SENSORY REACTIONS OF CHROMODORIS ZEBRA 271 



whereas if either one of them is itself touched twice in this way, 

 even if on the opposite faces, it alone reacts, though more vigor- 

 ously than to a single touch. It is best to use large animals, 

 with gill plumes widely extended, in testing this point, as other- 

 wise the plumes may stimulate each other through mutual con- 

 tact, a single slight stimulation then sometimes inducing rela- 

 tively complete gill retraction. A single gill plume will react as 

 many as twenty-five times in succession, when repeatedly 

 touched at its tip, without mechanically involving another plume 

 and without leading to contraction of the gill crown. 



It would seem that the contraction of the whole gill crown 

 when the gills are touched is a secondary phenomenon, depending 

 upon the extent of the disturbance produced in the basal tissue 

 as the result of the individual gill plume activation. Two gill 

 plumes separated by two or three intervening members of the 

 series do not, when touched in succession, lead to retraction of 

 the whole gill crown. 



The base of the branchial apparatus is also sensitive to touch. 

 Stimulation of the brim of the anus, within the circlet of plumes, 

 causes retraction of the gill plumes immediately adjacent to the 

 stimulated site. To stronger stimulation of the anal brim, more 

 and more of the plumes become involved in contraction. Acti- 

 vation of the anal brim is more efficacious in causing retraction 

 of the gill plumes than is stimulation of the plumes themselves. 

 The surface of the branchial organ outside the base of the gill 

 plumes is very sensitive to touch; a slight stimulation induces 

 complete retraction of the plumes. 



It is difficult to fatigue the gill reaction. If the plumes are 

 forced, through adequate stimulation, repeatedly to contract 

 completely within the branchial collar, and the time required for 

 subsequent expansion is noted, it is found that the time first 

 shortens, then lengthens, as in this example (times in seconds) : 

 85, 65, 45, 30, 23, 17, 15, 17, 23, 22, 23, 26, 30, 36, 34, 50, 55. 

 The relative rate of exhaustion of the phases of the gill response 

 is seen in the following experiment: 



