and which strikes us on first seeing a star-fish, or its relatives the
sea-urchin and the sea-anemone, consists of the fact that these animals
possess no head, which even the most insignificant worm or insect does
not lack, and that consequently its organs are in want of a guiding,
regulative member, possessing externally organs of sense and having
within a brain with the power to communicate the requisite commands for
the movement and the conduct of the same. On the contrary, each single
branch or ray possesses its own individual nervous system; and in the
case of the voluntary separation of the rays, which frequently occurs,
is able to continue life of its own independent accord, developing
itself by the growth of new rays into a new and complete star-fish.
(See Fig. 2.) But these five or more nervous systems do not radiate
from a common central nerve-ganglion which might be termed a central
brain, but are merely joined to a nerve-ring which lies in a common
central portion, encircling the esophagus; this nerve-ring in the
majority of cases forms a regular polygonic figure, and into each angle
of the polygon the nerve-cord of a ray enters. It will be seen from
this structural arrangement of things, that the psychical and mental
guidance of these animals is entrusted to a board of five members who
possess, it is true, sentient communication with each other, but act
without the intermediation of a presiding officer.
[Illustration: Fig. 2.—COMET-FORM OF ARM OF A STAR-FISH.
A cast-off arm re-forming by the sprouting of four new rays.]
We may well look forward with intense interest to the outcome of a
psychical administration of this kind, and to tell the truth, until
recently its importance has been greatly underestimated. Every
inference made with respect to the psychical excitability of an
animal must be derived from its movements and actions in various
natural and artificially produced positions, by observing what its
conduct under these conditions is. To start with, star-fishes, like
sea-urchins (which psychically are similarly governed), admit with
respect to the position of their bodies a distinction of top and
bottom; that is to say, the side on which the mouth lies situated in
the centre of the five rays belongs properly face downwards, while
the opposite surface is to be regarded as the dorsal side. But the
conceptions of a forepart and a hindpart, of a right and a left are
not applicable. The rays of the star-fish, like the central disc, also
plainly exhibit a distinction of lower and upper parts. Among the real
star-fishes (_Asteroidea_) the inferior or ventral surface of the
arms is supplied either with two or with four rows of sucker-feet or
pedicels, consisting of long, extensile, hollow sacs, which when filled
and extended by the water let into their widely ramified ambulacral
systems, protrude into the grooves of the arm through openings in the
hardened calcareous integument. To level surfaces they easily cling
fast by simply drawing back the terminal discs of their tubular feet
and thus creating a rarefied atmosphere in the space between the object
to which they adhere and the puffed out walls of the extremities of
the pedicels. Star-fishes may be seen climbing in this way, with their
hundreds and hundreds of tube-feet, up slippery cliffs and even the
perpendicular glass walls of aquariums, and they are even able to hang
suspended from a horizontal glass ceiling for a considerable length of
time after they have been taken out of the water. When they wish to
change their position they do it by alternately loosening and fastening
their extensile feet in such a way that those loosened reach forward
in one and the same direction uniform in all the arms, and fasten
themselves to the surface anew, whereupon the others also let loose and
go through the same movement in the same direction. The sucker-feet
also help to convey to the mouth the food seized at the end of the arms.
[Illustration: Fig. 3.—MODE OF LOCOMOTION OF SAND-STARS. (After
Preyer.)
In the cut to the left _1_ first advances, then (_5_) and (_2_); _3_
and _4_ remaining at rest. Whereupon (_5_) and (_2_) simultaneously
come back to the positions _5_ and _2_, _c_ is lifted and pushed
forwards with _1_, while the two rays _3_ and _4_ are pulled along
behind. In the figure to the right the same animal first shoves forward
the pairs (_1_) (_2_) and (_5_) (_3_), _4_ remaining at rest, and then
bends both pairs backwards, dragging only _4_ behind; _c_ is lifted and
thrown forward in the direction of the arrow.]
While in this instance, accordingly, the arms, although they are
not immovable and bend and approach each other, officiate rather as
the bearers of organs than as prehensile and locomotory apparatuses
themselves,—the sucker-feet performing the principal tasks and
requiring for their work a very finely ramified nervous system; in the
case of a certain other division of the star-fishes, the so-called
sand-stars (_Ophiuridae_), the arms are thinner and more supple, and
act as organs of prehension and locomotion, dispensing more or less
entirely with their suctorial pedicels. By alternately thrusting three
feet forward (Fig. 3) and then drawing back the two side feet of these
three, the five-footed sea-stars move more swiftly than the others,
sometimes proceeding by jumps even; but they cannot climb up smooth
surfaces, or cliffs, unless irregularities are present which may be
grasped by their pliant arms, whereas on the other hand the common
star-fishes, which are furnished with sucker-feet, climb best of all
on smooth and slippery surfaces, each one of their countless pedicels
being able to suspend a considerable weight, in some species as much as
twenty-five grammes. In other respects, especially with regard to the
ring-shaped connection of the five nerve-cords, their organisation is
essentially the same; only in the sand-stars the central portion forms
a disc more distinctly separate from the arms, in which former the
common organs of feeling and digestion have more fully retracted.
Recognising thus, that the star-fishes and all their relatives act
physically like a federal animal-union, composed of five independent
animal-states, I called attention in the first edition of my work
"Werden und Vergehen" (1876) to the psychological enigma that we
were here confronted with a five-fold Siamese monster, as it were,
in which five separate persons were brought mentally under the same
guidance, or where five minds had to pull, simultaneously, one rope.
On account of the absence of a head and a brain in these animals,
certain well-known modern animal psychologists have taken the position
that their powers of psychical performance are very scanty, and that,
in a much fuller sense than was predicated of all animals by the
Cartesians, these especially were irrational automatons, or, to use a
technical expression, were mere "reflex-organisms," animals in which
only direct external excitations evoke with unalterable regularity
responsive movements, so that, for example, if any unpleasant
excitation were brought to bear on them from any direction they would
move in the opposite direction, but world approach if anything became
perceptible that excited their desire for food. On this ground the
distinguished English animal psychologists Romanes and Ewart claim to
have established that these animals actually do respond like machines
to external excitations; if they were excited at any part of their body
by a wound, by the application of acids, an electric current, or any
other irritant, they would run without exception in a straight line in
the opposite direction, but if the excitation were applied to any two
parts of their body at some distance from each other they would move
in the line of the diagonal of the two directions, in accordance with
the principle of the parallelogram of forces. Similarly their movements
after prey and food (the presence of which at a distance was made
known by the emission of odors), their movements toward more brightly
illuminated parts of the containing vessel, their flight from the air
into the water, their recovery of their normal position when placed on
their backs, and finally their so-called autotomy or self-amputation,
that is the casting off of their members under the irritation of
powerful stimuli—were all held to represent mere automatic responses
to prearranged conditions without a trace of intelligence being
exhibited.
In view of this condition of things it was a very welcome announcement,
that one of the most brilliant representatives of modern experimental
physiology and psychology, Professor W. Preyer, at present of Berlin,
had determined to undertake a comprehensive series of experiments with
these very animals, and was able to carry out his intention at the
zoölogical station in Naples, so admirably adapted to the purpose. To
obtain clear ideas generally with regard to animal reflex-mechanisms,
fitter specimens for experiment could scarcely be presented than the
star-fishes, which unite a rare degree of decentralisation, power of
independent action, and absence of a cerebral centre, with a nervous
system of the minutest ramifications. Here, if anywhere, were simple,
clear and transparent results to be expected, and finally information
relating to the co-operative activity of different nervous systems.
Preyer published the results of his observations in the "Mittheilungen
der Zoologischen Station in Neapel" for the years 1886 and 1887, and
although he does not regard his labors as completed, the scientific
reading public may nevertheless take sufficient interest in the present
state of his researches to justify a presentment of the principal and
most general results obtained.
In confirmation of the view that previously obtained it was found that
these animals actually did respond in a rare degree to given stimuli in
a manner determined once for all; it could be foretold with a degree
of sureness verging on astronomical certainty, how, for example, the
sucker-feet of a star-fish would act if the animal in its normal and
sound condition was irritated at this or that place, powerfully or
weakly, one time or many times successively, by mechanical or chemical
applications, by electric currents, or heated instruments. With all
the means of irritation employed the result was always identical, and
consisted in the fact that the distensible feet were drawn in at the
point of application when the irritation did not extend beyond its
region, no matter whether it was applied at the inferior or superior
surface of the animal, but that protrusion of the feet never resulted
from local irritations of this character so long as they did not
exceed a certain intensity. A more powerful irritation, on the other
hand, radiating over a greater portion, or over the whole animal,
produces a general protrusion of the distensible pedicels, so far as
the irritation extends, with the single exception of the point of
application itself when the same lies on the inferior surface.
Inasmuch as this swarming protrusion of pedicels may spread over the
entire inferior surface when only a single arm is irritated in the
neighborhood of its extremity, it follows from this that the nervous
excitation must first be conveyed to the ring at the centre in order to
radiate thence to the pedicels of the other arms; and from the manner
in which the irritation is propagated, the course of the radiation can
be accurately followed. Thus, if the irritation of an arm proceeded
from the dorsal region, the distensible pedicels of this arm were the
first to protrude, then those of the two adjacent arms, and finally
those of the two remaining arms, but in the latter not quite out to
the extremities unless the irritation exceeded a certain intensity.
That is to say, the effect of the irritation was propagated through
the inner nerve-ring according to the same laws by which a fluid under
pressure or an electric current in a similar conductory system would
proceed. But if the connection of the ring was severed at both sides of
the irritated arm, the effect would remain confined to that arm. If the
connection was broken only on one side the irritation advanced round
the other side and reached the severed neighbor last. On the other
hand, a powerful irritation of the central disc immediately provoked
the extension of all the pedicels. The phenomena recorded occurred
moreover in accordance with simple mechanical laws as was expected from
the outset, and when the irritations were unusually powerful the effect
was manifested by a continuous alternate extension and contraction of
the pedicels.
Amputated arms of the common sucker-footed star-fish act like arms
isolated at both sides by severance of the nerve-ring, as just
explained. Upon local irritation they draw in their pedicels, and
protrude them upon being powerfully irritated; they creep forward in
a definite direction, and when placed upon their backs are able even
to turn themselves over like the uninjured animal. The severed arms
of sand-stars are less independent. They twist about aimlessly hither
and thither, but if any considerable portion of the central disc and
nerve-ring adheres to them they are able to perform adaptive movements.
Similarly the disc, with one or two arms attached, is not helpless; and
is able to get along quite alone without any arms. We could explain all
these movements by so-called reflex actions and might grant also that
the mechanism that effects these results operates in this case upon
a greater scale and with more independence than in other classes of
animals, for the reason that here a real guiding organ is not present.
But whatever might be inferred from the experiments just described in
favor of a senseless and unintelligent life of star-fishes, Professor
Preyer was nevertheless able by extending his experiments to win the
conviction that the old conception of star-fishes being real reflex
animals was wholly untenable, since a great number of capacities and
capabilities could be verified and provoked, which are intelligible
only on the basis of adaptive co-operation and mutual concerted action
in the five rays. We shall not discuss here whether this is also proved
by the wonderful fact that a star-fish, which fastens its arms to
everything possible, never seizes its own arm and thus, like Molière's
miser, in its visits to its oyster beds never catches itself for a
thief. We might say, indeed, that the arm seeking a hold does not seize
its companion because it feels it and has learned by experience that
it takes a Münchausen to pull one's self out of a swamp by the tops of
one's boots. But we find exactly the same phenomenon among creeping
plants, which clasp every kind of support in their way, but never,
as Darwin observed, take hold of their own stalks; whence we might
assume that there probably exists in these beings some sort of power of
reflex inhibition dependent upon a property of the body and developed
in consequence of the fact that clasping and grasping parts of itself
would involve a useless waste of energy. We shall see, however, that
under certain circumstances this instinctive "dread" of contact with
self is inoperative.
But to our main task. In the simplest changes of place and position,
intelligent co-operation of the arms is manifest. For if in moving from
one place to another, or in turning around each arm tended to perform
on its own account the necessary movements of extension or rotation,
without giving any heed to the others, the animal would endure the
torments of Tantalus before it could reach, if ever at all, the
choice bit of food that it had scented from afar, or the ray of light
towards which an obscure impulsion drove it. On the contrary, when a
star-fish is spying after food, we observe it lift the ends of its
pedicel-covered arms so that the downward deflected eye there situated
may obtain a good view of things in the neighborhood, and if in any
direction an object worth going after is discovered we see the many
hundreds of sucker-feet on the five arms push out in one and the same
direction,—a phenomenon that requires the presence of a very widely
ramified nervous system, since every tactile pedicel needs its separate
telegraph wire in order to be properly moved and not always in the same
direction, as for example when the animal wishes to perform a rotation
about its own axis. For these comical animals sometimes do rotate about
their axis, although our simple mind wonders why a Janus-head should
want to turn around, these animals being able to look simultaneously
in the four directions of the compass, and having still another eye
for looking downward. Similarly in the sand-stars, to which the
Medusa-heads with branched arms belong, an adaptive co-operation of the
arms in creeping and swimming occurs; which can be explained only as
the result of a common understanding issuing from the central ring.
It would seem to follow from Preyer's extensive observations, that as
a rule no one individual arm of a star-fish enjoys to the exclusion of
its fellows the prerogative of universal or even general precedence;
the lead of any one arm is rather solely determined by the object
sought, so that the one next to the object generally starts first
and assumes the lead of the little army of arms. Of course in the
case of new-growing star-fishes which have sprung from a single arm
by sprouting, this is different; for in this instance the old arm
will undoubtedly retain control of the others for some length of time
until the young ones have reached a certain size. Preyer does not seem
to have instituted observations to ascertain this, but it would be
interesting to determine whether an arm of this kind always takes the
lead, or in the proper cases acts as driver from behind and pushes the
baby-carriage with the children before it.
[Illustration: Fig. 4.—RECOVERY OF NORMAL POSITION BY ASTROPECTEN
AURANTIACUS. (After Preyer.)
Through the end of each ray of the animal a thread is drawn and
affixed to a cork; the animal lying back downwards. At first the
creature swung the corks alternately inwards and outwards, taking the
positions represented in the above figure. After the lapse of an hour
the ray with the smallest cork attached, upon which thus the least
upward pressure was exerted, was pulled downwards and sidewise and
brought beneath an adjacent ray; the two opposite rays were retracted
centrally, the disc lifted, the centre of gravity of the animal thus
displaced, and the turning effected.]
Examples of surprisingly dexterous co-operation and concerted adaptive
action are observed in these animals in their climbing on difficult
surfaces, and in their attempts, also, to regain their normal position
when placed on their backs or made to swim in reversed positions by
discs of cork fastened to the extremities of their arms. Scientists
have observed members of the orders _Asteroidea_ and _Ophiuroidea_,
in difficult positions of this kind, display an astounding sense of
equilibrium and a skilfulness in gaining firm holds, suggestive of the
athletic feats of monkeys, and that even when placed in very unusual
positions such as never occur in nature. Thus many star-fishes let
themselves drop from steep rocks and cliffs, if that happens to be the
best way of getting down; but in such cases before they let their whole
weight go hold fast to the last moment with one or two arms, as if it
were previously necessary to calculate the leap into the depths below.
To furnish the counter-test of this, and to prove that the central
nerve-ring is, as assumed, the indispensable and necessary condition
of this united co-operation, Preyer severed the ring in individual
specimens of the class between every two arms, sparing the other parts
as much as possible. In this way the nervous systems of the five
rays were disconnected. As was expected, it was found that the more
connections there were severed, the more difficult the animal found it
when placed on its back to regain its normal position. For since the
recovery of the normal position must be introduced by the groping about
and the fastening of the pedicels of one or of several adjacent and
half-turned arms, two arms or pairs of arms might for want of a mutual
understanding act directly in opposition to one another and thus make
the turning impossible. On the other hand, the central disc was able,
though deprived of all arms, to accomplish the turning, if only the
nerve-ring were preserved intact; and the more there remained of the
nerve-ring on a single arm the better the single arm was able to do it.
But in circumstances which were wholly new, the adaptive co-operation
of the arms demonstrated itself in so striking a manner that we may
say they are not to be easily put out of countenance or confounded.
When Professor Preyer, for example, slipped narrow rubber bands or
cylinders over their rough spiny arms, they rid themselves as a rule of
these unwonted fetters in a very short time, and in the most various
but always well calculated ways. Generally the two nearest ones seized
their poor imprisoned fellow "under the arms," bracing themselves with
their rough spiny surface against the rubber sleeve, and thus finally
stripping it off. (See Fig. 5; next page.) Sometimes, when the band was
loosely adjusted, twisting movements of the arm in the water sufficed
gradually to loosen it, until it could be finally cast off. Often the
peeling off was effected by pressing against a rough surface, whereby
sometimes an adjacent arm held the sleeve fast; and when no other
expedient was of avail the animal cast the arm, sleeve and all, away
from itself; and the latter may possibly have not gotten rid of it at
all. At times the casting off of the arm occurred subsequently, after
the obstacle had been entirely removed, and often even a day later, as
if the impeded arm was still sensible of some obstruction which caused
it to afterwards separate from its companions.
[Illustration: Fig. 5.—REMOVAL BY OPHIOMYXA OF A RUBBER SLEEVE. (After
Preyer.)
The figure represents the moment at which the band is about to be
removed. An adjacent arm is braced against the lower edge of the band,
forcing it off in the direction of the extremity of the ray.]
Attempts at flight and liberation from unwonted compulsory positions
or narrow confinement, also deserve special attention. Many a person
who has put a star-fish into a cage and fancied that he was assured
of its possession, has been disappointed on finding that the animal
had effected its escape through the meshes. But star-fishes have, in
consequence of their abhorrence of the air, been made to creep into
the narrow necks of bottles filled with water. Professor Preyer,
for example, thrust two of the arms of a common star-fish species
(_Asterias glacialis_) into a tube filled with salt water leaving
the three other arms exposed to the atmosphere outside; and although
it would have been impossible to force the animal into the tube
without crushing it, the three arms exposed to the air were also
pulled in within the space of three minutes. If the tube was placed
perpendicularly in water the animal quickly crept out again. The
performance seemed utterly impossible, for each single arm of the
star-fish was almost as thick at its base as the greatest width of
the tube, and yet three of these arms had to pass in side by side.
This was made possible by the animal emptying during the passage all
the numerous water-vesicles in the interior of the arms which serve
to fill and to empty the distensible pedicels therein; the star-fish,
after the expulsion of the water, becomes very soft in all its parts
and does not harden again until it has forced itself completely through
and refilled itself with water. In order to accomplish these emptyings,
bendings, turnings, and rollings, thousands of muscular fibres must
work in harmony within the body of the animal. This experiment was also
successfully carried out with other star-fishes, but I cannot agree
with the observer when he says that in so doing he brought the animals
into a completely new and hitherto unexperienced position. In their
haunts on rocky coasts they must assuredly often have to force their
way through narrow fissures and holes; and they must find occasion to
make use of the advantages of being able to evacuate water in the case
also of single arms, as when they search with them in narrow apertures
and snail-houses.