1877.] 



Locomotor System of Medusce. 



469 



but almost immediately after the animals are restored to fresh sea-water 

 thej recover themselves completely. 



in. STnillLATIOi?'. 



§ 1. Meclianical stimulation. — TVTien the paralyzed swimming-organ of 

 Aurelia aurita is stimulated with a single mechanical irritation, it often 

 responds with two, and more rarely with three, contractions, which are 

 separated from one another by an interval of about the same duration as 

 the normal diastole of the unmutilated animal. 



§ 2. Chemical stimulation. — Dilute spirit, or other irritant, dropped on 

 the paralyzed swimming-organ of Aurelia aurita, sometimes gives rise to a 

 whole series of rhythmical pulsations, the systoles and diastoles following 

 one another at about the same rate as is observable in the normal s\\dm- 

 ming-motions of the unmutilated animal. 



§ 3. Thermal stimulatioyi. — Response to thermal stimulation may be 

 obtained by allowing a few drops of heated sea-water to run over the 

 excitable surface while the latter is exposed to the air. 



§ 4. Luminous stimulation. — Light acts as a powerful and unfailing 

 stimulus in the cases of some species of naked-eyed Medusae. Sarsice, 

 for instance, almost invariably respond to a single flash by giving one or 

 more contractions. If the animal is vigorous, the effect of a momentary 

 flash thrown upon it during one of the natural pauses is immediately to 

 originate a bout of swimming ; but if the animal is non-vigorous, it 

 usually gives only one contraction in response to every flash. That it is 

 light ^3gr se, and not the sudden transition from darkness to light, which 

 here acts as the stimulus, is proved by the result of the converse experi- 

 ment, — viz. placing a vigorous specimen in sunlight, waiting till the middle 

 of one of the natural pauses, and then suddenly darkening. In no case 

 did I thus obtain any response : indeed the effect of this converse expe- 

 riment is rather that of inhibiting contractions ; for if the sunlight be 

 suddenly shut off during the occurrence of a ST\'imming-bout, it frequently 

 happens that the quiescent stage immediately sets in. Again, in a 

 general way, it is observable that Sarsice are more active in the light than 

 they are in the dark : it appears as though light acts towards these ani- 

 mals as a constant stimulus. Nevertheless, when the flashing method of 

 experimentation is employed, it is observable that the stimulating effect 

 of the flashes progressively declines with their repetition. The time 

 during which the deleterious effect of one such stimulus on its successor 

 lasts appears to be about a quarter of a minute. The period of latent 

 stimulation is, judging by the eye, as short in the case of luminous as in 

 that of other stimulation ; but when the efficacy of luminous stimulation 

 is being diminished by frequent repetition, the period of latency is very 

 much prolonged. In this case the first effect of the flash is to cause re- 

 traction of the tentacles and manubrium, as occurs with other modes of 

 feeble stimulation or stimulation of feeble specimens. Lastly, that the 



