270 W. J. CROZIER AND LESLIE B. AREY 



'15; Parker, '17) — with this important difference: in the actinian 

 tentacle it is the part proximal to the point of activation which 

 contracts. The polarization of the gill plume, which is a neuro- 

 muscular matter since it disappears under magnesium sulphate 

 anaesthesia, is further seen in the fact that the distal tip of a 

 plume, when touched, gives rise to only a slight longitudinal 

 contraction in the inmiediate region of the tip, although the 

 basal contraction may lead to the retraction of the plume as a 

 whole. Bionomically, the significance of the difference between 

 the neuromuscular polarizations within the actinian tentacle, on 

 the one hand, and the gill plume of Chromodoris on the other, 

 lies in the fact that the actinian tentacle carries food to the 

 animal's lips, hence the part between the disk and the point of 

 excitation is shortened; whereas the mode of retraction of the 

 gill plume probably saves it somewhat from being bitten by 

 fishes. The gill plumes are bitten at by fishes, and there is evi- 

 dence to show that some of the structural variations which they 

 present (Small wood, '10; Crozier, '17 e) originate as the result of 

 injury. 



The basal contraction of a plume spreads to other plumes in 

 proportion to the intensity of the stimulus and to the nearness of 

 its application to the base of the gill-plume; this is the reaction 

 which is responsible for the retraction of the whole gill crown. 

 Contraction at the base of a gill induces collapse of the whole 

 plume. 



Any desired degree of contraction of a single plume or of the 

 whole set may be induced by grading the intensity of the tactile 

 stimulus. Also by stimulating single plumes weakly and one 

 at a time, as many plumes as may be desired can be caused to 

 contract; e.g., all but one may be made to contract. Under 

 slightly stronger activation, especially in the case of the more 

 anterodorsal gill plumes, it can be demonstrated that the suc- 

 cessive stimulation of two adjacent gill plumes is much more 

 effective for the production of retraction of the whole gill crown 

 than is the equivalent stimulation of any single plume. Thus, 

 if two adjacent plumes are touched in quick succession or simul- 

 taneously the whole crown is retracted more or less completely; 



