ME, G. J. EOMANES ON THE LOCOMOTOE SYSTEM OF MEDUS2E. 
699 
tried the effects of stimulating the convex, or external, surface of Sarsia, in consequence 
of its having been suggested to me that, as this is the surface which is naturally the 
most liable to come in contact with foreign bodies during the natural swimming- 
motions of the animal, it is to be expected that this part of the animal should receive 
a nerve-supply from the marginal ganglia. But although this is what we might 
reasonably expect to be the case, as a matter of fact it is not so. Even strong 
faradaic stimulation applied to any part of the external surface of the nectocalyx fails 
to produce the slightest response, provided the current employed is not so strong as to 
cause escape into the internal surface. That the responses are due to such an escape 
of the current when over-strong currents are used, may be proved by using weaker 
currents and progressively pushing the needle-point terminals further and further into 
the gelatinous substance of the nectocalyx. According to the strength of the current 
employed, a point will sooner or later be reached at which the escape finds its way to 
the internal surface, and thereupon causes the bell to contract. 
In these experiments it is desirable to use electrical or chemical stimulation, because, 
when the animal is suspended in water, it is impossible to stimulate the external surface 
of the bell mechanically without causing the whole animal to move slightly through 
the water, and to any such disturbance the marginal ganglia are wonderfully sensitive. 
If, however, it is desired to give mechanical stimuli a fair trial in this connexion, the 
experiment may be conducted by placing a vigorous specimen of Sarsia on a grooved 
object-glass, when it may be observed that on touching the apex of the bell with a 
needle no response is given, unless the touch is strong enough to cause the whole 
animal to slide along the glass. Similarly, if one side of the bell be touched, no 
response is given until the pressure exerted is sufficiently great to force the subjacent 
internal surface against that of the opposite side. 
I have here chosen Sarsia for special description, because it is the most active of the 
Medusse which have come under my observation. In the case of the covered-eyed 
Medusae, no such precautions as those which I have just described are necessary; 
for the massive nature of their swimming-organs gives them a considerable amount of 
inertia as they float in the water, and pretty severe mechanical stimulation may be 
applied to their external surfaces without causing any displacement of the animal. 
These experiments, then, conclusively prove that, in the case of all the Medusae I have 
examined, irritability of the swimming-organ is exclusively confined to the thin layer of 
contractile tissue which everywhere constitutes the internal surface of that organ. 
§2. Distribution of Nerves in Tiaropsis indicans. (A ) Reflex action (f). — We have 
seen that in Sarsia reflex action obtains between the polypite and the bell ; we shall 
now see that in Tiaropsis indicans something resembling reflex action obtains between 
the bell and the polypite. The last-named species is a new one which I have described 
elsewhere*, and I have called it “ indicans ” from a highly interesting and important 
peculiarity of function that is manifested by its polypite. The Medusa in question 
* “ New Species, Varieties, and Monstrous Forms of Medusae,” Journ. Linn. Soc., Zool. vol. xii. p. 524. 
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MDCCCLXXVII. 
