which they stretch out, contract, and turn themselves every way. For
suppose the hydra or polype, _a b_, Fig. 16, to be fixed by the tail
_b_, having the body and the arms _a_ extended in the water; in order to
advance, it draws itself together, by bending itself so as to bring the
head and arms down to the substance on which it is to move; to do this,
it fixes the head or the arms as in Fig. 17; when these are well fixed,
it loosens the tail, and draws it towards the head, as in Fig. 18, which
it again loosens, and resting on the tail, stretches it out, as in Fig.
19. It is easy to see from this account, that their manner of walking is
very analogous to that of various terrestrial and aquatic animals. They
walk very slow, often stopping in the middle of a step, turning and
winding their body and arms every way. Their step is sometimes very
singular, as in the following instance: suppose the polype _a b_, Fig.
20, to be fixed by the tail _b_, the body and arms being extended in the
water, it first bends the fore-part towards the substance on which it is
moving, and fixes it thereto, as at _a_, Fig. 21; it then loosens the
lower end, and raises it up perpendicular, as in Fig. 22; now bending
the body to the other side, it fixes the tail, as in Fig. 23; then
loosening the anterior end, it rises up, as in Fig. 24.
They descend at pleasure to the bottom of the water, and ascend again,
either by the sides, or upon some aquatic plants; they often hang from
the surface of the water, resting as it were upon the tail; or, at other
times they are suspended by one arm from it. They walk also with ease
upon the surface of the water. If the extremity of the tail _b_, Fig. 7,
be examined with a magnifying glass, a small part of it will be found to
be dry, and above the surface of the water, and as it were in a little
concave space, of which the tail forms the bottom, so that it seems to
be suspended on the surface of the water, on the same principle that a
small pin or needle is made to swim.
Hence, when a polype means to pass from the sides of the glass to the
surface of the water, it has only to put that part out of the water by
which it means to be supported, and give it time to dry, which it always
does upon these occasions. They attach themselves so firmly by the tail
to aquatic plants, stones, &c. as not to be easily driven from the place
where they have fixed themselves; they often further strengthen these
attachments by means of one or two of their arms, which they throw out
and fix to adjacent substances, as so many anchors.
The mouth of the polype or hydra is situated at the fore-part of the
body, in the middle between the shooting forth of the arms. The mouth
assumes different appearances, according to the different purposes of
the insect; sometimes it is lengthened out, and forms a little conical
nipple, as in Plate XXIII. A. Fig. 13; sometimes it appears truncated,
as in Plate XXI. Fig. 8; at other times the interval between the arms
appears closed, as in Plate XXIII. A. Fig. 2 and 12; or hollow, as in
Fig. 11 of the same plate. If it be observed with a deep magnifier in
either of the two last cases, a small aperture may be discovered.
The mouth of the polype opens into the stomach, which is a kind of bag
or gut that goes from head to tail; this may be perceived by the naked
eye, when the animal is exposed to a strong light, or a candle placed on
the opposite side to the eye; for the colour of the polype does not
destroy the transparency thereof. The stomach will, however, be better
seen, if the eye be assisted by a deep magnifier; one of them is
represented as highly magnified in Plate XXI. Fig. 8. To be fully
satisfied whether they were perforated throughout, Trembley cut one
transversely into three parts; each piece immediately contracted itself,
and became very short; being then placed in a shallow glass full of
water, and viewed through the microscope, they were found to be visibly
perforated. Their microscopic appearance is represented in Plate XXIII.
A. Fig. 6, 7, 8; its mouth was at the anterior end _a_, Fig. 8, of one
of these parts. The tail was at the end _b_ of the third part, Fig. 6;
as this piece was also perforated, it plainly appears that the tail of
the hydra is open. The perforation, which is thus continued from one end
to the other, is called the stomach, because it contains and digests the
aliments. The skin which incloses the bag, and forms the stomach, is the
skin of the polype itself; so that the animal may be said to consist of
but one skin, disposed in the form of a tube or gut open at both ends.
On opening the polype, no vessels are to be distinguished; and whatever
be the nature of its organization, it must reside in the skin.
The skin must be so far organized, as to perform all the operations
necessary for the nutrition and growth of the animal, without
considering those that are necessary for its various motions. Whatever
are the means the Author of Nature has employed for these purposes, we
are ignorant of them. If their skin be examined by a microscope, it
appears like shagreen, or as if it were covered with little grains;
these are more or less separated from each other, according to the
degree in which the body is extended or contracted.
If the lips of a polype be cut transversely, and placed so that the cut
part of the skin may lie directly before the microscope, the skin
throughout its whole thickness will be found to consist of an infinite
number of these grains. To know whether the inside of the stomach was
formed of similar grains, several of them have been laid open and
examined by the microscope; the interior surface was then found to
consist of an immense number of them, being as it were more shagreened
than the exterior one, and less transparent. The grains are not strongly
united to each other, but may be separated without much trouble. Plate
XXIII. A. Fig. 10, represents a piece of skin thus laid open. To examine
these particulars further, a piece of skin a, Fig. 9, was laid in a few
drops of water, on a piece of glass before the microscope, and some of
the grains were separated from it, as at b c d, by pressing them with
the point of a pin; in endeavouring to open them, they spread themselves
into all parts of the water, and at last remained in heaps, as at e and
f.
If a polype be carefully placed before the microscope, without wounding
it, you will seldom be disappointed in seeing some of these grains
detach themselves from the superficies thereof, and that even in the
most healthy.
But if the grains separate themselves in large quantities, it is the
symptom of a very dangerous disorder; the surface of the polype thus
attacked becomes more and more irregular, and is no longer well
terminated and defined as before. The grains fall off on all sides, the
body and arms contract and dilate, it becomes of a white shining colour,
loses its form as at a, Fig. 4, and then dissolves into a heap of
grains, as at b, Fig. 5. The progress of this disorder is most easily
observed in the hydra viridis.
A very attentive and accurate examination shews that the skin is formed
of a kind of glareous substance, a species of gum, which fills up the
intervals between the grains, in which they are lodged, and by which
they are attached, though weakly, together. It has been already
observed, that it is to these grains that it owes its shagreen-like
appearance; it is from them also that it derives its colour; for, when
they are separated from the polype, they are the same colour with it,
whereas the glareous matter is without any distinguishing colour. The
construction of the polype seems then to be confined to these glandular
grains, to the viscous matter, and the invisible fibres which act upon
the glareous substance.
The structure of the arms of the polypes is very analogous to that of
their body. When they are examined by the microscope, either in a
contracted or dilated state, their surface is shagreened; if the arm be
much contracted, it appears more so than the body; on the contrary, it
appears less so in proportion as they are more extended; almost quite
smooth when at their full extension; so that in the hydra viridis the
appearance of the arms is continually varying, and these variations are
more sensible towards the extremity of the arm than at its origin, as,
in Plate XXI. Fig. 10; but more thinly scattered, or farther asunder, in
the parts further on, as at Fig. 9. The hairs which are exhibited in
this figure cannot be seen without a very deep magnifier, however they
indicate a further degree of organization in this little animal. The
extremity is often terminated by a knob.
All animals of this kind have a remarkable attachment to turn towards
the light, and this might naturally induce the inquirer to look for
their eyes; but how carefully soever this search has been pursued, and
however excellent the microscope with which every part has been
examined, yet no appearance of this organ has been found.
Notwithstanding this, they constantly turn themselves toward the light;
so that if that part of the glass in which we placed them be turned from
it, they will be found the next day to have removed themselves to the
side that is next the light, and the dark side will be quite
depopulated.
OF THE FOOD OF THE HYDRÆ, AND THEIR METHOD OF SEIZING AND SWALLOWING
THEIR PREY.
As the hydra fusca, Plate XXI. Fig. 7, has the longest arms, its manner
of feeding, and the different manœuvres it makes use of to seize and
manage its prey, are more remarkable than those of the two other
species; it will be, therefore, this kind only which will be principally
spoken of under the present head. To obtain a proper view, it should be
placed in a glass seven or eight inches deep. If the polype be fixed
near the top of the glass, the arms for the most part hang down toward
the bottom. This is a very convenient situation for feeding it, and
observing its management of the food.
The polypes are in general very voracious: an hungry one extends its
arms as a fisherman his nets; it spreads them every way, so that they
form a circle of considerable extent, every part of which is entirely
within the reach of one of them. In this expanded posture it lies in
expectation of its food; whatever comes within the verge of this circle
is seized by one or other of its arms: the arms are then contracted till
the prey is brought to the mouth, when it is soon devoured. While the
arms are contracting and exerting themselves vigorously to counteract
the efforts of the animal, which it has seized, to escape, they may be
observed to swell like the muscles of the human body when they are in a
state of exertion.
Though in general all ideas are derived from the senses, there are
certainly some that seem infused into us independently of the exertions
of any sense. This may be confirmed by many instances of animal
instinct; among others, it may be illustrated by the polype. Who taught
it, when just separated from the parent stock, to expand its arms, that
it might catch its prey? That its native element abounded with insects?
or that these were its proper food? No sense that we are acquainted with
could first give the information.
The polype does not always wait for its prey, it feels for it, and in a
manner follows it. It may be asked how can it perform this if destitute
of vision? or do the glandular grains answer the purpose of eyes? Who
can answer the question? what are our own eyes but glandular grains of a
larger size? If this should be the case, our hydra, like the libellulæ
and other insects, would realize, nay, exceed the fables of the
ancients, being an Argus entirely composed of eyes. Be this as it may,
they are certainly in possession of some sensation by which they are
informed of the approach of their prey, and which renders them attentive
to all that may confirm or destroy this perception.
When the arms of a polype are extended within a glass, put a centipe or
any kind of worm into it, see Plate XXIV. A. Fig. 1, and with the point
of a pin push it towards one of the arms; as soon as it touches this it
is seized; the worm or centipe endeavours by quick and strong efforts to
disengage itself, often swimming and dragging the arm from one side of
the glass to the other. This violent motion of the prey obliges the
polype to contract strongly the arm; in doing which, it often twists it
in the form of a cork-screw, as at o i, by which means it shortens it
more rapidly. The struggles of the devoted animal soon bring it in
contact with another arm; these contracting further, the little creature
is presently engaged with all the arms, and by degrees conveyed to the
mouth, against which it is held and subdued.
When a polype has nothing to eat, its mouth is generally open, but so
small, that it can scarce be perceived without the assistance of a
magnifying glass; but as soon as the arms have conveyed the prey to the
mouth, it opens itself wider, and this in proportion to the size of the
animal that is to be devoured; the lips gradually dilate, and adjust
themselves accurately to the figure of the prey. The greatest part of
the animals on which the polype feeds, are to its mouth, what an apple
the size of our heads would be to the mouth of a man.
The worms or other minute animals which are seized by the polype, are
not always brought to the mouth in the same situation; if they be
presented to it by one of their extremities, it is not requisite that
the polype should open its mouth considerably, and in effect it only
opens it so wide, as precisely to give entrance to the worm, Fig. 5. If
it be not too long for the stomach, it remains there extended; but if it
be longer, the end which first enters is bent, so that when the worm is
entirely swallowed, it may be seen lying folded in the stomach, Plate
XXIV. B. Fig. 12.
If the middle, or any other part of the worm, be presented to the mouth
of the polype, it seizes this part with the lips, extending them on both
sides, and applying them against the worm, so that the mouth assumes the
form of a boat, pointed at each end, Plate XXIV. A. Fig. 2; the polype
gradually closes the two points of its boat-like lips, and by this
motion and suction swallows the worm, Fig. 4.
The polypes kill worms so speedily, that Fontana thinks they must
contain the most active and powerful venom; for the lips of a polype
scarce touch the worm, but it expires, so great is the energy of the
poison it conveys into it, though no wound can be observed in the dead
animal.
As soon as the stomach is filled, its capacity is enlarged, the body is
shortened, Plate XXIV. A. Fig. 6, the arms are for the most part
contracted, the polype hangs down without motion, and appears to be in a
kind of stupor, and very different from its shape when extended; but in
proportion as the food is digested, and it has voided the
excrementitious parts, the body lengthens, and gradually recovers its
usual form.
The transparency of the polype permits us to see distinctly the worm it
has swallowed, Plate XXIV. B. Fig. 12, which gradually loses its form.
It is at first macerated in the stomach of the polype, and when the
nutritious juices are separated from it, the remainder is discharged by
the mouth, Fig. 13. It is with these, as with other voracious animals,
as they devour a great quantity of food at once, so also they can fast
for a long time. The history of insects furnishes many examples of this
kind.
One circumstance is observable, which probably contributes much to the
digestion of their food, namely, that the aliments are continually
pushed backward from one extremity of the stomach to the other; this
motion may be easily observed with a microscope, in a polype which is
not too full, and in which the food has been already divided into little
fragments. For these observations, it is best to feed the polype with
such food as will give a lively-coloured juice; as for example, those
worms whose intestines are filled with red substances: for by these
means we shall see that the nutritious juices are conveyed not only to
the extremity of the body, but also into the arms; from whence it is
probable that each of the arms form also a kind of gut, which
communicates with that of the body. Some bits of a small black snail
that is frequently to be found in our ditches, was given to a polype.
The substance of this skin was soon reduced into a pulp, consisting of
little black fragments; on examining the polype with the microscope,
these particles were perceived to be driven about the stomach, and to
pass from head to tail, and into their arms, even where these were as
fine as a thread; they were afterwards forced into the stomach, and from
thence to the tail, from whence they were again driven into the arms,
and so on.
The grains take their tinge from the food which nourishes the polypes;
these grains become red or black, if the polype be fed with juices that
are either red or black; and they are more or less tinged with these
different colours, in proportion to the strength and quantity of the
nutritive juices. It is also observable, that they lose their colour if
fed with aliments that are not of the same colour with themselves.
The polypes feed on the greater part of those insects that are to be
found in fresh water. They may be nourished with worms, the larvæ of
gnats, &c. they will also eat larger animals if they are cut into small
pieces, as snails, large aquatic insects, small fish, butcher’s meat,
&c. Sometimes two polypes seize the same worm, and each begins to
swallow its own end, continuing so to do till their mouths meet, Plate
XXIV. A. Fig. 8; in this position they remain for some time, at last the
worm breaks, and each has its share; sometimes the combat does not end
here, for each continuing to dispute the prize, one of the polypes
opens its mouth advantageously, and swallows the other with its portion
of the worm, Plate XXIV. B. Fig. 14; this combat ends more fortunately
for the devoured polype than might be at first expected, for the other
often gets the prey out of its stomach, but lets it out again sound and
safe, after having imprisoned it above an hour. From hence we learn,
that the stomach of the polype, which so soon dissolves the animal
substances which are conveyed into it, is not capable of digesting that
of another polype.
Plate XXIV. A. Fig. 5, represents a polype with one half a centipe in
its mouth, as at a; the other part without, as at m. Fig. 1 represents
one suspended in water by a piece of packthread; c n, a centipe seized
by it, and drawn partly towards the mouth; i o, the bendings in the arm;
p, an arm in search of a small aquatic insect. Fig. 2, a polype
stretching itself into a boat-like form, to take or swallow a worm lying
sideways. Fig. 4, the same polype with the worm swallowed and bent
within it. Fig. 6, is a polype in the situation they generally assume
when they have satisfied their voracious appetite. Fig. 7, one that has
swallowed a small monoculus. Fig. 9, a, one whose arms are loaded with
monoculi. Fig. 10, a polype full of them from head to tail. Fig. 3, one
that has only swallowed a few of them. Fig. 8, represents two polypes
engaged in combat for a worm, of which both of them have swallowed a
part.
Plate XXIV. B. Fig. 11, represents a polype engaged with a very large
worm. Fig. 12, a worm seen within the skin of a polype. Fig. 13, a
polype disgorging the excrementitious parts of a worm.
Plate XXI. Fig. 12, a polype that has swallowed a small fish, and taken
the shape thereof.
OF THE GENERATION OF THE HYDRÆ.
As the hydra fusca and the hydra grisea are considerably larger than the
hydra viridis, it is more easy to observe the manner of their producing
their young. It is upon these, therefore, that most of the observations
here recited have been made. If one of them be examined in summer, when
the animals are most active, and more particularly prepared for
propagation, it will be found to shoot forth from its side several
little tubercles, or knobs, which grow larger and larger every day;
after two or three days inspection, what at first appeared but a small
excrescence, takes the figure of a small animal, entirely resembling its
parent. It does not inclose a young polype, but is the real animal in
miniature, united to the parent, as a sucker to the tree.
When a young polype first begins to shoot, the excrescence terminates in
a point, as at e, Plate XXIV. B. Fig. 24; so that it is rather of a
conical figure, and of a deeper colour than that of the body. This cone
soon becomes truncated, and in a little time appears cylindrical. The
arms then begin to shoot from the anterior end c i. The tail adheres to
the body of the parent, but grows gradually smaller, till at last it
only adheres by a point b, Fig. 23, it is then ready to be separated;
for this purpose the mother and young ones fix themselves to the glass,
or other substance upon which they may be situated. They have then only
to give a sudden jerk, and they are divided from each other. There are
some trifling differences to be observed now and then in their
performing this operation, which it would be too tedious to enumerate
here. A polype, a b, Fig. 20, with a young one, c d, places its body in
an arch of a circle a d b, against the sides of the glass, the young one
being fixed at the top d of the arch, with its head also fixed against
the glass; so that the mother, by contracting the body, and thus
becoming straight, loosens herself from the young one.
The young ones shoot in proportion to the warmth of the weather, and the
nature of the food eaten by the mother; some have been observed to be
perfectly formed in twenty-four hours, while others have required
fifteen days for the same purpose; the first were produced in the midst
of summer, the latter in a cold season.
The tail of the young polype communicates with, and partakes of the food
from the parent in the same manner as its own arms do, and the food lies
in the same manner as in the arms. When this fœtus is furnished with
arms, it catches its prey, swallows, digests, and distributes the juices
thereof even to the parent body; every good is common to each. Here then
we have evident communication between the fœtus and the mother; this
communication was further proved by the following experiment. A large
polype, one of the hydra fusca, was placed on a slip of paper, in a
little water; the middle of the body of the young one was cut, and the
superior part of that end which remained fixed to the parent was found
to be open. The parent polype was then cut on each side of the shoot.
Thus a short cylinder was obtained, which was open at both ends. This
being viewed through a microscope, the light was seen to come through
the side slip, or young one, into the stomach of the old one. For
further conviction, the cylindrical portion was cut lengthways; on
observing these parts, not only the hole t of the communication, Plate
XXIV. B. Fig. 17, was distinctly seen, but one might see through the end
o of the young one. On changing the situation of these two pieces of
prepared polypes, and looking through the opening e, Fig. 18, the
day-light was seen through the hole of communication i.
This communication, between the parent polype and its young ones may be
seen on feeding them; for, after the parent a b, Plate XXIV. B. Fig. 22,
has eaten, the bodies of the young ones swell, being filled with the
aliments as if they themselves had been eating. In the hydra fusca the
young ones do not proceed from the tail part b c, Plate XXIII. B. Fig.
16, but only from the part a c, with this exception, there is no
particular part of the body before the rest, on which they produce their
young. Some of them have been so closely observed, and have so greatly
multiplied, that there would be scarce any impropriety in saying they
produced their young ones from all the exterior parts of their body. A
polype puts forth frequently five or six young ones at the same time.
Trembley has had some that have produced nine or ten at the same time,
and when one dropped off another came in its place.
Though this gentleman had for two years thousands of them under his eye,
and considered them with the most scrupulous attention, he never
observed any thing like copulation. To be more certain on this head, he
took two young ones the instant they came from their parent, and placed
them in separate glasses; they both multiplied, not only themselves, but
their offspring, which were separated and watched in the same manner to
the seventh generation; nay, they have even the faculty of multiplying
while they adhere to the parent. The arms of the young ones do not
sprout till the body has attained some length.