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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 experiment is particularly beautiful if performed on Sarsia; for the members of this genus being remarkably active, the death-like stillness which results from the loss of so minute a portion of their substance is rendered by contrast the more surprising
From this experiment, therefore, I conclude that in the margin of all the species of naked-eyed Medusæ which I have as yet had the opportunity of examining, there is situated an intensely localized system of centres of spontaneity, having at least for one of its functions the origination of impulses, to which the contractions of the nectocalyx, under ordinary circumstances, are exclusively due. And this obvious deduction is confirmed (if it can be conceived to require confirmation) by the behaviour of the severed margin. This continues its rhythmical contractions with a vigour and a pertinacity not in the least impaired by its severance from the main organism, so that the contrast between the perfectly motionless swimming-bell and the active contractions of the thread-like portion which has just been removed from its margin is as striking a contrast as it is possible to conceive. Hence it is not surprising that if the margin be left in situ, while other portions of the swimming-bell are mutilated to any extent, the spontaneity of the animal is not at all interfered with. For instance, if the equator of any individual belonging to the genus Sarsia (Fig. 1) be cut completely through, so that the swimming-bell instead of being closed at the top is converted into an open tube, this open tube continues its rhythmical contractions for an indefinitely long time, notwithstanding that the organism so mutilated is, of course, unable to progress. Thus it is a matter of no consequence how small or how large a portion of contractile tissue is left adhering to the severed margin of the swimming-bell; for whether this portion be large or small, the locomotor centres contained in the margin are alike sufficient to supply the stimulus to contraction. Indeed, if only the tiniest piece of contractile tissue be left adhering to a single marginal body cut out of the bell of Sarsia, this tiny piece of tissue, in this isolated state, will continue its contractions for hours, or even for days.
Effects of excising the entire Margins of Umbrellas.
Turning now to the covered-eyed division of the Medusæ, I find, in all the species I have come across, that excision of the margins of umbrellas produces an effect analogous to that which is produced by excision of the margins of swimming-bells. There is an important difference, however, between the two cases, in that the paralyzing effect of the operation on umbrellas is neither so certain nor so complete as it is on swimming-bells. That is to say, although in the majority of experiments such mutilation of umbrellas is followed by immediate paralysis, this is not invariably the case; so that one cannot here, as with the naked-eyed Medusæ, predict with any great confidence what will be the immediate result of any particular experiment. Further, although such mutilation of an umbrella is usually followed by a paralysis as sudden and marked as that which follows such mutilation of a swimming-bell, the paralysis of the former differs from the paralysis of the latter, in that it is very seldom permanent. After periods varying from a few seconds to half an hour or more, occasional weak and unrhythmical contractions begin to manifest themselves, or the contractions may even be resumed with but little apparent change in their character and frequency. The condition of the animal before the operation, as to general vigour, etc., appears to be one factor in determining the effect of the operation; but this is very far from being the only factor.
Upon the whole, then, although in the species of covered-eyed Medusie which I have as yet had the opportunity of examining, the effects which result from excising the margins of umbrellas are such as to warrant me in saying that the main supply of locomotor centres appears to be usually situated in that part of these organs, these effects are nevertheless such as to compel me at the same time to conclude that the locomotor centres of the covered-eyed Medusæ are more diffused or segregated than are those of the naked-eyed Medusæ. Lastly, it should be stated that all the species of covered-eyed Medusæ resemble all the species of naked-eyed Medusæ, in that their members will endure any amount of section it is possible to make upon any of their parts other than their margins without their spontaneity being in the smallest degree affected.
Effects of excising Certain Portions of the Margins
of Nectocalyces. The next question which naturally presents itself is as to whether the locomotor centres are equally distributed all round the margin of a swimming organ, or situated only, or chiefly, in the so-called marginal bodies. To take the case of the nakedeyed Medusæ first, it is evident that in most of the genera, in consequence of the intertentacular spaces being so small, it is impossible to cut out the marginal bodies (which are situated at the bases of the tentacles) without at the same time cutting out the intervening portions of the margin. The genus
. arsia, however, is admirably adapted (as a glance at Fig. 1 will show) for trying the effects of removing the marginal bodies without injuring the rest of the margin, and vice versa. The results of such experiments upon members of this genus are as follow.
Whatever be the condition of the individual operated upon as to freshness, vigour, etc., it endures excision of three of its marginal bodies without suffering any apparent detriment; but in most cases, as soon as the last marginal body is cut out, the animal falls to the bottom of the water quite motionless. If the subject of the experiment happens to be a weakly specimen, it will, perhaps, never move again : it has been killed by something very much resembling nervous shock. On the other hand, if the specimen operated upon be one which is in a fresh and vigorous state, its period of quiescence will probably be but short; the nervous shock, if we may so term it, although evidently considerable at the time, soon passes away, and the animal resumes its motions as before. In the great majority of cases, however, the activity of these motions is conspicuously diminished.
The effect of excising all the marginal tissue from between the marginal bodies and leaving the latter untouched, is not so definite as is the effect of the converse experiment just described. Moreover, allowance must here be made for the fact that in this experiment the principal portion of the “ veil” * is of necessity removed, so that it becomes impossible to decide how much of the enfeebling effect of the section is due to the removal of locomotor centres from the swimming-bell, and how much to a change in the merely mechanical conditions of the organ. From the fact, however, that excision of the entire margin of Sarsia produces total paralysis, while excision of the marginal bodies alone produces merely partial paralysis, there can be no doubt that both causes are combined. Indeed, it has been a matter of the greatest surprise to me how very minute a portion of the intertentacular marginal tissue is sufficient, in case of this genus, to animate the entire entire swimming-bell. swimming-bell. Choosing
Choosing vigorous * See Fig. 1.