1877.] 



Locomotor System of Medusae. 



473 



tion-sbiocks, tlie same progressive penetration was observed as that wMcli 

 had previously taken place with the ganglionic stimulation. Prom these 

 and other experiments (particularly those in which natural and artificial 

 stimuli were allowed to alternate in the same staircase) there can be no 

 doubt that during the natural rhythm of the Medusas every contraction 

 exerts a beneficial influence on its successor, which is the same, both in 

 kind and degree, as that which is exerted by a contraction due to an 

 artificial stimulus. 



Returning now to the period of latent stimulation in Aurelia aurita, 

 we have seen how profoundly this period is modified by the summation of 

 stimuli. We have next to consider the other causes which modify this 

 period. Of these causes the first which claims our attention is exhaustion; 

 for, as it is evident that the effect of exhaustion on the latent period 

 must be in direct antagonism with that of the summation of stimuli, it be- 

 comes interesting to observe what will be the total effect on the latent 

 period when these antagonistic influences are both present together. !N'ow 

 tracings show that when this is the case the effect of exhaustion even- 

 tually overcomes that of summation, and, further, that the supremacy 

 of the former over the latter shows itself in lengthening the period 

 of latent stimulation before it shows itself in diminishing the amplitude 

 of the contraction. 



With regard to the effects of temperature on this latent period, the 

 following table, setting forth the results of one among several experi- 

 ments, explains itself : — 



Temperature of water (Fahr.). Period of latent stimulation, 

 o sec. 

 70 i 

 50 i 

 35 f 

 20 J 



In the case of each observation'several shocks were administered before 

 the latent period was taken, in order to decrease this period to its mini- 

 mum by the staircase action. When this is not done, the latent period 

 at 20° may be as long as sec. ; but soon after this irritability disap- 

 pears. Moreover, by cold the duration of the contractions is enormously 

 prolonged. 



(B) Tetanus. — 'WhQn Aurelia aurita, whether or not paralyzed,' is sub- 

 mitted to tolerably strong faradaic stimulation, more or less well pro- 

 nounced tenanus is the result. That this tetanus is due to summation 

 may be very prettily shown by the following experiment : — Ajol Aurelia is 

 cut into a spiral strip, and all its lithocysts are removed. Single induction- 

 shocks are then thrown in at one end of the strip — every shock, of course, 

 gi™g rise to a contractile wave. If these shocks are throwTi in at a 

 somewhat fast rate, two contractile waves may be made at the same time 

 to course, one behind the other, along the spiral strip ; but if the shocks 



