SENSORY REACTIONS OF CHROMODORIS ZEBRA 269 



gill crown. Less vigorous stimulation has. no effect on gill 

 contraction. 



The gill plumes are individually sensitive, and react sepa- 

 rately to slight stimulation. More violent activation (e.g., a 

 sharp tap or gentle pinching) of a single plume spreads through 

 the other plumes, according to its intensity. A single plume 

 presents a smooth, narrow, distally tapering outer edge, a simi- 

 lar inner edge, and running between them two broad blade-like 

 faces from which jut out the thin gill plates. Tactile excitation 

 of the outer or of the inner faces leads to similar reactions of 

 about equal magnitudes. The gill-bearing faces of the plume 

 are less sensitive; frequently, an individual gill may be bent 

 back and forth without eliciting a response. Presumably this 

 occurs naturally in tidal currents, and during the movements of 

 the gill crown as a whole. 



To a light touch, the common form of response is a local 

 constriction of the plume, usually not equal on the two sides, 

 accompanied by local longitudinal contraction, so that a slight 

 swaying movement of the plume results. The plume as a whole 

 may or may not be pulled down at its base. To stronger stimu- 

 lation the characteristic response involves the following events: 

 local constriction, spreading distally from the point of activation, 

 leading to the collapse and ' shriveling' of the plume distal to the 

 point of activation; this is succeeded by the retraction of the 

 plume through the traction of muscles not intrinsic to the plume 

 itself, but situated in the basal tissue of the gill crown. Still 

 stronger activation leads to longtitudinal contraction of the gill 

 plume, both distally and proximally to the site of touching. The 

 reaction of the plume distal to the point of activation is nicely 

 demonstrated by plumes which have acquired a branching or 

 dichotomously divided form (Crozier, '17 e). If one of the 

 branches of such a plume be touched on the side, this branch 

 alone, and only distal to the stimulated point, contracts, unless 

 the stimulation be too strong. 



The type of polarity evidenced in the reactions of a gill plume 

 is curiously akin to that seen in the responses of an actinian 

 tentacle under similar conditions of local activation (Rand, '09, 



