by 

 gravity. 



Loeb ('91, p. 70) has said that Cerianthus and Actinia equina 

 went from smooth glass to a mussel shell or piece of ulva more 

 readily than in the reverse direction in his experiments. This 

 indicates a certain "contact irritability," which seems to be pos- 

 sessed also by .$. davisi, as the latter moves about more freely 

 on smooth glass than on rough surfaces. The reaction to the 

 contact stimulus, however, is not so strong as the orienting re- 



1 Besides the pelagic species which Andres describes among the Minyadidce, all 

 non-adherent, there is an interesting polyp abundant in the harbor of Honolulu which, 

 I am told by my friend Mr. Loye H. Miller, leads both a sedentary and a free exis- 

 tence. It appears to have no pedal float, sustaining itself by means of rhythmic move- 

 ments of the tentacles which send it along foot foremost at a fair rate of speed. 



