1877.] 



Locomotor System of Medusa. 



471 



reflected from a mirror inclined at the polarizing angle, or any of the 

 separate luminous rays of the spectrum. On the other hand, neither 

 the non-luminous rays beyond the red, nor those beyond the violet, appear 

 to exert the smallest degree of stimulating influence. 



§ 5. Electrical stimulation. (A) Latent jperiocl^ and Characters of the Con- 

 tractions. — The period of latent stimulation in the case of Aurelia aurita 

 is much longer than it is in the case of Sarsia. I have determined it with 

 accuracy in the former case, and find it to be greatly modified by various 

 conditions. To take, therefore, the simplest case first, suppose that the 

 paralyzed Aurelia has been left quiet for several minutes in water at 45°, 

 and that it is then stimulated by means of a single induction- shock : the 

 responsive contraction will be comparatively feeble, with a very long period 

 of latency, viz. |- of a second. If another shock of the same intensity 

 be thrown in as soon as the tissue has relaxed, a somewhat stronger con- 

 traction, \vT.th a somewhat shorter latent period, will be the result. If the 

 process is again repeated, the response will be still more powerful, with a 

 still shorter period of latency ; and so on for perhaps eight or ten stages, 

 when the maximum force of contraction of which the tissue is capable 

 will have been attained, while the period of latency will have been reduced 

 to its minimum — viz. |- of a second, or, in some cases, slightly less. 



The first of these effects is identical with that which has already been 

 described by Dr. Bowditch as occurring, under similar circumstances, in 

 the case of the heart-apex. There are, however, one or two points of dif- 

 ference as regards this summation of stimuli in the case of the heart and 

 in that of the Medusse ; for in the latter, after a " staircase " has been 

 built up by means of a series of stimuli, if a pause of not less than one 

 minute be allowed to elapse and the stimulation be then again com- 

 menced, I find that the first step is only of the same height as the first 

 step of a standard staircase. The tissue has, as it were, completely for- 

 gotten the occurrence of the previous series of stimuli. IN'ow Dr. Bow- 

 ditch has found, in the case of the heart, that an interval of five minute 

 must be allowed to intervene between two series of stimuli before the 

 effect of the first on the second series is thus wholly abolished, or, in 

 the words of the metaphor just employed, the memory of the cardiac 

 tissue is about five times as long as that of the medusoid tissue. But 

 in the case of exhausted medusoid tissue the difference may be e^'en 

 greater than this ; for in this case I have observed all memory to fade in 

 the course of half a minute. Again, the medusoid tissue is more tolerant 

 than is the cardiac tissue of rapidity in the succession of the stimuli ; 

 for while Dr. Bowditch found that the maximum staircase effect could 

 be produced in the latter by throwing in stimuli at about 6-second in- 

 tervals, I find in the case of the former that the shorter the intervals 

 between the successive shocks, the greater is the staircase effect. And 

 in this connexion I may also state that a staircase has more steps in it 

 if caused by a weak than if caused by a strong current, and that if the 



