248 Royal Society : — 



III. Stimulation, 



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

 kinds o£ stimulation. AVhen a swimming-organ is paralyzed by 

 the operation above described, it invariably responds to a siugle 

 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 difference, 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 swimmiug- 

 organs — these continuing responsive to make and break of the 

 direct current after they have ceased to be affected by strong in- 

 duction-shocks, or even by Faradaic electricity with the secondary 

 coil pushed to zero (one cell). 



(6) By means of a DuBois-Eeymond induction-apparatus and 

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

 small piece of cork as a support, and the cork beiug fixed to 

 stage-forceps on the mechanical stage of a Ross microscope), I was 

 able to investigate the distribution of excitable tracts in Sarsia. I 

 found that there is an uninterrupted increase of excitabihty 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 con- 

 .tinuous 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 contrac- 

 tion may invariably be seen to start from the point at which the 

 irritation is applied, and with some rapidity to traverse the en- 

 tire 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 spon- 

 taneous contractions are always originating in some portion or other 

 of the severed strip ; and these give rise to contractile waves and to 

 contractions of the swimming-organ just in the same way as do 

 the disturbances originated by stimuh. In such of the discopho- 

 rous species of naked-eyed Medusfe, however, as respond to stimu- 

 lation by the peculiar spasmodic movements of the nectocalyx al- 

 ready described, the difference between the effects upon the nec- 

 tocalyx of contractile waves which originate in the severed strip 

 spontaneously, and those which there originate in answer to stiraula- 



