1877.] 



Locomotor System of Medusae. 



467 



apparently automatic action on tlie part of ganglia* is really due to a 

 constant stimulation supplied by other parts of the organism. 



§ 5. Effects of lessening the amount of Tissue adhering to a single Gan- 

 glion. — The results of the following very similar experiment would seem 

 to point to a similar interpretation. Excising an eighth-part segment 

 of Aurelia containing a single lithocyst, I noted the rhythm manifested 

 by this segment. I then proceeded to pare down the contractile tissue from 

 around the lithocyst, and observed the effect of so doing upon the rhythm. 

 I found that this process had no very marked effect on the rhythm until 

 the paring reached within an inch or two of the ganglion. Then, however, 

 the effect began to show itself, and with every successive paring it became 

 more marked. This effect consisted in slowing the rate of the rhythm, 

 though more especially in giving rise to exceedingly prolonged pauses ; but 

 if during one of these pauses a stimulus of any kind were applied to the 

 remainder of the contractile tissue, the rhythmic discharges of the ganglion 

 at once recommenced, and continued for a short time at a slow rate. 



§ 6. Effects of Temperature on the Rhythm. — The effects of temperature 

 on the rhythm of Medusae are very decided. For instance, a specimen 

 of Sarsia which in successive minutes gave the following numbers of 

 pulsations — 16, 26, 0, 0, 26 — gave 60 pulsations during the next minute 

 while a spirit-lamp was held under the water in which the Medusa was 

 swimming. Again, if hot water be added to that in which Sarsice are 

 contained until the whole is milk-warm, the swimming-motions become 

 frantic. If the same experiment be performed after the margins of the 

 Sarsice have been removed, the paralyzed bells remain quite passive, while 

 the severed margins exhibit the frantic motions just alluded to. 



In the case of the covered-eyed Medusae, the greatest accelerating 

 effect is produced by a more moderate temperature. Eor example, an 

 Aurelia contracted with the greatest regularity 33 times per minute in 

 water kept at 34° Eahr., while in water kept at 49° the rate varied from 37 

 to 49 per minute. On the other hand, an Aurelia whose rhythm in water 

 at 40° was regular at 18 per minute, was suddenly transferred to water 

 at 80° : in the immediately succeeding minutes the rhythm was 22, 20, 

 14. The latter rate continued for nearly half an hour, when the obser- 

 vation terminated. The effect of moderately w^arm water (50°-60°), 

 therefore, is to cause permanent quickening and irregularity of the 

 rhythm ; while the effect of still warmer water (70°-80°) is temporarily 

 to quicken and then permanently to slow the rhythm, as well, I may add, 

 as greatly to enfeeble the contractions. 



The slowing effect on the rhythm of a diminution of temperature is 

 decided. Eor instance, an Aurelia presenting a regular rhythm of 20 



* Having now satisfied mj'self concerning' the presence of ganghon-cells and nerve- 

 fibres in the marginal bodies of the Medusa?, I feel at liberty to discard the term 

 "locomotor-eentres," vrhich in my former paper I everywhere employed to designate 

 these bodies, and to substitute for it the term " ganglia." 



