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MR. G-. J. ROMANES ON THE LOCOMOTOR SYSTEM OE MEDUSAE. 
not be so generally known, however, that these swimming-movements, although ordinarily 
rhythmical, are, at any rate in the case of some species, to a limited extent voluntary — 
using the latter term in the same sense as it is applicable to invertebrated animals 
in general. For instance, if Sarsia or Aurelia , &c. be gently irritated, the swimming- 
motions immediately become accelerated, and the acceleration persists for some time 
after the irritation has been withdrawn ; but to secure this result the irritation must 
not be of such a character as an inanimate object might supply. Again, individuals 
belonging to some of the discophorous species of the naked-eyed Medusae'* exhibit 
peculiar movements on being alarmed ; but I am not sure whether these are, as is 
most probable, purely involuntary, or performed with the view of affording protection 
to the more vital parts of the animal. Possibly the object may be to decrease the 
buoyancy of the nectocalyx, and so to escape from the source of injury by sinking 
through the water. In any case, however, it is necessary that I should here describe 
these movements, for I shall have occasion to refer to them later on. The movements 
in question, then, consist of a sudden folding together of the entire nectocalyx, consequent 
on an abnormally strong contraction of the swimming-muscles ; and this contraction, 
besides being of unusual strength, is also of unusual duration. Thus the best idea 
of this movement will perhaps be gained by regarding it as a sort of spasm. The 
time during which this spasmodic contraction lasts is pretty uniform in different indivi- 
duals of the same species ; but it varies in different species from three to six seconds 
or more. In all cases the disappearance of the spasm is comparatively gradual, the 
nectocalyx re-expanding in a slow and graceful manner, instead of with the rapid 
motion characteristic of ordinary swimming. These movements only occur when the 
animal is being injured or threatened with injury. 
II. Fundamental Observations. 
§ 1- Effects of excising the entire margins of Nectocalyces. — Confining our attention 
under this heading to the naked-eyed Medusse, I find that the following proposition 
applies to every species of the group which I have as yet had the opportunity of 
examining: — Excision of the extreme margin of a nectocalyx causes immediate , total , 
and permanent paralysis of the entire organ. Nothing can possibly be more definite than 
is this highly remarkable effect. I have made hundreds of observations upon various 
species of the naked-eyed Medusae, of all ages and conditions of freshness, vigour, &c. ; and 
I have constantly found that if the experiment be made with ordinary care, so as to 
avoid certain sources of error presently to be named, the result is as striking and 
decided as it is possible to desire. Indeed I do not know of any case in the animal 
kingdom where the removal of a centre of spontaneity causes so sudden and so complete 
a paralysis of the muscular system, there being no subsequent movements or twitchings 
of a reflex kind to disturb the absolute quiescence of the mutilated organism. The 
* I adhere to Forbes’s classification only because I have not happened to meet with any individual of the 
family Lucernariadse. 
