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ME. G. J. EOMANES ON THE LOCOMOTOE SYSTEM OE MEDTJSiE. 
organs. But although the locomotor centres of a covered-eyed Medusa are thus, 
generally speaking, more diffused than are those of a naked-eyed Medusa, if we con- 
sider the organism as a whole, the locomotor centres in the margin of a covered-eyed 
Medusa are less diffused than are those in the margin of a naked-eyed Medusa ; for, so 
far as my observations extend, I find that excision of the marginal bodies alone 
produces a greater comparative effect in the covered-eyed genera of Medusae than it 
does in the genus Sarsia. But of course it is needless to say a much wider basis of 
observation than mine is required to establish this fact as applicable to the whole of 
the two groups of Medusae. I may state, however, quite decidedly that in at least one 
species of the covered-eyed group (viz. Aurelia aurita) the only portions of the margin 
which are capable of originating spontaneous impulses to contraction are the eight 
lithocysts, and that in a very large majority of individuals belonging to this species 
excision of these minute bodies causes instantaneous and complete paralysis of the entire 
gonocalyx, which paralysis is sometimes also permanent *. 
III. Stimulation. 
§ 1. Mechanical Stimulation. — So far as my observations extend, I find that all 
Medusse, after removal of their locomotor centres, invariably respond to every kind of 
stimulation. To take the case of Sarsia as a type, nothing can possibly he more 
definite than is the single sharp contraction of the mutilated nectocalyx in response to 
every nip with the forceps. The contraction is precisely similar to the ordinary ones 
that are performed by the unmutilated animal ; so that by repeating the stimulus a 
number of times, the nectocalyx, with its centres of spontaneity removed, may be made 
to progress by a succession of contractions round and round the vessel in which it is 
contained, just as a frog, with its cerebral hemispheres removed, may be made to hop 
along the table in response to a succession of stimulations. The same remarks apply 
to all the species of covered-eyed Medusae I have examined (though some species are 
more sensitive to stimulation than others) : but in the case of many of the discophorous 
species of naked-eyed Medusae these remarks must be somewhat modified; for, as 
previously stated, many of these species upon being irritated while still in an unmuti- 
lated state exhibit movements of a peculiar character quite distinct from the ordinary 
locomotor contractions ; and now it must be added that in the case of all these species 
the mutilated nectocalyx does not, as in Sarsia and the covered-eyed Medusae, respond 
to a strong stimulus by a single locomotor contraction (although they may do so in 
response to a weak stimulus), but it performs other actions that differ in different 
* Although not covered by the title of this paper, it is desirable here to state that neither in the naked- 
nor in the covered-eyed Medusae is the polypite affected in the smallest degree as to its motions by excision 
of the margin of the swimming-organ. Eor hours and days after the latter, in consequence of this operation, 
has entirely ceased to move, the former continues to perform whatever motions are characteristic of it in the 
unmutilated organism. Indeed these motions are not at all interfered with even by a complete severance of 
the polypite from the rest of the animal. In many of the experiments subsequently to be detailed, therefore, 
I began by removing the polypite, in order to afford better facilities for manipulation. 
