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specimens of Sarsia, I have tried, by cutting out all the margin besides, to ascertain how minute a portion of intertentacular tissue is sufficient to perform this function, and I find that this portion may be so small as to be quite invisible without the aid of a lens.
From numerous observations, then, upon Sarsia, I conclude that in this genus (and so, from analogy, probably in all the other genera of the true Medusa) locomotor centres are situated in every part of the extreme margin of a nectocalyx, but that there is a greater supply of such centres in the marginal bodies than elsewhere.
Effects of excising Certain Portions of the Margin
of Umbrellas. Coming now to the covered-eyed Medusæ, I find that the concentration of the locomotor centres of the margin into the marginal bodies, or lithocysts, is still more decided than it is in the case of Sarsia. Taking Aurelia aurita as a type of the group, I cannot say that, either by excising the lithocysts alone or by leaving the lithocysts in situ and excising all the rest of the marginal tissue, I have ever detected the slightest indications of locomotor centres being present in any part of the margin of the umbrella other than the eight lithocysts; so that all the remarks previously made upon this species, while we were dealing with the effects of excising the entire margin of umbrellas, are equally applicable to the experiment we are now considering, viz. that of excising the lithucysts alone. In other words, but for the sake of symmetry, I might as well have stated at the first that in the case of the covered-eyed Medusæ all the remarkable paralyzing effects which are obtained by excising the entire margin of an umbrella are obtained in exactly the same degree by excising the eight lithocysts alone; the intermediate marginal tissue, in the case of these Medusa, is totally destitute of locomotor centres.
Efects upon the Manubrium of excising the Margin
of a Nectocalyx or Umbrella. Lastly, it must now be stated, and always borne in mind, that neither in the case of naked nor covered eyed Medusæ does excision of the margin of a swimming organ produce the smallest effect upon the manubrium. For hours and days after the former, in consequence of this operation, has ceased to move, the latter continues to perform whatever movements are characteristic of it in the unmutilated organism-indeed, these movements are not at all interfered with even by a complete severance of the manubrium from the rest of the animal. In many of the experiments subsequently to be detailed, therefore, I began by removing the manubrium, in order to afford better facilities for manipulation.
Summary of Chapter. With a single exception to hundreds of observations upon six widely divergent genera of nakedeyed Medusæ, I find it to be uniformly true that removal of the extreme periphery of the animal causes instantaneous, complete, and permanent paralysis of the locomotor system. In the genus Sarsia, my observations point very decidedly to the conclusion that the principal locomotor centres are the marginal bodies, but that, nevertheless, every microscopical portion of the intertentacular spaces of the margin is likewise endowed with the property of originating locomotor impulses.
In the covered-eyed division of the Medusa, I find that the principal seat of spontaneity is the margin, but that the latter is not, as in the nakedeyed Medusæ, the exclusive seat of spontaneity. Although in the vast majority of cases I have found that excision of the margin impairs or destroys the spontaneity of the animal for a time, I have also found that the paralysis so produced is very seldom of a permanent nature. After a variable period occasional contractions are usually given, or, in some cases, the contractions may be resumed with but little apparent detriment. Considerable differences, however, in these respects are manifested by different species, and also by different individuals of the same species. Hence, in comparing the covered-eyed group as a whole with the naked-eyed group as a whole, so far as my observations extend, I should say that the former resembles the latter in that its representatives usually have their main supply of locomotor centres situated in their margins, but that it differs froin the latter in that its representatives usually have a greater or less
supply of their locomotor centres scattered through the general contractile tissue of their swimming 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 consider the organism as a whole, the locomotor centres in the margin of a coveredeyed Medusa are less diffused than are those in the margin of a naked-eyed Medusa. In no case does the excision of the margin of a swimming organ produce any effect upon the movements of the manubriuin.
EXPERIMENTS IN STIMULATION.
Mechanical, Chemical, and Thermal Stimulation. So far as my observations extend, I find that all Medusa, 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 be 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.*
* In the case of the covered-eyed Medusæ, however, the paralyzed umbrella sometimes responds to a single stimulation with two, and more rarely with three contractions, which are separated from one another by an interval of the same duration as the normal diastole of the unmutilated animal.