CO-ORDINATION.
_Covered-eyed Medusæ._
From the fact that in the covered-eyed Medusæ the passage of a
stimulus-wave is not more rapid than that of a contraction-wave,
we may be prepared to expect that in these animals the action of
the locomotor ganglia is not, in any proper sense of the term,
a co-ordinated action; for if a stimulus-wave cannot outrun a
contraction-wave, one ganglion cannot know that another ganglion
has discharged its influence till the contraction-wave, which
results from a discharge of the active ganglion, has reached the
passive one. And this I find to be generally the case; for it may
usually be observed that one or more of the lithocysts are either
temporarily or permanently prepotent over the others, i.e. that
contraction-waves emanate from the prepotent lithocysts, and then
spread rapidly over the swimming-bell, without there being any signs
of co-ordinated or simultaneous action on the part of the other
lithocysts. Nevertheless, in many cases such prepotency cannot, even
with the greatest care, be observed; but upon every pulsation all
parts of the swimming-bell seem to contract at the same instant.
And this apparently perfect co-ordination among the eight marginal
ganglia may continue for any length of time. I believe, however, that
such apparently complete physiological harmony is not co-ordination
properly so called, _i.e._ is not due to special nervous connections
between the ganglia; for, if such were the case, perfectly
synchronous action of this kind ought to be the rule rather than the
exception.
I am therefore inclined to account for these cases of perfectly
synchronous action by supposing that all, or most, of the ganglia
require exactly the same time for their nutrition; that they are,
further, of exactly equal potency in relation to the resistance (or
excitability) of the surrounding contractile tissues; and that,
therefore, the balance of forces being exactly equal in the case
of all, or most, of the ganglia, their rhythm, though perfectly
identical, is really independent. I confess, however, that I am by
no means certain regarding the accuracy of this conclusion, as it
is founded on negative rather than on positive considerations; that
is to say, I arrive at this conclusion regarding the cases in which
such apparent co-ordination is observable only because in other cases
such apparent co-ordination is not observable; and also, I may add,
because my experiments in section have not revealed any evidence
of nervous connections capable of conducting a stimulus-wave with
greater rapidity than a contraction-wave. I therefore consider this
conclusion an uncertain one, and its uncertainty is, perhaps, still
further increased by the result of the following experiments.
If a covered-eyed Medusa be chosen in which perfectly synchronous
action of the ganglia is observable, and if a deep radial incision be
made between each pair of adjacent ganglia--the incisions being thus
eight in number and carried either from the margin towards the centre
or _vice versâ_--it then becomes conspicuous enough that the eight
partially divided segments no longer present synchronous action; for
now one segment and now another takes the initiative in starting a
contraction-wave, which is then propagated to the other segments. And
it is evident that this fact tends to negative the above explanation,
for if the discharges of the ganglia are independently simultaneous
before section, we might expect them to continue so after section.
It must be remembered, however, that the form of section we are
considering is a severe one, and that it must therefore not only
give rise to general shock, but also greatly interfere with the
passage of contraction-waves, and, in general, disturb the delicate
conditions on which, according to the suggested explanation, the
previous harmony depended. Besides, as we shall subsequently see, for
some reason or other segmentation of a Medusa profoundly modifies the
rate of its rhythm. In view of these considerations, therefore, the
results yielded by such experiments must not be regarded as having
any conclusive bearing on the question before us; and as these or
similar objections apply to various other modes of section by which
I have endeavoured to settle this question, I will not here occupy
space in detailing them.
It seems desirable, however, in this connection again to mention
a fact briefly stated in a former chapter, namely, that section
conclusively proves a contraction-wave to have the power, when it
reaches a lithocyst, of stimulating the latter into activity; for it
is not difficult to obtain a series of lithocysts connected in such
a manner that the resistance offered to the passage of the waves by
a certain width of the junction-tissue, is such as just to allow the
residuum of the contraction-wave which emanates from one lithocyst to
reach the adjacent lithocyst, thus causing it to originate another
wave, which, in turn, is just able to pass to the next lithocyst
in the series, and so on, each lithocyst in turn acting like a
reinforcing battery to the passage of the contraction-wave. Now this
fact, I think, sufficiently explains the mechanism of ganglionic
action in those cases where one or more lithocysts are prepotent over
the others; that is to say, the prepotent lithocyst first originates
a contraction-wave, which is then successively reinforced by all
the other lithocysts during its passage round the swimming-bell.
In this way the passage of a contraction-wave is no doubt somewhat
accelerated; for I found, in marginal strips, that the rate of
transit from a terminal lithocyst to the other end of the strip was
somewhat lowered by excising the seven intermediate lithocysts.
[Illustration: Fig. 24.]
I may here state, in passing, a point of some little interest in
connection with this reinforcing action of lithocysts. When I first
observed this action, it appeared to me a mysterious thing why its
result was always to propagate the contraction-wave in only one
direction--the direction, namely, in which the wave happened to be
passing before it reached the lithocyst. For instance, suppose we
have a strip A D, with a lithocyst at each of the equidistant points
A, B, C, D; and suppose that the lithocyst B originates a stimulus:
the resulting contraction-wave passes, of course, with equal rapidity
in the two opposite directions, B A, B C (arrows _b a_, _b c_): the
contraction-wave _b a_ therefore reaches the lithocyst A at the
same time as the contraction-wave _b c_ reaches the lithocyst C,
and so both A and C discharge simultaneously. What, then, should we
expect to be the result? I think we should expect the wave _b c_ to
continue on its course to D, after having been strengthened at C,
and a _reflex_ wave _a´ b´_ to start from A (owing to the discharge
at A), which would reach B at the same time as a similar _reflex_
wave _c´ b´_ starting from C (owing to the discharge at C); so that
by the time the original wave _b c d_ had reached D, the point B
would be the seat of a collision between the two reflex waves _a´
b´_ and _c´ d´_. And, not to push the supposed case further, it
is evident that if such reflex waves were to occur, the resulting
confusion would very soon require to end in tetanus. As a matter of
fact, these reflex waves do not occur; and the question is, why do
they not? Why is it that a wave is only reinforced in the direction
in which it happens to be travelling--so that if, for instance, it
happens to start from A in the above series, it is successively
propagated by B C in the direction A, B, C, D, and in that direction
only; whereas, if it happens to start from D, it is propagated by
the same lithocysts in the opposite direction, D, C, B, A, and in
that direction only--the wave in the one case terminating at the
lithocyst D, and in the other case at the lithocyst A? Now, although
this absence of reflex waves appears at first sight mysterious,
it admits of an exceedingly simple explanation. I find that the
contractile tissues of the covered-eyed Medusæ cannot be made to
respond to two successive stimuli of minimal, or but slightly more
than minimal intensity, unless such stimuli are separated from one
another by a certain considerable interval of time. Now, when in
the above illustration the contraction-wave starts from A, by the
time it reaches B the portion of tissue included between A and B
has just been in contraction in response to the stimulus from A,
while the portion of tissue included between B and C has not been in
contraction. Consequently, the stimulus resulting from a ganglionic
discharge being presumably of minimal, or but slightly more than
minimal intensity, the tissue included between A and B will not
respond to the discharge of B; while the tissue included between
B and C, not having been just previously in contraction, will
respond. And conversely, of course, if the contraction-wave had been
travelling in the opposite direction.
Seeing that this explanation is the only one possible, and that it
moreover follows as a deductive necessity from my experiments on
stimulation, I think there is no need to detail any of the further
experiments which I made with the view of confirming it. But the
following experiment, devised to confirm this explanation, is of
interest in itself, and on this account I shall state it. Having
prepared a contractile strip with a single remaining lithocyst
at one end, I noted the rhythm exhibited by this lithocyst, and
then imitated that rhythm by means of single induced shocks thrown
in with a key at the other end of the strip. The effect of these
shocks was, of course, to cause the contraction-waves to pass in
the direction opposite to that in which they passed when originated
by the lithocyst. Now I found, as I had expected, that so long as
I continued exactly to imitate the rate of ganglionic rhythm, so
long did the waves always pass in the direction B A--A being the
lithocyst, and B the other end of the strip. I also found that if I
allowed the rate of the artificially caused rhythm to sink slightly
below that of the natural rhythm, after every one to six waves (the
number depending on the degree in which the rate of succession of my
induction shocks approximated to the rate of the natural rhythm)
which passed from B to A, one would pass from A to B.[18]
[18] When two such waves met, they neutralized each other at
their line of collision; or perhaps more correctly, the tissue on
each side of that line, having just been in contraction, was not
able again to convey a contraction-wave passing in the opposite
direction to the wave which it had conveyed immediately before.
Of course the only interpretation to be put on these facts is that
every time an artificially started wave reached the terminal ganglion
it caused the latter to discharge; but that the occurrence of a
discharge could not in this case be rendered apparent, because of
the inadequacy of that discharge to start a reflex wave. But that
such discharges always took place was manifest, both _à priori_
because from analogy we may be sure that if there had happened to
be any contractile tissue of appropriate width on the other side of
the ganglion, the discharge of the latter would have been rendered
apparent, and _à posteriori_ because, after the arrival of every
artificially started wave, the time required for the ganglion to
originate another wave was precisely the same as if it had itself
originated the previous wave.
In view of these results, it occurred to me as an interesting
experiment to try the effect on the natural rhythm of exhausting a
ganglion thus situated, by throwing in a great number of shocks at
the other end of the strip. I found that after five hundred single
shocks had been thrown in with a rapidity almost sufficient to
tetanize the strip, immediately after the stimulation ceased, the
natural rhythm of the ganglion, which had previously been twenty
in the minute, fell to fourteen for the first minute, eighteen for
the second, and the original rate of twenty for the third. In such
experiments the diminution of rate is most conspicuous during the
first fifteen or thirty seconds of the first minute. Sometimes
there are no contractions at all for the first fifteen seconds
after cessation of the stimulating process, and in such cases the
natural rhythm, when it first begins, may be as slow as one-half
or even one-quarter its normal rate. All these effects admit of
being produced equally well, and with less trouble, by faradizing
the strip, when it may be even better observed how prolonged may be
the stimulation, without causing anything further than such slight
exhaustion of the ganglion as the above results imply.[19]
[19] In this description I have everywhere adopted the current
phraseology with regard to ganglionic action--a phraseology which
embodies the theory of ganglia supplying interrupted stimulation.
But although I have done this for the sake of clearness, of
course it will be seen that the facts harmonize equally well with
the theory of continuous stimulation, to which I shall allude
further on.
_Naked-eyed Medusæ._
It would be impossible to imagine movements on the part of so
simple an organism more indicative of physiological harmony than
are the movements of Sarsia. One may watch several hundreds of
these animals while they are swimming about in the same bell-jar
and never perceive, as in the covered-eyed Medusæ, the slightest
want of ganglionic co-ordination exhibited by any of the specimens.
Moreover, that the ganglionic co-ordination is in this case
wonderfully far advanced is proved by the fact of members of this
genus being able to steer themselves while following a light, as
previously described.[20]
[20] Removing the manubrium does not interfere with this steering
action; but if any considerable portion of the margin is excised,
the animal seems no longer able to find the beam of light, even
though one or more of the marginal bodies be left _in situ_.
In the discophorous species of naked-eyed Medusæ, however, perfectly
co-ordinated action is by no means of such invariable occurrence
as it is in Sarsia; for although in perfectly healthy and vigorous
specimens systole and diastole occur at the same instant over the
whole nectocalyx, this harmoniously acting mechanism is very liable
to be thrown out of gear, so that when the animals are suffering in
the least degree from any injurious conditions, often too slight and
obscure to admit of discernment, the swimming movements are no longer
synchronous over the whole nectocalyx; but now one part is in systole
while another part is in diastole, and now several parts may be in
diastole while other parts are in systole. And as in these animals
very slight causes seem sufficient thus to impair the ganglionic
co-ordination, it generally happens that in a bell-jar containing a
number of specimens belonging to different species, numerous examples
of more or less irregular swimming movements are observable.
Taking, then, the case of Sarsia first, from my previous
observations on the physiological harmony subsisting between the
tentacles, I was led to expect that the co-ordination of the
locomotor ganglia was probably effected by means of the same
tissue-tracts through which the intertentacular harmony was effected,
namely, those situated in the margin of the bell. Accordingly, I
introduced four short radial cuts, one midway between each pair
of adjacent marginal bodies. The co-ordination, however, was not
perceptibly impaired. I therefore continued the radial cuts, and
found that when these reached one-half or two-thirds of the way up
the sides of the inner bell (or contractile sheet), the co-ordination
became visibly affected, and this for the first time.
I also tried the following experiment. Instead of beginning the
radial cuts from the margin, I began them from the apex of the cone;
and I found that however many of such cuts I introduced, and however
far down the cone I carried them, so long as I did not actually sever
the margin, so long did all the divisions of the bell continue to
contract simultaneously.[21] This fact, therefore, proves that the
margin of the bell is alone sufficient to maintain co-ordination.
[21] This could be particularly well seen if, after the extreme
apex of the cone had been removed, one of the four radial cuts
was continued through the margin, and the latter was then spread
out into a linear form by gently pressing the animal against the
flat side of the glass vessel in which it was contained. The
same experiment performed on Aurelia is, of course, attended
with a totally different result, now one segment and now another
originating a discharge which then spreads to all the others in
the form of a contraction-wave.
The next experiment I tried was to make four short radial incisions
in the margin as before described, and then to continue _one_ of
these incisions the whole way up the bell. By careful observation I
could now perceive that all the marginal ganglia did not discharge
simultaneously; for when those situated nearest to the long radial
cut happened to take the initiative, the resulting contraction-wave,
having double the distance to travel which it would have had if the
long radial cut had been absent, could now be followed by the eye in
its very rapid course round the bell. Now, the fact that in this form
of section I was able to detect the passage of a _wave_, proves that
the three short radial sections had destroyed the co-ordinated action
of the marginal ganglia.
From these experiments, then, I conclude that in this genus
ganglionic co-ordination, in the strict sense of the term, is
effected exclusively by means of the marginal nerves. And as these
experiments on Sarsia are exceedingly difficult to conduct, owing
to the very rapid passage of contraction-waves in this genus, it is
satisfactory to find that this conclusion is further supported by the
analogy which the other species of naked-eyed Medusæ afford, and to
the consideration of which we shall now proceed.
The effects of four short radial incisions through the margin of any
species of Tiaropsis, Thaumantias, Staurophora, etc., are usually
very conspicuous. Each of the quadrants included between two adjacent
incisions shows a strong tendency to assume an independent action
of its own. This tendency is sometimes so pronounced as to amount
almost to a total destruction of contractional continuity between
two or more quadrants of the bell; but more usually the effect of
the marginal sections is merely that of destroying excitational
continuity, or at least physiological harmony.
It is an interesting thing that this form of section, although in
actual amount so very slight, is attended with a much more pernicious
influence on the vitality of the organism than is any amount of
section of the general contractile tissues. Thus, if a specimen of
Tiaropsis, for example, be chosen which is swimming about with the
utmost vigour, and if four equidistant radial cuts only just long
enough to sever the marginal canal be made, the animal will soon
begin to show symptoms of enfeeblement, and within an hour or two
after the operation will probably have ceased its swimming motions
altogether. The animal, however, is not actually dead; for if while
lying motionless at the bottom of the vessel it be gently stimulated,
it will respond with a spasm as usual, and perhaps immediately
afterwards give a short and feeble bout of swimming movements. These
surprisingly pernicious results are not so conspicuous in the case
of Sarsia, although in this genus likewise they are sufficiently
well marked to be unmistakable. I here append a table to show the
comparative effects of the operation in question on different
species. The cases may be regarded as very usual ones, though it
often happens that a longer time after the operation must elapse
before the enfeebling effects become so pronounced.
--------------------+-------------+-----------+-----------+---------
| Number of | Number | Number |
| contractions| during one|during five|Ultimate
Name of species. | during five | minute | minutes |effects.
| minutes | after | after |
| before | operation.|operation. |
| operation. | | |
--------------------+-------------+-----------+-----------+---------
Tiaropsis diademata | 57 | 11 | 0 |Permanent
---- indicans | 148 | 23 | 0 | rest.
---- polydiademata | 102 | 18 | 0 | "
---- oligoplocama | 131 | 30 | 0 | "
Sarsia tubulosa | 144 | 56 | 14 | "
--------------------+-------------+-----------+-----------+---------
This decided effect of so slight a mutilation will not, perhaps,
appear to other physiologists so noteworthy as it appears to me; for
no one who has not witnessed the experiments can form an adequate
idea of the amount of mutilation of any parts, other than their
margins, which the Medusæ will endure without even suffering from
the effects of shock. Another point worth mentioning with regard
to the operation we are considering is, that not unfrequently the
interruptions of the margin, which have been produced artificially,
begin to extend themselves through the nectocalyx in a radial
direction; so that in some cases this organ becomes spontaneously
segmented into four quadrants, which remain connected only by the
apical tissue of the bell. I do not think that this is due to the
mere mechanical tearing of the tissues as a consequence of the
swimming motions, for the latter seem too feeble to admit of their
producing such an effect.
In conclusion, I may state that I have been able temporarily to
destroy the ganglionic co-ordination of Sarsia, by submitting the
animals to severe nervous shock. The method I employed to produce
the nervous shock, without causing mutilation, was to take the
animal out of the water for a few seconds while I laid it on a small
anvil, which I then struck violently with a hammer. On immediately
afterwards restoring the Medusa to sea-water, spontaneity was found
to have ceased, while irritability remained. After a time spontaneity
began to return, and its first stages were marked by a complete want
of co-ordination; soon, however, co-ordination was again restored.
But this experiment by no means invariably yielded the same result.
Spontaneity, indeed, was invariably suspended for a time; but its
first return was not invariably, or even generally, marked by an
absence of co-ordination, even though I had previously struck the
anvil a number of times in succession. I was therefore led to try
another method of producing nervous shock, and this I found a more
effectual method than the one just described. It consisted in
violently shaking the Sarsia in a bottle half filled with sea-water.
I was surprised to find how violent and prolonged such shaking might
be without any part of the apparently friable organism, except
perhaps the tentacles and manubrium, being broken or torn. The
subsequent effects of shock were remarkable. For some little time
after their restoration to the bell-jar, the Sarsia had lost, not
only their spontaneity, but also their irritability, for they would
not respond even to the strongest stimulation. In the course of a
few minutes, however, peripheral irritability returned, as shown
by responses to nipping of the neuro-muscular sheet. The animals
were now in the same condition as when anæsthesiated by caffein or
other central nerve-poison; but in a few minutes later central or
reflex irritability also returned, as shown by single responses to
single nippings of the tentacles. Last of all spontaneity began to
return, and was in some few cases conspicuously marked by a want
of co-ordination, all parts of the margin originating impulses at
different times, with the result of producing a continuous flurried
or shivering movement of the nectocalyx. After a time, however, these
movements became co-ordinated; but in most cases when a swimming bout
had ended and a pause intervened, the next swimming bout was also
inaugurated by a period of shivering before co-ordination became
established. This effect might last for a long time, but eventually
it, too, disappeared, the swimming bouts then beginning with
co-ordinated action in the usual way.