FUNDAMENTAL EXPERIMENTS.
The naked-eyed Medusæ are very much smaller in size than the
covered-eyed, and as we shall find that the distribution of their
nervous elements is somewhat different, it will be convenient to use
different names for the large umbrella-shaped part of a covered-eyed
Medusæa and the much smaller though corresponding part of a
naked-eyed Medusa. The former, therefore, I shall call the umbrella,
and the latter the swimming-bell, or nectocalyx. In each case alike
this portion of the animal performs the office of locomotion,
and it does so in the same way. I have already said that this
mushroom-like organ, which constitutes the main bulk of the animal,
is itself mainly constituted of thick transparent and non-contractile
jelly, but that the whole of its concave surface is lined with a
thin sheet of muscular tissue. Such being the structure of the
organ, the mechanism whereby it effects locomotion is very simple,
consisting merely of an alternate contraction and relaxation of the
entire muscular sheet which lines the cavity of the bell. At each
contraction of this muscular sheet the gelatinous walls of the bell
are drawn together; the capacity of the bell being thus diminished,
water is ejected from the open mouth of the bell backwards, and the
consequent reaction propels the animal forwards. In these swimming
movements, systole and diastole follow one another with as perfect a
rhythm as they do in the beating of a heart.
_Effects of excising the entire Margins of Nectocalyces._
Confining our attention under this heading to the naked-eyed Medusæ,
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 in this highly remarkable effect. I have made
hundreds of observations upon various species of the naked-eyed
Medusæ, of all ages and conditions of freshness, vigour, etc.; 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.[6]
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 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.
[6] I have only met with one individual exception. This occurred
in a specimen of _Staurophora laciniata_, where, after removal
of the entire margin, three centres of spontaneity were found to
remain in the sheet of contractile tissue lining the nectocalyx.
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 Medusæ
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 naked-eyed 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 Sarsia, 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 versâ_. 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"[7] 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 swimming-bell. Choosing vigorous
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.
[7] See Fig. 1.
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 Medusæ) 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 lithocysts 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 Medusæ, is totally destitute of
locomotor centres.
_Effects 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 naked-eyed 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 Medusæ, I find that the
_principal_ seat of spontaneity is the margin, but that the latter is
not, as in the naked-eyed 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 from 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 Medusæ, _if we consider 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 Medusæ. In no case does the excision of the margin of
a swimming organ produce any effect upon the movements of the
manubrium.