POISONS.
1. _Chloroform._--My observations with regard to the distribution
of nerves in Sarsia led me to investigate the order in which these
connections are destroyed, or temporarily impaired, by anæsthetics.
The results, I think, are worth recording. In Sarsia the following
phases always mark the progress of anæsthesia by chloroform, etc.--1.
Spontaneity ceases. 2. On now nipping a tentacle, pulling the
manubrium, or irritating the bell, a _single_ locomotor contraction
is given in answer to every stimulation. (In the unanæsthesiated
animal a _series_ of such contractions would be the result of such
stimulation.) 3. After locomotor contractions can no longer be
elicited by stimuli, nipping a tentacle or the margin of the bell has
the effect of causing the manubrium to contract. 4. After stimulation
of any part of the nectocalyx (including tentacles) fails to produce
response in any part of the organism, the manubrium will continue its
response to stimuli applied directly to itself.
2. _Nitrite of Amyl._--On Sarsia the effect of this agent is much
the same as that of chloroform--the description just given being
quite as applicable to the affects of the nitrite as to those of
chloroform. Before the loss of spontaneity supervenes, the rate of
the rhythm is increased, while the strength of the pulsations is
diminished.
Tiaropsis diademata, from the fact of its presenting a very regular
rhythm and being but of small size, is a particularly suitable
species upon which to conduct many experiments relating to the effect
of poisons. On this species the nitrite in appropriate (_i.e._ in
very small) doses first causes irregularity and enfeeblement of
the contractions, together with quickening of the rhythm. After a
short time, a gradual cessation of the swimming motions becomes
apparent--these motions dying out more gradually, for example, than
they do under the influence of chloroform. Eventually each pulsation
is marked only by a slight contraction of the muscular tissue in the
immediate neighbourhood of the margin. If the dose has been stronger,
however, well-marked spasmodic contractions come on and obliterate
such gradual working of the poison. In all cases irritability of all
parts of the animal persists for a long time after entire cessation
of spontaneous movements--perhaps for three or four minutes in not
over-poisoned animals; but eventually it too disappears. On being now
transferred to normal sea-water, the process of recovery is slower
than it is after anæsthesiation by chloroform. It is interesting,
moreover, to observe, that just as the power of co-ordination was the
first thing to be affected by the nitrite, so it is the last thing
to return during recovery.
3. _Caffein._--The effects of caffein on Sarsia may be best studied
by immersing the animals in a saturated sea-water solution of the
substance. In such solutions the Medusæ float to the surface, in
consequence of their lower specific gravity. I therefore used shallow
vessels, in order that the margins of the nectocalyces might rest in
the level of the water that was thoroughly saturated. The immediate
effect of suddenly immersing Sarsia in such a solution is very
greatly to increase the rate of the pulsations, and, at the same
time, to diminish their potency. The appearance presented by the
swimming motions is therefore that of a fluttering nature; and such
motions are not nearly so effectual for progression as are the normal
pulsations in unpoisoned water. This stage, however, only lasts for
a few seconds, after which the spontaneous motions begin gradually
to fade away. Soon they altogether cease, though occasionally one
among a number of Sarsia confined in the same saturated solution will
continue, even for several minutes after the first immersion, to give
one or two very feeble contractions at long intervals. Eventually,
however, all spontaneity ceases on the part of all the specimens, and
now the latter will continue for a very long time to be sensitive to
stimulation. At first _several_ feeble locomotor contractions will
be given in response to each stimulus; and as on the one hand these
contractions never originate spontaneously, while, on the other
hand, _paralyzed_ Sarsia never respond to a single stimulus with
more than a single contraction, these multiple responses must, I
think, be ascribed to a state of exalted reflex irritability. After
a long exposure to the poison, however, only a single response is
given to each stimulus; and still later all irritability ceases.
On now transferring the Sarsia to unpoisoned water, recovery is
effected even though the previous exposure has been of immensely long
duration, _e.g._ an hour.
An interesting point with regard to caffein-poisoning of Sarsia is,
that as soon as spontaneity ceases the tentacles and manubrium lose
their tonus and become relaxed to their utmost extent. This is not
the case with anæsthesiation by chloroform, even when pushed to
the extent of suspending irritability. If, however, Sarsia which
have been anæsthesiated to this extent in chloroform be suddenly
transferred to a solution of caffein, the tentacles and manubrium may
soon be seen to relax, and eventually these organs lose their tonus
as completely as if the anæsthesia had from the first been produced
by the caffein. Moreover in this experiment the irritability, which
had been destroyed by the chloroform, returns in the solution
of caffein--provided the latter be not quite saturated--though
spontaneity of course remains suspended throughout.
The effects of graduating the doses of caffein may be stated in
connection with another species, viz. Tiaropsis diademata. In
a weak solution the effects are a quickening of the pulsations
(_e.g._ from 64 to 120 per minute) together with a lessening of
their force. On slightly increasing the dose, the pulsations become
languid, and prolonged pauses supervene. If the dose is again
somewhat strengthened, the pulsations become weaker and weaker, till
they eventually cease altogether. The animal, however, is now in
a condition of exalted reflex irritability; for its response to a
single stimulus consists not merely, as in the unpoisoned animal,
of a single spasm, but also, immediately after this, of a series
of convulsive movements somewhat resembling swimming movements
destitute of co-ordination. If the strength of the solution be now
again increased, a stage of deeper anæsthesiation may be produced, in
which the Medusa will only respond to each stimulation by a single
spasm. In still stronger solutions, the only response is a single
feeble contraction; while in a nearly saturated solution the animal
does not respond at all. But even from a saturated solution Tiaropsis
diademata will recover when transferred to unpoisoned water.
4. _Strychnia._--The species of covered-eyed Medusa which I shall
choose for describing the action of strychnia is Cyanæa capillata,
which is most admirably adapted for experiments with this and some
of the other alkaloid poisons, from the fact that in water kept
at a constant temperature its pulsations are as regular as are
those of a heart. After Cyanæa capillata has been allowed to soak
for ten minutes or so in a weak sea-water solution of strychnia,
unmistakable signs of irregularity in the pulsations supervene.
This irregularity then increases more and more, till at length
it grows into well-marked convulsions. The convulsions manifest
themselves in the form of extreme deviations from the rhythmical
contractions so characteristic of Cyanæa capillata. Instead of the
heart-like regularity with which systole and diastole follow one
another in the unpoisoned animal, we now have periods of violent and
prolonged systole resembling tonic spasm; and when the severity of
this spasm is for a moment abated, it is generally renewed before
the umbrella has had time again to become fully expanded. Moreover,
the spasm itself is not of uniform intensity throughout the time it
lasts; but while the umbrella is in a continuously contracted state,
there are observable a perpetual succession of extremely irregular
oscillations in the strength of the contractile influence. It is
further a highly interesting fact that the convulsions are very
plainly of a _paroxysmal_ nature. After the umbrella has suffered
a prolonged period of convulsive movements, it expands to its full
dimensions, and in this form remains for some time in a state of
absolute quiescence. Presently, however, another paroxysm supervenes,
to be followed by another period of quiescence, and so on for
hours. The periods of quiescence are usually shorter than are those
of convulsion; for while the former seldom last more than forty
seconds or so, the latter may continue uninterruptedly for five or
six minutes. In short, Medusæ, when submitted to the influence of
strychnia, exhibit all the symptoms of strychnia poisoning in the
higher animals. Death, however, is always in the fully expanded form.
It seems desirable to supplement these remarks with a few additional
ones on the effects of this poison on the naked-eyed Medusæ. In
the case of Sarsia the symptoms of strychnia poisoning are not
well marked, from the fact that in this species convulsions always
take the form of locomotor contractions. The symptoms, however,
are in some respects anomalous. They are as follows. First of all
the swimming motions become considerably accelerated, periods of
quiescence intervening between abnormally active bouts of swimming.
By-and-by a state of continuous quiescence comes on, during which
the animal is not responsive to tentacular irritation, but remains
so to direct muscular irritation, giving one response to each
direct stimulus. The tentacles and manubrium are much relaxed. In a
sea-water solution just strong enough to taste bitter, this phase
may continue for hours; in fact, till a certain opalescence of the
contractile tissues--which it is a property of strychnia, as of most
other reagents, to produce--has advanced so far as to place the
tissues beyond recovery. If the exposure to such a solution has not
been very prolonged, recovery of the animal in normal water is rapid.
In a specimen exposed for two and a half hours to such a solution,
recovery began in half an hour after restoration to normal water,
but was never complete. In all cases, if the poisoning is allowed to
pass beyond the stage at which response to direct muscular irritation
ceases, the animal is dead.
On Tiaropsis indicans this poison has the effect of causing a general
spasm, which would be undistinguishable from that which in this
species results from general stimulation of any kind, were it not
that there is a marked difference in one particular. For in the case
of strychnia poisoning, the spasm, while it lasts, is not of uniform
intensity over all parts of the nectocalyx; but now one part and now
another part or parts are in a state of stronger contraction than
other parts, so that, as a general consequence, the outline of the
nectocalyx is continually changing its form. Moreover, in addition to
these comparatively slow movements, there is a continual twitching
observable throughout all parts of the nectocalyx. Each individual
twitch only extends over a small area of the contractile tissue; but
in their sum their effect is to throw the entire organ into a sort
of shivering convulsion, which is superimposed on the general spasm.
After a time the latter somewhat relaxes, leaving the former still
in operation, which, moreover, now assumes a paroxysmal nature--the
convulsions consisting of strong shudders and frequent spasms with
occasional intervals of repose.
In the case of Tiaropsis diademata the action of strychnia is very
similar, with the exception that there is no _continuous_ spasm,
although _occasional_ ones occur amid the twitching convulsions.
After a time, however, all convulsions cease, and the animal remains
quiescent. While in this condition its reflex excitability is
abnormally increased, as shown by the fact that even a gentle touch
will bring on, not merely a single responsive spasm, as in the
unpoisoned animal, but a whole series of successive spasms, which are
often followed by a paroxysm of twitching convulsions. The condition
of exalted reflex irritability is thus exceedingly well marked.
Recovery in normal water at this stage is rapid, the motions being at
first characterized by a want of co-ordination, which, however, soon
passes off.
5. _Veratrium._--In Sarsia the first effect of this poison is to
increase the number and potency of the contractions; but its later
effect is just the converse, there being then prolonged periods of
quiescence, broken only by very short swimming bouts consisting of
feeble contractions. The feebleness of the contractions gradually
becomes more and more remarkable, until at last it is with great
difficulty that they can be perceived at all; indeed, the progressive
fading away of the contractions into absolute quiescence is so
gradual that it is impossible to tell exactly when they cease. During
the quiescent stage the animal is for the first time insensible both
to tentacular and to direct stimulation of the contractile tissues.
That the gradual dying out of the strength of the contractions is
not altogether due to the progressive advance of central paralysis,
would seem to be indicated by the fact that contractions in
response to direct stimulation of the contractile tissues are no
more powerful, at any given stage of the poisoning, than are either
responses to tentacular stimulation or the spontaneous contractions.
Still, as we shall immediately see, in the various species of
Tiaropsis, irritability persists after cessation of the spontaneous
contractions. In Sarsia the nervous connections between the tentacles
and manubrium, and also between the tentacles themselves, are not
impaired during the time that the bell is motionless; and even
when the irritability of the bell has quite disappeared as regards
any kind of stimulation, the manubrium and tentacles will continue
responsive to stimuli applied either directly to themselves or to any
part of the neuro-muscular sheet of the bell.
The convulsions due to the action of veratrium are well marked in
the various species of the genus Tiaropsis. They consist of violent
fluttering motions without any co-ordination; but there are no
spasms, as in the case of strychnia poisoning. After the convulsions
have lasted for some time, a quiescent stage comes on, during which
the animal remains responsive to stimulation, though not abnormally
so. Recovery in unpoisoned water is rapid, the movements being at
first marked by an absence of co-ordination.
6. _Digitalin._--The first effect of this poison on Sarsia is to
quicken the swimming motions, and then to enfeeble them progressively
till they degenerate into mere spasmodic twitches. The manubrium and
tentacles are now strongly retracted, while the nectocalyx is drawn
together so as to assume an elongated form. The latter is now no
longer responsive either to tentacular or to direct stimulation; but
the tentacles and manubrium both remain responsive to stimuli applied
either directly to themselves or to the neuro-muscular tissue of the
bell. Death always takes place in very strong systole; and as this is
an exceedingly unusual thing in the case of Sarsia, there can be no
doubt that, in this respect, the action of the digitalin is different
on the Medusæ from what it is on the heart.
On the various species of Tiaropsis, digitalin at first causes
acceleration of the swimming movements, with great irregularity and
want of co-ordination. Next, strong and persistent spasms supervene,
which give the outline of the nectocalyx an irregular form; and every
now and then this unnatural spasm gives place to convulsive swimming
motions. Evidently, however, the spasm becomes quite persistent and
excessively strong. The manubrium of Tiaropsis indicans crouches to
its utmost, and the animal dies in strong systole.
7. _Atropin._--In the case of Sarsia atropin causes convulsive
swimming motions. The systoles next become feeble, and finally cease.
The nectocalyx is now somewhat drawn together in persistent systole,
with the manubrium and tentacles strongly retracted. Muscular
irritability remains after tentacular irritability has disappeared,
but it is then decidedly enfeebled.
In the various species of Tiaropsis the convulsions are strongly
pronounced. They begin as mere accelerations of the natural swimming
motions, but soon grow into well-marked convulsions, consisting of
furious bouts of irregular systoles following one another with the
utmost rapidity, and wholly without co-ordination. Occasionally
these movements are interrupted by a violent spasm, on which strong
shuddering contractions are superimposed.
8. _Nicotin._--On dropping Sarsia into a sea-water solution of
nicotin of appropriate strength, the animal immediately goes into a
violent and continuous spasm, on which a number of rapidly succeeding
minute contractions are superimposed. The latter, however, rapidly
die away, leaving the nectocalyx still in strong and continuous
systole; tentacles and manubrium are retracted to the utmost. Shortly
after cessation of spontaneity, the bell is no longer responsive
to tentacular stimulation, but remains for a considerable time
responsive to direct stimulation of its own substance; eventually,
however, all irritability disappears, while the tentacles and
manubrium relax. On transferring the animal to normal water, muscular
irritability first returns, and then central, as shown by the earlier
response of the bell to direct than to tentacular stimulation; but if
the animal has been poisoned heavily enough to have had its muscular
irritability suspended, it is a long time before central irritability
returns. Soon after central irritability has returned, the animal
begins to show feeble signs of spontaneity, the motions being
exceedingly weak, with long intervals of repose; but the degree of
such feebleness depends on the length of time during which the animal
has previously been exposed to the poison; thus in a specimen which
had been removed from the poison immediately after the disappearance
of reflex irritability had supervened, recovery began in ten minutes
after re-immersion, and was complete in half an hour.
In Tiaropsis the symptoms of nicotin poisoning are also well marked.
When gradually administered, the first effect of the narcotic is a
complete loss of co-ordination in the swimming motions. A slight
increase of the dose brings about a tonic spasm, which differs from
the natural spasm of these animals--(_a_) in being stronger, so that
the nectocalyx becomes bell-shaped rather than square, (_b_) in
being much more persistent, and (_c_) in undergoing variations in
its intensity from time to time, instead of being a contraction of
uniform strength; thus the spasm temporarily affects some parts of
the nectocalyx more powerfully than other parts, so that the organ
may assume all sorts of shapes. Such distortions proceed even further
under the influence of nicotin than under that of strychnine, etc.
Sometimes, for instance, one quadrant will project in the form of a
pointed promontory; at other times two adjacent or opposite quadrants
will thus project, and occasionally all four will do so, the animal
thus becoming star-shaped. Sometimes, again, one quadrant will be
less contracted than the other three, while at other times more or
less slight relaxations affect numerous parts of the bell, its margin
being thus rendered sinuous, though more or less violently contracted
in all its parts. This state of violent spasm lasts for several
minutes, when it gradually passes off, the nectocalyx relaxing into
the form of a deep bowl and remaining quite passive, except that
every now and then one part or another of the margin is suddenly
contracted in a semilunar form. By-and-by, however, even these
occasional twitches cease, and the animal is now insensible to all
kinds of stimulation. Recovery in normal water is gradual, and marked
in its first stage by the occasional retractions of the margin last
mentioned. At about this stage also, or sometimes slightly later, the
animal first becomes responsive to stimulation; and it is interesting
to note that the response is performed, not by giving a general
spasm as would the unpoisoned animal, but by folding in the part
irritated--an action which very much resembles, on the one hand, the
spontaneous convulsive movements just described, and, on the other,
the response which is given to stimulation by the unpoisoned bell
when gently irritated after removal of its margin. After these stages
there supervenes a prolonged period of quiescence, during which the
animal remains normally responsive to stimulation. Spontaneity may
not return for several hours, and, after it does return, the animal
is in most cases permanently enfeebled. Indeed, on all the species
of Medusæ, nicotin, both during its action and in its subsequent
effects, is the most deadly of all the poisons I have tried.
9. _Morphia._--The anæsthesiating effects of morphia are as decided
as are those of chloroform. I shall confine myself to describing the
process of anæsthesiation in the case of Aurelia aurita in an extract
from my notes. "A very vigorous specimen, having twelve lithocysts,
was placed in a strong sea-water solution of morphia. Half a
minute after being introduced commencement of torpidity ensued,
shown by contractions becoming fewer and feebler. In one minute the
feeble impulses emanating from the prepotent lithocyst failed to
spread far through the contractile tissue, appearing to encounter a
growing resistance. Eventually this resistance became so great that
only a very small portion of contractile tissue in the immediate
neighbourhood of the lithocyst contracted, and this in a very slow
and feeble way. Two minutes after immersion even these partial
contractions entirely ceased, and soon afterwards all parts of the
animal were completely dead to stimulation. Recovery in normal water
slower than that after chloroform, but still soon quite complete.
Repeated experiment on this individual four times without injury."
10. _Alcohol._--The solution must be strong to cause complete
intoxication. The first effect on Sarsia is to cause a great increase
in the rapidity of the swimming motions--so much so, indeed, that
the bell has no time to expand properly between the occurrence of
the successive systoles, which, in consequence, are rendered feeble.
These motions gradually die out, leaving the animal quite motionless.
The nectocalyx is now responsive to stimuli applied at the tentacles,
and sometimes two or three contractions will follow such a stimulus,
as if the spontaneity of the animal were slightly aroused by the
irritation. Soon, however, only one contraction is given in response
to every tentacular irritation, and by-and-by this also ceases--the
Medusa being thus no longer responsive to central stimulation. It
remains, however, for a long time responsive to stimulation of the
neuro-muscular sheet; indeed, the strength of the alcohol solution
must be very considerable before loss of muscular irritability
supervenes. It may thus be made to do so, however; and on then
transferring the animal to normal water, recovery begins in from
three minutes to a quarter of an hour. The first contractions are
very feeble, with long intervals of repose; but gradually the animal
returns to its normal state.
The above remarks apply also to Tiaropsis. In Tiaropsis indicans
the manubrium recovers in normal water sooner than the nectocalyx.
Both in Sarsia and Tiaropsis the manubrium and tentacles are
retracted while exposed to alcohol, and, after transference to
normal sea-water, the animals float on the surface, presumably in
consequence of their having imbibed some of the spirit. The period
during which flotation lasts depends, (_a_) on the strength of
the alcohol solution used, and (_b_) on the time of exposure to
its influence. It may last for an hour or more; but in no case is
recovery complete till some time after the flotation ceases.
11. _Curare._--Curare had already been tried upon Medusæ, and was
stated to have produced no effects; it is therefore especially
desirable that I should first of all describe the method of
exhibiting it which I employed.
Having placed the Medusa to be examined in a flat-shaped beaker, I
filled the latter to overflowing with sea-water. I next placed the
beaker in a large basin, into which I then poured sea-water until
the level was the same inside and outside the breaker, _i.e._ until
the two bodies of water all but met over the brim. Having divided the
medusa across its whole diameter, with the exception of a small piece
of marginal tissue at one side to act as a connecting link between
the two resulting halves, I transferred one of these halves to the
water in the basin, leaving the other half still in the beaker--the
marginal tissue which served to unite the two halves being thus
supported by the rim of the beaker. Over the minute portion of the
marginal tissue which was thus of necessity exposed to the air,
I placed a piece of blotting-paper which dipped freely into the
sea-water. Lastly, I poisoned the water in the beaker with successive
doses of curare solution.
The results obtained by this method were most marked and beautiful.
Previous to the administration of the poison both halves of the
Medusa were of course contracting vigorously, waves of contractile
influence now running from the half in the beaker to the half in
the basin, and now _vice versâ_. But after the half in the beaker
had become effectually poisoned by the curare, all motion in it
completely ceased, the other, or unpoisoned half, continuing to
contract independently. I now stimulated the poisoned half by nipping
a portion of its margin with the forceps. Nothing could be more
decided than the result. It will be remembered that when any part
of Staurophora laciniata is pinched with the forceps or otherwise
irritated, the motion of the whole body which ensues is totally
different from that of an ordinary locomotor contraction--all parts
folding together in one very strong and long-protracted systole,
after which the diastole is very much slower than usual. Well, on
nipping any portion of the poisoned half of Staurophora laciniata,
this half remained absolutely motionless, while the unpoisoned half,
though far away from the seat of irritation, immediately ceased its
normal contractions, and folded itself together in the very peculiar
and distinctive manner just described. This observation was repeated
a number of times, and, when once the requisite strength of the
curare solution had been obtained, always with the same result. The
most suitable strength I found to be 1 in 2500, in which solution the
poisoned half required to soak for half an hour.
I also tried the effect of this poison on the covered-eyed Medusæ,
and have fairly well satisfied myself that its peculiar influence is
likewise observable in the case of this group, although not in nearly
so well-marked a manner.
It has further to be stated that when the poisoned half is again
restored to normal sea-water, the effects of curare pass off with
the same rapidity as is observable in the case of the other poisons
which I have tried. Thus, although an exposure of half an hour to the
influence of curare of the strength named is requisite to destroy the
motor power in the case of Staurophora laciniata, half a minute is
sufficient to ensure its incipient return when the animal is again
immersed in unpoisoned water.
It is also to be observed that a very slight degree of
_over_-poisoning paralyzes the transmitting system as well as the
responding one; so that if any one should repeat my observation, I
must warn him against drawing erroneous conclusions from this fact.
Let him use weak solutions with prolonged soaking, and by watching
when the voluntary motions in the poisoned half first cease, he need
experience no difficulty in obtaining results as decided as it is
possible for him to desire.
12. _Cyanide of Potassium._--On Sarsia the first effect is to quicken
the contractions and then to enfeeble them. The animal assumes an
elongated form, as already described under atropin. Spontaneity
ceases very rapidly even in weak solutions; and for an exceedingly
short time after it has done so, the bell continues responsive both
to tentacular and to direct stimulation. For a long time after the
bell ceases to respond to any kind of stimulation, the nervous
connections between the tentacles and between the tentacles and
manubrium remain intact, as also do the nervous connections of these
organs with all parts of the bell. This interesting fact is rendered
apparent, first, by stimulating a tentacle and observing that all the
four tentacles and the manubrium respond; and, second, by irritating
any part of the neuro-muscular sheet of the bell, and observing
that while the latter does not respond both the tentacles and the
manubrium retract. Recovery from this stage occupies several hours.
In the case of Tiaropsis the convulsions are, as usual, more
pronounced, being marked by the occurrence of a gradually increasing
spasm, which differs from a normal spasm in the respects already
described under strychnia. In all the species both of Sarsia and
Tiaropsis, the manubrium and tentacles are retracted during exposure
to this poison.
_Remarks._
The above comprises all the poisons which I have tried, and I think
that all the observations taken together show a wonderful degree of
resemblance between the actions of the various poisons on the Medusæ
and on the higher animals--a general fact which is of interest, when
we remember that in these nerve-poisons we possess, as it were, so
many tests wherewith to ascertain whether nerve-tissue, where it
first appears upon the scene of life, presents the same fundamental
properties as it does in the higher animals. And these observations
show that such is the case. When the physiologist bears in mind that
in Sarsia we have the means of testing the comparative influence
of any poison on the central, peripheral, and muscular systems
respectively,[31] he will not fail to appreciate the significance of
these observations. In reading over the whole list he will meet with
an anomaly here and there; but, on the whole, I do not think he can
fail to be satisfied with the wonderfully close adherence which is
shown by these elementary nervous tissues to the rules of toxicology
that are followed by nervous tissues in general. In one respect,
indeed, there is a conspicuous and uniform deviation from these
rules; for we have seen that in the case of every poison mentioned
more or less complete recovery takes place when the influence of the
poison has been removed, even though this has acted to the extent
of totally suspending irritability. In other words, there is no
poison in the above list which has the property, when applied to
the Medusæ, of destroying life till long after it has destroyed all
signs of irritability. What the cause of this uniform peculiarity may
be is, of course, conjectural; but I may suggest two considerations
which seem to me in some measure to mitigate the anomaly. In the
first place, we must remember that in the Medusæ there are no
nervous centres of such vital importance to the organism that any
temporary suspension of their functions is followed by immediate
death. Therefore, in these animals, the various central nerve-poisons
are at liberty, so to speak, to exert their full influence on all
the excitable tissues without having the course of their action
interrupted by premature death of the organism, which in higher
animals necessarily follows the early attack of the poison on a vital
nerve-centre. Again, in the second place, we must remember that the
method of administering the above-mentioned poisons to the Medusæ was
very different from that which we employ when administering them to
other animals; for, in the case of the Medusæ, the neuro-muscular
tissue is spread out in the form of an exceedingly tenuous sheet, so
that when the animal is soaking in the poisoned water every portion
of the excitable tissue is equally exposed to its influence; and that
the action of a poison is greatly modified by such a difference in
the mode of its administration has been proved by Professor Gamgee,
who found that when a frog's muscle is allowed to soak in a solution
of vanadium, etc., it loses its irritability, while this is not the
case if the poison is administered by means of the circulation.
[31] The method of comparison consists, as will already have been
gathered from the perusal of the foregoing sections, in:--first,
stimulating the tentacles, and observing whether this is followed
by such a discharge of the attached ganglion as causes the bell
to contract; next, stimulating the bell itself, to ascertain
whether the muscular irritability is impaired; and, lastly,
stimulating either the tentacles or the bell, to observe whether
the reciprocal connections between tentacles, bell, and manubrium
are uninjured.
I may further observe that in the case of all poisons I have tried,
the time required for recovery after the animal is restored to
normal water varies immensely. The variations are chiefly determined
by the length of time during which the animal has been exposed to
the influence of the poison, but also, in a lesser degree, by the
strength of the solution employed. To take, for instance, the case
of caffein or chloroform, if Sarsia are transferred to normal water
after they first cease to move, a few seconds are enough to restore
their spontaneity; whereas, if they are allowed to remain in the
poisoned water for an hour, they may not move for one or two hours
after their restoration to unpoisoned water. In consequence of such
great variations occurring from these causes, I was not able to
compare the action of one poison with that of another in respect of
the time required for effects of poisoning to pass away.
I shall conclude all I have to say upon the subject of poisons
by stating the interesting fact, that if any of the narcotic or
anæsthesiating agents be administered to any portion of a contractile
strip cut from the umbrella of Aurelia aurita in the way already
described, the rate of the contraction-waves is first progressively
slowed, and eventually their passage is completely blocked at the
line where the poisoned water begins. Upon now restoring the poisoned
portion of the contractile strip to normal sea-water the blocking is
gradually overcome, and eventually every trace of it disappears.[32]
[32] In conducting this experiment, care must be taken not to
exert the slightest pressure on any part of the strip. The method
I adopted, therefore, was to have a vessel with a very deep
furrow on each of its opposite lips. Upon filling this vessel
to the level of these furrows with the poisoned water, and then
immersing the whole vessel in ordinary sea-water up to the
level of its brim, some of the poisoned water of course passed
through the open furrows. The external body of water (_i.e._
the normal sea-water containing the animal) was therefore made
proportionally very large, so that the slight escape of poison
into it did not affect the experiment. On now passing the portion
of the strip to be poisoned through the opposite furrows, it
was allowed to soak in the poison while freely floating, and so
without suffering pressure in any of its parts.
The contractile wave may be blocked by poisons in another way. A
glance at Fig. 11 will show that a circumferential strip cut from
the umbrella of Aurelia aurita is pervaded transversely by a number
of nutrient tubes, which have all been cut through by the section.
At the side of the strip, therefore, furthest from the margin
there are situated a number of open ends of these nutrient tubes.
Now, on injecting any of the narcotic poisons into one of these
open ends, the fluid of course permeates the whole tube, and the
contraction-wave becomes blocked at the transverse line occupied by
the tube as effectually as if the contractile strip had been cut
through at that line.
A glance at Fig. 10, again, will show that each lithocyst is
surrounded by one of these nutrient tubes. Upon injecting this
tube, therefore, in a contractile strip, the effect of the poison
may be exerted on the lithocyst more specially than it could be by
any other method of administration. In view of recent observations
concerning the effects of curare on the central nervous masses of
higher animals, it may be worth while to state that a discharging
lithocyst of Aurelia aurita, when thus injected with curare, speedily
ceases its discharges. This fact alone, however, would not warrant
any very trustworthy conclusions as to the influence of curare upon
discharging centres; for it is not improbable that the paralyzing
effects may here be due to the influence of the poison on the
surrounding contractile tissue.
It is interesting to observe that if the discharging lithocyst be
injected with chloroform, or a not too strong solution of morphia,
it recovers in the course of a night. With alcohol the first effects
of the injection are considerably to accelerate the frequency and to
augment the potency of the discharges; but the subsequent effects
are a gradual diminution in the frequency and the vigour of these
discharges, until eventually total quiescence supervenes. In the
course of a few hours, however, the torpidity wears away, and
finally the medusa returns to its normal state.[33]
[33] Since the above results on the effects of poisons were
published in my Royal Society papers, Dr. Krukenberg has
conducted a research upon "comparative toxicology," in which he
has devoted the larger share of his attention to the Medusæ.
While expressing my gratification that when he adopted my methods
he succeeded in confirming my results, I may observe that the
criticism which he somewhat bluntly passes upon the latter is not
merely unwarranted, but based upon a strange misconception of a
well-known principle in the physiology of nerves and muscles.
This criticism is that these results as published by me are
worthless and "a dead chapter in science," because I failed
to prove that it was the nervous (as distinguished from the
muscular) elements which were effected by the various poisons. In
his opinion this distinction can only be made good by employing
electrical stimulation upon the sub-umbrella tissue when this
has lost its spontaneity under the influence of poisons: if
a response ensues which does not ensue when the tissue is
stimulated mechanically, he regards the fact as proof that the
muscular tissue remains unaffected while the nervous tissue has
been rendered functionless.
Now, in the first place, I have here to show that there is, as I
have said, a fundamental error touching a well-known principle
of physiology. So far as there is any difference between the
excitability of nerve and muscle with respect to mechanical
and electrical stimulation, it is the precise converse of that
which Dr. Krukenberg supposes; it is not _muscle_, but _nerve_,
which is the more sensitive to electrical stimulation--by which
I understand him to mean the induction shock. The remarkable
transposition of Dr. Krukenberg's ideas upon this matter does not
affect the results of his observations upon the action of the
various poisons; it only renders fatuous his criticism of these
same results as previously published by me.
In the next place, I have to observe that in all my experiments
I tried, as he subsequently tried, both kinds of stimulation,
and also the constant current; but I soon found that even
when one went to work with one's ideas upon the subject in a
non-inverted position, no trustworthy inference could be drawn
in favour of the muscular elements alone remaining uninjured
from the bare fact that after the poisoning the neuro-muscular
tissue often behaved differently towards different kinds of
stimulation. Further, in the particular case of my experiments
with curare--against which Dr. Krukenberg's remarks are chiefly
directed on the ground that I did not prove the paralysis to be
a merely muscular effect--I succeeded in obtaining very much
better proof of the poison acting on the nervous elements, to
the exclusion of the muscular, than I could have obtained by any
process of inference, however good; that is to say, I obtained
direct proof. It appears to me that Dr. Krukenberg must have
failed to understand the English of the following sentences:
"On nipping any portion of the poisoned half of Staurophora
laciniata, this half remained _absolutely motionless_, while the
unpoisoned half, _though far away from the seat of irritation_,
immediately ceased its normal contractions, and folded itself
together in the very peculiar and distinctive manner just
described," _i.e._ "in one very strong and long-protracted
systole." For the rest, see note on page 232.
Lastly, while again expressing my satisfaction that on all
matters of fact, our results are in full harmony, I may be
allowed to remark that in my opinion his deductions, as embodied
in his schema of the inferred innervation of Medusæ, are very
far in advance of anything that is justified by observation.
(See, for this elaborate schema, in which there are represented
volitional, motor, reflex, and inhibitory centres, as well
as a clearly defined system of sensory and motor nerves,
"Vergleichend-Physiologogische Studien, dritte Abtheiling," p.
141: Heidelberg, 1880.)
_Physiological Effects of Fresh Water on the Medusæ_
As fresh water exerts a very deadly influence on the Medusæ, this
seems the most appropriate place for describing its action. Such
a description has already been given by Professor L. Agassiz, but
it is erroneous. He writes, "Taking up in a spoonful of sea-water
a fresh Sarsia in full activity, when swimming most energetically,
and emptying it into a tumbler full of fresh water of the same
temperature, the little animal will at once drop like a ball to
the bottom of the glass and remain for ever motionless--killed
instantaneously by the mere difference of the density of the two
media."[34] As regards the appearance presented by Sarsia when
subjected to "this little experiment," the account just quoted is
partly correct; but Professor Agassiz must have been over-hasty
in concluding that, because the animals seemed to be thus "killed
instantaneously," such was really the case. Nothing, indeed, could
be more natural than his conclusion; for not only is the contrast
between the active swimming motions of the Sarsia in the sea-water
and their cessation of all motion in the fresh water very suggestive
of instantaneous death; but a short time after immersion in the
latter their contractile tissues, as Professor Agassiz observed,
become opalescent and whitish. Nevertheless, if he had taken the
precaution of again transferring the Sarsia to sea-water, he would
have found that the previous exposure to fresh water had not had the
effect which he ascribes to it. After a variable time his specimens
would have resumed their swimming motions; and although these might
have had their vigour somewhat impaired, the animals would have
continued to live for an indefinite time--in fact, quite as long as
other specimens which had never been removed from the sea-water. Even
after five minutes' immersion in fresh water, Sarsia will revive
feebly on being again restored to sea-water, although it may be two
or three hours before they do so; they may then, however, live as
long as other specimens. In many cases Sarsia will revive even after
ten minutes' exposure; but the time required for recovery is then
very long, and the subsequent pulsations are of an exceedingly feeble
character. I never knew a specimen survive an exposure of fifteen
minutes.[35] In not a few cases, after immersion in fresh water, the
animal continues to pulsate feebly for some little time; and, in all
cases, irritability of the contractile tissues persists for a little
while after spontaneity has ceased. The opalescence above referred
to principally affects the manubrium, tentacles, and margin of the
nectocalyx. While in fresh water the manubrium and tentacles of
Sarsia are strongly retracted.
[34] "Mem. American Acad. Arts and Sciences," 1850, p. 229.
[35] The covered-eyed Medusæ survive a longer immersion than
the naked-eyed--Aurelia aurita, for instance, requiring from a
quarter to half an hour's exposure before being placed beyond
recovery. Moreover, the cessation of spontaneity on the first
immersion is not so sudden as it is in the case of the naked-eyed
Medusæ--the pulsations continuing for about five minutes, during
which time they become weaker and weaker in so gradual a manner
that it is hard to tell exactly when they first cease.
Thinking it a curious circumstance that the mere absence of the few
mineral substances which occur in sea-water should exert so profound
and deadly an influence on the neuro-muscular tissues of the Medusæ,
I was led to try some further experiments to ascertain whether it is,
as Agassiz affirms, to the mere difference in density between the
fresh and the sea water, or to the absence of the particular mineral
substances in question, that the deleterious influence of fresh
water is to be ascribed. Although my experiments lead to no very
instructive conclusion, they are, I think, worth stating.
I first tried dissolving chloride of sodium in fresh water till the
latter was of the same density as sea-water. Sarsia dropped into
such a solution continued to live for a great number of hours; but
they were conspicuously enfeebled, keeping for the most part at
the bottom of the vessel, and having the vigour of their swimming
motions greatly impaired. The tentacles and manubrium were strongly
retracted, as in the case of exposure to fresh water, and the tissues
also became slightly opalescent. Thinking that perhaps a fairer test
would be only to add as much chloride of sodium to the fresh water as
occurs in sea-water, I did so; but the results were much the same.
On now adding sulphate of magnesium, however, to the amount normally
present in sea-water, the Sarsia became more active. I next tried the
effects of chloride of sodium dissolved in fresh water to the point
of saturation, or nearly so. The Sarsia, of course, floated to the
surface, and they immediately began to show symptoms of torpidity.
The latter became rapidly more and more pronounced, till spontaneity
was quite suspended. The animals, however, were not dead, nor did
they die for many hours, their irritability continuing unimpaired,
although their spontaneity had so completely ceased. The tentacles
and manubrium were exceedingly relaxed, which is an interesting fact,
as being the converse of that which occurs in water containing too
small a proportion of salt. Lastly, to give the density hypothesis
a still more complete trial, I dissolved various neutral salts and
other substances, such as sugar, etc., in fresh water till it was
of the density of sea-water; but in all cases, on immersing Sarsia
in such solutions, death was as rapid as that which followed their
immersion in fresh water.
_The Fresh-water Medusa._
On June 10, 1880, it was noticed that the fresh water in the large
tank of the lily-house of the Royal Botanical Society, Regent's Park,
was swarming with a small and active species of Medusa, previously
unknown to science--it being, indeed, at that time unknown to
science that any species of Medusa inhabited fresh water, although
it was well known that some of the other Hydrozoa do so. Examination
showed that the new species belonged to the order Trachomedusæ,
and the Petasidæ of Haeckel's classification--its nearest known
relative, according to Professor Ray Lankester, being the genus
Aglauropsis, which occurs on the coast of Brazil. The Medusa was
called Limnocodium ([Greek: limnê], a pond, and [Greek: kôdôn], a
bell) sorbii by Professors Allman and Lankester. I am indebted to the
kindness of Professor Allman for permission to reproduce his drawing
of the animal. (Fig. 31.) It is remarkable that, although this Medusa
has reappeared every June in the same tank, no one has yet succeeded
in tracing its life-history. Nor is it known from what source the
tank first became impregnated with this organism. No doubt the germs
must have been conveyed by the roots or leaves of some tropical
plant that at some time was placed in the tank; but the Botanical
Society has no record of any plant which can be pointed to as thus
having probably served to import the organism.
[Illustration: Fig. 31.]
I shall now proceed to give an account of my observations on the
physiology of this interesting animal, by quoting _in extenso_ my
original paper upon the subject (_Nature_, June 24, 1880). Before
doing so, however, I may state that Professors A. Agassiz, Moseley,
and others have since informed us that sundry species of sea-water
Medusæ have been observed by them living and thriving in the brackish
waters of estuaries--a fact which strongly corroborates the inference
at the end of the present paper.
"The natural movements of the Medusa precisely resemble those of its
marine congeners. More particularly, these movements resemble those
of the marine species which do not swim continuously, but indulge in
frequent pauses. In water at the temperature of that in the Victoria
lily-house (85° Fahr.), the pauses are frequent, and the rate of
the rhythm irregular, suddenly quickening and suddenly slowing even
during the same bout, which has the effect of giving an almost
intelligent appearance to the movements. This is especially the case
with young specimens. In colder water (65° to 75°) the movements are
more regular and sustained; so that, guided by the analogy furnished
by my experiments on the marine forms, I infer that the temperature
of the natural habitat of this Medusa cannot be so high as that of
the water in the Victoria lily-house. In water of that temperature
the rate of the rhythm is enormously high, sometimes rising to three
pulsations per second. But by progressively cooling the water,
this rate may be progressively lowered, just as in the case of the
marine species; and in water at 65°, the maximum rate that I have
observed is eighty pulsations per minute. As the temperature at which
the greatest activity is displayed by the fresh-water species is a
temperature so high as to be fatal to all the marine species which I
have observed, the effects of cooling are, of course, only parallel
in the two cases when the effects of a series of higher temperatures
in the one case are compared with those of a series of lower
temperatures in the other. Similarly, while a temperature of 70° is
fatal to all the species of marine Medusæ which I have examined,
it is only a temperature of 100° that is fatal to the fresh-water
species. Lastly, while the marine species will endure any degree of
cold without loss of life, such is not the case with the fresh-water
species. Marine Medusæ, after having been frozen solid, will, when
gradually thawed out, again resume their swimming movements; but this
fresh-water Medusa is completely destroyed by freezing. Upon being
thawed out, the animal is seen to have shrunk into a tiny ball, and
it never again recovers either its life or its shape.
"The animal seeks the sunlight. If one end of the tank is shaded, all
the Medusæ congregate at the end which remains unshaded. Moreover,
during the daytime they swim about at the surface of the water; but
when the sun goes down they subside, and can no longer be seen. In
all these habits they resemble many of the sea-water species. They
are themselves non-luminous.
"I have tried on about a dozen specimens the effect of excising the
margin of the nectocalyx. In the case of all the specimens thus
operated upon, the result was the same, and corresponded precisely
with that which I have obtained in the case of marine species; that
is to say, the operation produces immediate, total, and permanent
paralysis of the nectocalyx, while the severed margin continues to
pulsate for two or three days. The excitability of a nectocalyx thus
mutilated persists for a day or two, and then gradually dies out,
thus also resembling the case of the marine naked-eyed Medusæ. More
particularly, the excitability resembles that of those marine species
which sometimes respond to a single stimulation with two or three
successive contractions.
"A point of specially physiological interest may be here noticed.
In its unmutilated state the fresh-water Medusa exhibits the power
of localizing with its manubrium a seat of stimulation situated in
the bell; that is to say, when a part of the bell is nipped with the
forceps, or otherwise irritated, the free end of the manubrium is
moved over and applied to the part irritated. So far the movement of
localization is precisely similar to that which I have previously
described as occurring in Tiaropsis indicans (_Phil. Trans._, vol.
clxvii.). But further than this, I find a curious difference. For
while in Tiaropsis indicans these movements of localization continue
unimpaired after the margin of the bell has been removed, and will
be ineffectually attempted even after the bell is almost entirely
cut away from its connections with the manubrium, in the fresh-water
Medusa these movements of localization cease after the extreme
margin of the bell has been removed. For some reason or another the
integrity of the margin here seems to be necessary for exciting the
manubrium to perform its movements of localization. It is clear that
this reason must either be that the margin contains the nerve-centres
which preside over these localizing movements of the manubrium,
or, much more probably, that it contains some peripheral nervous
structures which are alone capable of transmitting to the manubrium
a stimulus adequate to evoke the movements of localization. In its
unmutilated state this Medusa is at intervals perpetually applying
the extremity of its manubrium to one part or another of the margin
of the bell, the part of the margin touched always bending in to meet
the approaching extremity of the manubrium. In some cases it can
be seen that the object of this co-ordinated movement is to allow
the extremity of the manubrium--_i.e._ the mouth of the animal--to
pick off a small particle of food that has become entangled in the
marginal tentacles. It is therefore not improbable that in _all_
cases this is the object of such movements, although in most cases
the particle which is caught by the tentacles is too small to be
seen with the naked eye. As it is thus no doubt a matter of great
importance in the economy of the Medusa that its marginal tentacles
should be very sensitive to contact with minute particles, so that a
very slight stimulus applied to them should start the co-ordinated
movements of localization, it is not surprising that the tentacular
rim should present nerve-endings so far sensitive that only by their
excitation can the reflex mechanism be thrown into action. But if
such is the explanation in this case, it is curious that in Tiaropsis
indicans every part of the bell should be equally capable of yielding
a stimulus to a precisely similar reflex action.
"In pursuance of this point, I tried the experiment of cutting
off _portions_ of the margin, and stimulating the bell _above the
portions of the margin which I had removed_. I found that in this
case the manubrium did not remain passive as it did when the _whole_
margin of the bell was removed; but that it made ineffectual efforts
to find the offending body, and in doing so always touched some part
of the margin which was still unmutilated. I can only explain this
fact by supposing that the stimulus supplied to the mutilated part is
spread over the bell, and falsely referred by the manubrium to some
part of the sensitive--_i.e._ unmutilated--margin.
"But to complete this account of the localizing movements, it is
necessary to state one additional fact which, for the sake of
clearness, I have hitherto omitted. If any one of the four radial
tubes is irritated, the manubrium will correctly localize the seat of
irritation, whether or not the margin of the bell has been previously
removed. This greater case, so to speak, of localizing stimuli in
the course of the radial tubes than anywhere else in the nectocalyx,
except the margin, corresponds with what I found to be the case
in Tiaropsis indicans and probably has a direct reference to the
distribution of the principal nerve-tracts.
"On the whole, therefore, contrasting this case of localization with
the closely parallel case presented by Tiaropsis indicans, I should
say that the two chiefly differ in the fresh-water Medusa, even when
unmutilated, not being able to localize so promptly or so certainly,
and in the localization being only performed with reference to the
margin and radial tubes, instead of with reference to the whole
excitable surface of the animal.
"All marine Medusæ are very intolerant of fresh water, and,
therefore, as the fresh-water species must presumably have had marine
ancestors,[36] it seemed an interesting question to determine how
far this species would prove tolerant of sea-water. For the sake of
comparison, I shall first briefly describe the effects of fresh water
upon the marine species.[37] If a naked-eyed Medusa which is swimming
actively in sea-water is suddenly transferred to fresh water, it
will instantaneously collapse, become motionless, and sink to the
bottom of the containing vessel. There it will remain motionless
until it dies; but if it be again transferred to sea-water it will
recover, provided that its exposure to the fresh water has not been
of too long duration. I have never known a naked-eyed Medusa survive
an exposure of fifteen minutes; but they may survive an exposure
of ten, and generally survive an exposure of five. But although
they thus continue to live for an indefinite time, their vigour is
conspicuously and permanently impaired; while in the fresh water
irritability persists for a short time after spontaneity has ceased,
and the tentacles and manubrium are strongly retracted.
[36] Looking to the enormous number of marine species of Medusæ,
it is much more probable that the fresh-water species was derived
from them than that they were derived from a fresh-water ancestry.
[37] For full account, see _Phil. Trans._, vol. clxvii. pp. 744,
745.
"Turning now to the case of the fresh-water species, when first it is
dropped into sea-water at 85° there is no change in its movements for
about fifteen seconds, although the tentacles may be retracted. But
then, or a few seconds later, there generally occurs a series of two
or three tonic spasms, separated from one another by an interval of
a few seconds. During the next half-minute the ordinary contractions
become progressively weaker, until they fade away into mere twitching
convulsions, which affect different parts of the bell irregularly.
After about a minute from the time of the first immersion all
movement ceases, the bell remaining passive in partial systole.
There is now no vestige of irritability. If transferred to fresh
water after five minutes' exposure, there immediately supervenes a
strong and persistent tonic spasm, resembling rigor mortis, and the
animal remains motionless for about twenty minutes. Slight twitching
contractions then begin to display themselves, which, however, do not
affect the whole bell, but occur partially. The tonic spasm continues
progressively to increase in severity, and gives the outline of the
margin a very irregular form; the twitching contractions become
weaker and less frequent, till at last they altogether die away.
Irritability, however, still continues for a time--a nip with the
forceps being followed by a bout of rhythmical contractions. Death
occurs in several hours in strong and irregular systole.
"If the exposure to sea-water has only lasted two minutes, a similar
series of phenomena is presented, except that the spontaneous
twitching movements supervene in much less time than twenty minutes.
But an exposure of one minute may determine a fatal result a few
hours after the Medusa has been restored to fresh water.
"Contact with sea-water causes an opalescence and eventual
disintegration of the tissues, which precisely resemble the effects
of fresh water upon the marine Medusæ. When immersed in sea-water
this Medusa floats upon the surface, owing to its smaller specific
gravity.
"In diluted sea-water (fifty per cent.) the preliminary tonic spasms
do not occur, but all the other phases are the same, though extended
through a longer period. In sea-water still more diluted (1 in 4
or 6) there is a gradual loss of spontaneity, till all movement
ceases, shortly after which irritability also disappears; manubrium
and tentacles expanded. After an hour's continued exposure, intense
rigor mortis slowly and progressively developes itself, so that at
last the bell has shrivelled almost to nothing. An exposure of a
few minutes to this strength places the animal past recovery when
restored to fresh water. In still weaker mixtures (1 in 8, or 1 in
10) spontaneity persists for a long time; but the animal gradually
becomes less and less energetic, till at last it will only move in a
bout of feeble pulsations when irritated. In still weaker solutions
(1 in 12, or 1 in 15) spontaneity continues for hours, and in
solutions of from 1 in 15, or 1 in 18, the Medusa will swim about for
days.
"It will be seen from this account that the fresh-water Medusa
is even more intolerant of sea-water than are the marine species
of fresh water. Moreover, the fresh-water Medusa is beyond all
comparison more intolerant of sea-water than are the marine species
of brine; for I have previously found that the marine species will
survive many hours' immersion in a saturated solution of salt. While
in such a solution they are motionless, with manubrium and tentacles
relaxed, so resembling the fresh-water Medusa shortly after being
immersed in a mixture of one part sea-water to five of fresh; but
there is the great difference that, while this small amount of salt
is very quickly fatal to the fresh-water species, the large addition
of salt exerts no permanently deleterious influence on the marine
species.
"We have thus altogether a curious set of cross relations. It
would appear that a much less profound physiological change would
be required to transmute a sea-water jelly-fish into a jelly-fish
adapted to inhabit brine, than would be required to enable it to
inhabit fresh water. Yet the latter is the direction in which the
modification has taken place, and taken place so completely that
the sea-water is now more poisonous to the modified species than
is fresh water to the unmodified. There can be no doubt that the
modification was gradual--probably brought about by the ancestors
of the fresh-water Medusa penetrating higher and higher through the
brackish waters of estuaries into the fresh water of rivers--and
it would, I think, be hard to point to a more remarkable case
of profound physiological modification in adaptation to changed
conditions of life. If an animal so exceedingly intolerant of fresh
water as is a marine jelly-fish may yet have all its tissues changed
so as to adapt them to thrive in fresh water, and even die after an
exposure of one minute to their ancestral element, assuredly we can
see no reason why any animal in earth or sea or anywhere else may not
in time become fitted to change its element."[38]
[38] While these sheets are passing through the press, a paper
has been read before the Royal Society by Mr. A. G. Bourne,
describing the hydroid stage of the fresh-water Medusa (_Proc.
Roy. Soc._, Dec. 11, 1884). He has discovered the hydroids on
the roots of the _Pontederia_, which have been growing in the
Lily-tank for several years, and which are therefore probably the
source from which the tank became impregnated with the Medusæ.