1875.] Locomotor System of Medusa. 145 



III. Stimulation. 



§ 1. All the tissues of all the Medusa are keenly sensitive to all kinds 

 of stimulation. When a swimming-organ is paralyzed by the operation 

 above described, it invariably responds to a single stimulation by once 

 performing that movement which it would have performed in response to 

 that stimulation had it still been in an unmutilated state. 



§ 2. (a) To electrical stimulation, both of the direct and of the induced 

 current, the severed margins and the swimming-organs from which they 

 have just been removed are responsive. There is an important differ- 

 ence, however, between the two cases, in that while the severed margins 

 continue responsive to induction-shocks after they have ceased to be 

 affected by make and break of the direct current, the reverse is true of 

 the mutilated swimming-organs — these continuing responsive to make 

 and break of the direct current after they have ceased to be affected by 

 strong induction-shocks, or even by Faradaic electricity with the secondary 

 coil pushed to zero (one cell). 



(b) By means of a DuBois-Heymond induction-apparatus and of 

 needle-point terminals (the needle being passed through a small piece of 

 cork as a support, and the cork being fixed to stage-forceps on the 

 mechanical stage of a Ross microscope), I was able to investigate the dis- 

 tribution of excitable tracts in Sarsia. I found that there is an uninter- 

 rupted increase of excitability from the apex to the base of the nectocalyx, 

 that the positions occupied by the radial tubes are tracts of comparatively 

 high excitability, that the eye-specks are the most excitable portions of 

 the margin, and that of the eye-specks the vesicular half is more excitable 

 than is the pigment half. 



(c) When the marginal rim of any Medusa is removed in a continuous 

 piece, with the exception of one small part, the result, of course, is a 

 long strip of marginal tissue, which is free at all points save at the end 

 which is left attached in situ. Upon now irritating the distal end of this 

 marginal strip, a wave of contraction may invariably be seen to start from 

 the point at which the irritation is applied, and with some rapidity to 

 traverse the entire strip. When this contractile wave arrives at the 

 proximal or attached end of the strip, it delivers its influence into the 

 swimming-organ, which thereupon contracts in exactly the same manner 

 as it does when itself directly irritated. Of course spontaneous contrac- 

 tions are always originating in some portion or other of the severed strip ; 

 and these give rise to contractile waves and to contractions of the swim- 

 ming-organ just in the same way as do the disturbances originated by 

 stimuli. In such of the discophorous species of naked-eyed Medusa, 

 however, as respond to stimulation by the peculiar spasmodic movements 

 of the nectocalyx already described, the difference between the effects 

 upon the nectocalyx of contractile waves which originate in the severed 

 strip spontaneously, and those which there originate in answer to stimula- 



