Having in the two preceding chapters given the reader such a general idea of the history and oeconomy of a great variety of those minute animated bodies, called insects, as I am inclined to hope has not only afforded him entertainment and instruction, but tended to excite an emulation for further researches; I shall endeavour to gratify so laudable a disposition, by introducing him to a class of beings whose oeconomy and singular properties equally engage the attention of the philosopher and the natural historian; a scene which opposes our general system of vitality, and which presents to the eye of the mind, as well as that of the body, a series of astonishing wonders. It is among the minutiÆ of nature that we find her models most diversified, and displaying the marvellous fecundity of its powers.

The polypes described in this chapter are fresh-water insects, of the genus of hydra, of the order of zoophytes, and class of vermes. The body consists of a single tube, furnished at one end with long arms, by these it seizes small worms, and conveys them to its mouth. It has, according to our general notions, neither head, heart, stomach, nor intestines of any kind; and is without the distinction of sexes, yet extremely prolific. From the simplicity of its structure those of its oeconomy and functions are probably derived. When they are cut or divided into a number of pieces, the separated parts in a very little time become so many perfect and distinct animals; each piece having a power of producing a head, a tail, and the other organs necessary for its existence.

They are generally known by the name of polype; but as this was thought by many to be improper, because that, strictly speaking, they have no feet, LinnÆus called the genus hydra, probably from their property of re-producing the parts which are cut off, a circumstance that naturally brings to mind the fabulous story of the Lernean hydra. Dr. Hill called them biota, on account of the strong principle of life with which every part is endued.

Leeuwenhoeck, whose indefatigable industry in his researches after small insects permitted very few things to escape his notice, discovered these animals, and gave some account of them in the Philosophical Transactions for the year 1703. There is also in the same volume a letter from an anonymous hand on this subject. We had, however, no regular account of them, their various habits, their different species, or of their wonderful properties, till the year 1740, when they first engaged the attention of M. Trembley, to whose assiduity and observations we are indebted for the display of their nature and oeconomy.

Previous to the successful experiments of this gentleman, Leibnitz and Boerhaave, as well as some of the ancient philosophers, reflecting on the various gradations in the scale of animated nature, had endeavoured to prove that there might be degrees of life between the animal and the plant, and that animals might be found which would propagate by slips, like plants. These conjectures were verified by Trembley, but not in consequence of any pre-conceived ideas in favour of such a supposition; on the contrary, it was only by repeated observations that he could destroy his own prejudices, and join these wonderful beings to the animal kingdom.[106]

Though natural history is so fruitful in extraordinary facts, it has hitherto produced none so singular as the various properties of the different species of the hydra.

I shall endeavour, first, to trace the progress of this discovery, in which we shall see with what sage caution and accuracy Trembley, and other naturalists examined this wonderful phÆnomenon, and what accumulated evidence was judged necessary to establish the fact.

We find M. Trembley writing in January, 1741, to M. Bonnet, that he did not know whether he should call the object which then engaged his attention, a plant or an animal. “I have studied it,” says he, “ever since June last, and have found in it striking characteristics of both plant and animal. It is a little aquatic being. At first sight, every one imagines it to be a plant; but if it be a plant, it is sensitive and ambulant; if it be an animal, it may be propagated by slips or cuttings, like many plants.” It was not till the month of March, in the same year, that he could satisfy himself as to their nature.

When Reaumur saw, for the first time, two polypes formed from one that he had divided into two parts, he could hardly believe his eyes; and even after having repeated the operation an hundred times, and again examined it an hundred more, he says that the sight was not become familiar to him.

The first account the Royal Society received of the surprizing properties of the hydra, was in a letter from M. Buffon, dated the 18th of July, 1741, to Martin Folkes, Esq. their president, acquainting them with the discovery of a small insect called a polypus, which is found sticking about the common duck weed, and which, being cut in two, puts forth from the upper part a tail, and from the lower end a head, so as to become two animals instead of one. If it be cut into three parts, the middlemost puts out from one end a head, and from the other a tail, so as to become three distinct animals, all living like the first, and performing the various offices of their species: which observations are, adds Buffon, well averred.

There is no phÆnomenon in all natural history more astonishing than this, that man, at pleasure, should have a kind of creative power, and out of one life make two, each completely formed with all its apparatus and functions, its perceptions and powers of motion and self-preservation; and as complete in all respects as that from which they derived their existence, and equally enjoying the humble gratifications of their nature.[107]

Mr. Folkes, in confirmation of the foregoing article, communicated to the Society a letter from the Hon. W. Bentinck, Esq. at the Hague, dated September, describing the insects discovered by Trembley, adding, that he himself had seen them. In November, a letter was read from Dr. Gronovius, of Leyden, giving an account of a water insect not yet known to, or described by any author; after describing it, he adds, “but what is more surprizing, if this animal is cut into five or six pieces, in a few hours there will be as many animals, exactly similar to their parent.” The accounts of this animal were so extraordinary, that they were not credited until Professors Albinus and Musschenbroeck were provided with some specimens, and found all that had been related thereof to be exactly true.

November 25, a letter from Cambridge was read to the Royal Society, in which the author endeavours to lessen, by reason, the prejudices which then combated the belief of these facts. “Some of our friends,” says the author, “who are firmly attached to the general metaphysical notions they have formerly learned, reason strongly against the possibility of such a fact: but I have myself owned on other occasions, my distrust of the truth, or certainty at least, of some of those principles, and I shall make no scruple of acknowledging, that I have already seen so many strange things in nature, that I am become very diffident of all general assertions, and very cautious in affirming what may or may not possibly be. The most common operations both of the animal and vegetable world, are all in themselves astonishing, and nothing but daily experience and constant observation can make us see without amazement an animal bring forth another of the same kind, or a tree blossom and bear leaves and fruit.

“The same observation and experience make it also familiar to us, that, besides the first way of propagating vegetables from their respective fruit and seed, they are also propagated from cuttings, and every one knows that a twig of a willow particularly, cut off and only stuck into the ground, does presently take root and grow, and become as real and perfect a tree as the original one from which it was taken. Here then we find in the vegetable kingdom quite common, the very thing of which we have an example before us in the animal kingdom, in this new-discovered insect. The best philosophers have long observed strong analogies between these two classes of beings; and the more they have penetrated into nature, the more they have extended this analogy: now in such a scale, who is the man that will be bold to say, just here animal life entirely ends, and here vegetable life begins? or, just so far, and no farther, one sort of operation goes; and just here another sort, quite different, takes its place? or again, who will venture to say, life in every animal is a thing absolutely different from that which we dignify by the same name in every vegetable?” Thus does the author endeavour to persuade the prejudiced, and lead them to pay attention to the facts which were now laid open to their view, and which were further confirmed by a letter from M. Trembley, in January 1740; which letter was strengthened by an extract from the preface to the sixth volume of Reaumur’s history of insects. In March, 1742, Mr. Folkes gave an account of them to the Royal Society, from observations made on several polypes which had been sent by M. Trembley from Holland to him. The insects now began to be known, and were soon found in England, and the experiments that had been made on them abroad were published by Mr. Folkes,[108] my father,[109] and Mr. Baker:[110] conviction now became too strong for argument, and metaphysical objections gave way to facts. The animal is described in the following manner:

HYDRA.[111]

Flos: os terminale, cinctum cirris setaceis. Stirps vaga, gelatinosa, uniflora, basi se affigens.[112]

This animal fixes itself by its base, it is gelatinous, linear, naked, can contract itself, and change its place. Its mouth, which is at one end, is surrounded by hair, like feelers. It sends forth its young ones from its sides, which drop off.

1. Hydra viridis, tentaculis subdenis brevioribus.

Green polype, generally with about ten short arms; it is represented in Plate XXI. Fig. 5.

2. Hydra fusca, tentaculis suboctonis longissimis.

This polype has very long arms, often eight in number; it is represented at Plate XXI. Fig. 7. The arms are several times longer than the body.

3. Hydra grisea, tentaculis subseptenis longioribus.

This polype has also generally long arms, in number about seven; it is of a yellowish colour, small towards the bottom; it is represented at Plate XXI. Fig. 6.

4. Hydra pallens, tentaculis subsenis mediocribus.

The arms of this polype are generally about six in number, and of a moderate length.

5. Hydra hydatula, tentaculis quaternis obsoletis corpore vesicario. Plate XXI. Fig. 1, 2, 3, 4.

This polype has a vesicular body, and four obsolete arms; is found in the abdomen of sheep, swine, &c.

6. Hydra stentorea, tentaculis ciliaribus corpore infundibuliformi.

This polype has been called tunnel-shaped; the mouth is surrounded with a row of hairs; it is represented at Plate XXII. Fig. 27 and 28.

7. Hydra socialis, mutica torosa rugosa.

Bearded, thick, and wrinkled. Plate XXI. Fig. 11.

OF THE HYDRA VIRIDIS, HYDRA FUSCA, AND HYDRA GRISEA.

Plate XXI. and XXIII.

These three species of the hydra having been those on which the greatest number of experiments have been made, and of which we have the best information, it is of these only I shall speak in the following account, unless it is particularly mentioned otherwise.

There are few animals more difficult to describe than the hydra, as it has scarce any thing constant in its form, varying continually in its figure: they are often so beset with young, as to appear ramose and divaricated, the young ones constituting as it were a part of the parent’s body.

Whoever has looked with care at the bottom of a wet shallow ditch, when the water is stagnant, and the sun has been powerful, may remember to have seen many little transparent lumps, of a jelly-like appearance, about the size of a pea, and flatted on one side; the same appearances are also often to be seen on the under side of the leaves of those weeds or plants that grow on the surface of the water; these are the hydrÆ gathered up into a quiescent state, and seemingly inanimate, because either undisturbed or not excited by the calls of appetite to action. They are generally fixed by one end to some solid substance, at the other end there is a large opening, round about which the arms are placed as so many rays round a center, which center is the mouth.

They are slender and pellucid, formed of a tender kind of substance, in consistence something like the horns of a snail, and can contract the body into a very small compass, or extend it to a considerable length. They can do the same with the arms; with these they seize minute worms and various kinds of aquatic insects, bring them to the mouth, and swallow them. After the food is digested, and the nutritive parts which are employed in sustaining its life are separated from the rest, they reject the remainder by the mouth.

The first polype which Trembley discovered was one of the hydra viridis, represented in Plate XXI. Fig. 5. These are generally of a fine green colour. The indications of spontaneous motion were first perceived in the arms of these little creatures; they can extend or contract, bend and wind them divers ways. Upon the slightest touch they contract themselves so much, as to appear little more than a grain of a green substance, the arms disappearing entirely. He soon after found the hydra grisea, Fig. 6, and saw it eat, swallow, and digest worms much larger than itself. This discovery was soon followed by that of the hydra fusca, Fig. 7.

The most general attitudes of these hydrÆ are those which are represented in Fig. 5 and 6 of the same plate. They fix the posterior extremity b against a plant or other substance, as e f; the body a b; and the arms a c, being extended in the water. There is a small difference in the attitudes of the three kinds which we are now describing.

The bodies of the hydra viridis, Fig. 5, and of the hydra grisea, Fig. 6, diminish from the anterior to the posterior extremity by an almost insensible gradation. The hydra fusca does not diminish in the same gradual manner, but from the anterior extremity a, to the part d, which is often two-thirds of the length of their body, it is nearly of an equal size; from this part it becomes abruptly smaller, and goes on from thence of a regular size to the end. The number of arms in these three kinds are at least six, and at most twelve or thirteen, though eighteen may sometimes be found on the hydra grisea. They can contract their bodies till they are not above one-tenth of an inch in length; they can also stop at any intermediate degree, either of contraction or extension, from the greatest to the least. The species represented at Fig. 5, are generally about half an inch long when stretched out. Those exhibited at Fig. 6 and 7, are about three-fourths of an inch, or one inch in length, though some are to be found at times about an inch and half long. The arms of the hydra viridis, Fig. 5, are seldom longer than their bodies; those of Fig. 6 are commonly one inch long, while those of Fig. 7 are generally about eight inches; whence Trembley has called it the long-armed polype.

The bulk of the hydrÆ decreases, in proportion as they extend themselves, and vice versa. They may be made to contract themselves, either by touching them, or agitating the water in which they are contained. They all contract themselves so much when, taken out of the water, as to appear only like a little lump of jelly. They can contract or extend their arms without extending or contracting the body, or the body, without making any alteration in the arms; or they can contract or dilate only some of the arms, independent of the rest: they can also bend their body and arms in all possible directions. Those represented at Fig. 7 let their arms in general hang down, making different turns and returns, often directing some of them back again to the top of the water. They can also dilate the body at different places, sometimes at one part, and then again at another; sometimes they are thick set with folds, which, if carelessly viewed, might be taken for rings.

They have a progressive motion, which is performed by that power by 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 manoeuvres 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 foetus 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 foetus 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.

Several excrescences or buds often appear at the same time on a polype, which are so many polypes growing from one trunk; whilst these are developing, they also bud, which buds again put forth little ones, the parent and the young ones forming a singular kind of animal society, in which all participate of the same life, and the same wants. In this state, the parent appears like a shrub thick set with branches. Several generations are often thus attached to one another, and all to the parent polype; after a time, this tree of polypes or hydrÆ is decomposed, and gives birth to new generations, or fresh genealogical trees. Here we see a surprizing chain of existence continued, and numbers of animals naturally produced, without any union of sexes; every polype raising a numerous posterity by a kind of animal vegetation.

From Fig. 16, Plate XXIII. B. the reader may form an idea of the promptitude with which these creatures increase and multiply; the whole group formed by the parent and its young was about an inch and an half long, and one inch broad, the arms of the mother and her nineteen little ones hanging down towards the bottom of the vessel; the animal would eat about twelve monoculi per day, and the little ones about twenty among them, or rather more than thirty for the group.

OF THE RE-PRODUCTION OF THE HYDRÆ.

So strange is the nature of this creature’s life, that the method by which other animals are killed and destroyed becomes a means of propagating these. When divided and cut to pieces in every direction that fancy can suggest, it not only continues to exist, but each section becomes an animal of the same kind.

A polype cut transversely or longitudinally, in two or three parts, is not destroyed; each part in a little time becomes a perfect polype. This species of fecundity is so great in these animals, that even a small portion of their skin will become a little polype, a new animal rising as it were from the ruins of the old, each small fragment yielding a polype. If the young ones be mutilated while they grow upon the parent, the mutilated parts are re-produced; the same changes succeed also in the parent. A truncated portion will put forth young before it is perfectly formed itself, or has acquired its new head and tail; sometimes the head of the young one supplies the place of that which would grow out of the anterior part of the trunk.

If a polype be slit, beginning at the head, and proceeding to the middle of the body, a polype will be formed with two heads, and will eat at the same time with both. If the polype be slit into six or seven parts, it becomes a hydra with six or seven heads. If these be again divided, we shall have one with fourteen; cut off these, and as many new ones will spring up in their place, and the heads thus cut off will become new polypes, of which so many new hydrÆ may again be formed; so that in every respect it exceeds the fabulous relation of the Lernean hydra.

As if the wonders already related of the polype were not sufficient to engage our attention to these singular animals, new circumstances, as surprizing as the foregoing, present themselves to convince us of the imperfection of our ideas of animality, and of the greatness of the power of our Lord and Saviour, who is the source and origin of every degree of life, in all its immense gradations, as unity is the origin of number in all its varied series, multiplied proportions and combinations; and as numbers may be considered as recipient of unity, in order to make manifest the wonderful powers thereof, so the universe and its parts are adapted to receive life from the source of all life, and thus become representatives of his immensity and eternity.

The polypes may be as it were grafted together. If the truncated portions of a polype be placed end to end, and then pushed together with a gentle force, they will unite, and form a single one. The union is at first made by a fine thread, and the portions are distinguished by a narrow neck, which gradually fills up and disappears, the food passing from one portion to another. Portions not only of the same, but pieces of different polypes may be thus united together. You may fix the head of one polype to the trunk of another; and that which is thus produced, will grow, eat, and multiply like another.

There is still another method of uniting these animals together, more wonderful in its nature, and less analogous to any known principles of animation, and more difficult to perform. It is effected by introducing one within the other, forcing the body of one into the mouth of the other, and pushing it down so that their heads may be brought together: in this state it must be kept for some time; the two individuals are at last united, and grafted into each other; and the polype, which was at first double, is converted into one, with a great number of arms, and performs all its functions like another.

The hydra fusca furnishes us with another prodigy, to which we know nothing that is similar either in the animal or vegetable kingdom. They may be turned inside out like a glove, and, notwithstanding the apparent improbability of the circumstance, they live and act as before. The lining or coating of the stomach now forms the epidermis, and the former epidermis now constitutes the coating of the stomach. A polype thus turned, may often have young ones attached to its side. If this be the case, after the operation they are of course inclosed in the stomach. Those which have acquired a certain size extend themselves towards the mouth, that they may get out when separated from the body; those which are but little grown, turn themselves inside out, and by these means place themselves again on the outside of the parent polype.

The polype thus turned combines itself a thousand different ways. The fore-part often closes itself, and becomes a supernumerary tail. The polype which was at first straight, now bends itself, so that the two tails resemble the legs of a pair of compasses, which it can open and shut. The old mouth is at the joint as it were of the compasses; it cannot, however, act as one, so that a new one is formed near it, and in a little time a new species of hydra is formed with several mouths.

Plate XXIII. B. Fig. 18, represents the upper part of a polype that has been divided into two parts; a, the upper, c, the lower part, the end c being something larger than that of a common polype, and is sensibly perforated; in the summer time this part often walks and eats the same day it is cut. Fig. 17, the other part of the same polype; the anterior end is very open, and the edges of it turned a little outwards, which afterwards folding inwards, close the aperture. This end now appears swelled, as at c, Fig. 21; the arms shoot out from this end: at first three or four points only begin to shoot, as at c, Fig. 20, and while these increase in size, others appear between them; they can seize their prey and eat before their arms have done growing. In the height of summer the arms will often begin to shoot in twenty-four hours; but in cold weather it will be fifteen or twenty days before the head is formed. Fig. 22, represents a polype that was cut close under the arms; this became also a complete animal in a little time.

The sides of a polype that has been cut longitudinally, roll themselves up in different ways, generally beginning at one of the extremities, rolling itself up in a heap, as in Plate XXIII. B. Fig. 19, with the outside of the skin inwards; it soon unrolls itself, and the cut sides form themselves into a tube, whereof the edges a b and e i, Fig. 15, on both sides meet each other and unite. Sometimes they begin to join at the tail end, at other times the whole sides gradually approach each other. The sides join so close, that from the first moment of their junction no scar can be discovered. Fig. 14, represents a polype partly joined, as at i b, the part c a e not yet closed. Fig. 29, represents a polype, the heads of which have been repeatedly divided, by which means it becomes literally a hydra. Fig. 24, represents a polype that has been turned, endeavouring to turn itself back again, the skin of the anterior part lying back upon the other; the arms varying in their direction, being sometimes turned towards the head, see Fig. 24 and 26, at others, towards the tail. The anterior extremity c, formed by the edges of the reversed part a, remained open for some days, and then began to close; new arms shot out near the old ones, and several mouths were formed at those parts where the arms joined the body. Fig. 23, 25, 27, 28, represent the different changes that took place in another polype that had been turned inside out, and the different revolutions it went through before it acquired a fixed state; a c always shews the part the polype had turned back, and a b the part it could not turn back.

A polype, which has been partly turned back, remains but a little time in that situation. Fig. 28, a, the part where the portion it had turned back joined to the body a b; this became straight, and formed a right angle with a b; the same day another head appeared at e, and several arms, a o, a n, began to shoot from the mouth a; at the other side of this mouth there were the old arms a d. The next day the portion a c was drawn near the body, and formed an acute angle with it, as at Fig. 25. Fig. 27, represents the same swelled, after having swallowed a worm. Four days afterwards its form had varied considerably, as may be seen by comparing Fig. 25 and 28, having now one common mouth, and two small polypes growing on it.

We may now be permitted to make a few reflections on this singular animal. On considering the various properties that have been already described, many particulars will be found in them that are very analogous to others that are continually carrying on around us; we perceive that there is a successive unfolding of new parts. In every organized frame there is a continual effort to extend its sphere of action, and enlarge the operation of that portion of life which is communicated to it. This gradual evolution requires a secret and curious mechanism, to regulate and modify by re-action the continued conatus of the forming principle within it. The polype is an organized whole, of which each part, each molecule, each atom, tends to produce another; it is, if we may so speak, one entire ovary, a compound of germ, or seed. In cutting a polype to pieces, the nourishing juices, which would have been employed in supporting the whole, are made to act upon each portion.

When a polype is divided longitudinally, it forms two half tubes; the opposite edges of these approach, and in a very short time form a perfect tube. The sides are made to touch each other by certain motions and contractions of the piece; but as soon as the edges come in contact, a slight adhesion takes place, the corresponding vessels unite, and new ones are unfolded, as in a vegetable graft; by these means the points of connection and cohesion are multiplied, the motion of the fluids is re-established, and with them the vital oeconomy. This is performed with more rapidity than in vegetables, because the polype is nearly gelatinous, and its parts are extremely ductile; this ductility is supported and preserved by the element which it inhabits. The same reasoning applies equally to explain the formation of so many heads to a polype, as constitute it a real hydra.

A new polype is formed out of small portions or fragments, in a very different manner, the operations in nature being always varied, according as the circumstances differ; each fragment is puffed up, the skin separated, and an empty space is formed within it; this part is to become the stomach of the rising polype, which soon sends forth arms, and is formed to the perfection proper to its kind. We learn from this instance that the skin of the polype is not so simple as was at first imagined; for we find it dividing itself into two membranes, and forming thereby a cavity fit to perform all the functions of a stomach; but why these membranes are separated in the small portions, and not in the larger, we cannot tell; but though we are ignorant of this, and many more circumstances relative to the re-production of these little animals, yet the foregoing facts enable us to understand better the nature of the existence of these polypes which have been turned inside out.

For as that part which formed the interior skin of the stomach in the little fragments before-mentioned, became the exterior part of the animal, the inside of the polype is consequently so similar to the exterior skin, that one may be substituted for the other, without injuring the vital functions; from hence we might, in some measure, have inferred the possibility of the polypes living, after they have been turned inside out, independent of the fact itself.

The viscera of the animal are situated in the thickness of the skin, and absorbing pores are placed both on the inside and outside, so that the animal can live whether the skin be turned one way or the other. The Author of nature did not create the polype to be turned as we turn a glove; but he formed an animal whose viscera were lodged in the thickness of the skin, and with powers to resist the various accidents to which it was unavoidably exposed by the nature of its life; and the organization necessary for this purpose was so constructed, that the skin might be turned without destroying life.

Every portion of a divided polype has, like the vegetable bud, all the viscera necessary to its existence; it can, therefore, live by itself, and push forth a head and tail, when placed end to end against another piece. The vegetation consists in uniting the portions, the vessels of each part increase in length, and a communication is soon formed between them, which unites the whole. The ease with which the parts unite, is as has been observed before, probably owing to their gelatinous nature; for we find many similar instances in tender substances. The solid parts of the embryo, as the fingers, unite in the womb; tender fruit and leaves may be also thus united.

A portion of these creatures is capable of devouring its prey almost as soon as it is divided from the rest. In the structure of those animals which are most familiar to us, a particular place is appropriated for the developement and passage of the embryo. But on the body of an animal, which, like a tree, is covered with prolific gems, it is not surprizing that the young ones should proceed from its sides, like branches from a tree. The mother and her young ones form but one whole; she nourishes them, and they contribute to her existence, as a tree supports, and is reciprocally supported by its branches and leaves.

OF THE HYDRA PALLENS.

The hydra pallens has been fully described only by M. RÖsel;[113] it is very seldom to be met with, is of a pale yellow colour, and grows smaller gradually from the bottom, the tail is somewhat round or knobbed, the arms are about the length of the body, of a white colour, and generally seven in number, apparently composed of a chain of globules; it brings forth the young from all parts of its body. LinnÆus defines it as, hydra pallens tentaculis subsenis mediocribus;[114] Pallas as, hydra attenuata corpore flavescente, sursum attenuato.[115]

OF THE HYDRA HYDATULA.

Plate XXI. Fig. 1, 2, 3, and 4.

The next in order is the hydra hydatula, which we have already defined from LinnÆus as a hydra with four obsolete arms, and a vesicular body: it is spoken of by several medical writers, who are enumerated in the Systema NaturÆ, p. 1321. It is described also by Hartman, Misc. Nat. Cur. Dec. I. An. 7, Obs. 206, Dec. II. An. 4, Obs. 73, as hydatis animata; also in the Dissert. de Inf. Viv. p. 50; n. 6, tÆnia hydatoidea. Pallas defines it as tÆnia hydatigena rugis imbricata corpore postice bulla lymphaticÆ terminato. The following description is extracted from that in the Philosophical Transactions, No. 193, by Dr. Tyson, who names it lumbricus hydropicus.

In the dissection of a gazella or antelope, Dr. Tyson observed several hydatides or films filled with water, about the size of a pigeon’s egg, and of an oval form, fastened to the omentum, and some in the pelvis, between the bladder of urine and the rectum; and he then suspected them to be a particular sort of insect, bred in animal bodies, or at least the embryos or eggs of them: 1. Because he observed them included in a membrane, like a matrix, so loosely, that by opening it with a finger or knife, the internal bladder, containing the serum or lympha, seemed no where to have any connection with it, but would very readily drop out, still retaining its liquor, without spilling any of it. 2. He observed that this internal bladder had a neck or white body, more opake than the rest of the bladder, and protuberant from it, with an orifice at its extremity, by which, as with a mouth, it exhausted the serum from the external membrane, and so supplied its bladder or stomach. 3. Upon bringing this neck near the candle, it moved and shortened itself. Fig. 1, represents one of these watery bladders inclosed in its external membrane, its shape was nearly round, being only a little depressed or flatted, as a drop of quicksilver will be by lying on a plane. In Fig. 2, the neck is better seen; the external membrane being taken off, an open orifice is found at its extremity; it consists of circular rings or incisures, which are more visible when magnified, as in Fig. 3; it then appears granulated with a number of little eminences all over the surface; the orifice at the extremity seems to be formed by retracting itself inwards, and upon trial it was found to be so; for in Fig. 4, the neck of this polype is represented magnified and drawn out its whole length; on opening it there were found within the two strings a, a, which probably convey into the stomach the moisture and nourishment, which the animal, by protruding its neck, extracts from the external membrane.[116]

OF THE HYDRA STENTOREA.

Plate XXII. Fig. 27 and 28.

Hydra tentaculis ciliaribus corpore infundibuliformi.

The arms of this hydra are rows of short hairs, the body trumpet-shaped.

This species of hydra is very common, and has been described by almost every writer on these subjects; it is placed by MÜller among the vorticellÆ.

Vorticella stentorea caudata, elongata, tubÆformis limbo ciliato. MÜller animalcula infusoria.

Mr. Baker originally named it the funnel-like polype, which Messrs. Trembley and Reaumur changed to the tunnel-like polype, under which name it appears in the Philosophical Transactions, No. 474.

There are three kinds of them, which are of different colours, green, blue, and white. The white ones are the most common. It is necessary to observe them often, and in various attitudes, in order to obtain a tolerable idea of their structure. They do not form clusters, but adhere singly by their tail to whatever comes in their way; their anterior end is wider than the posterior, and being round, gives the animal somewhat of a funnel form, though it is not completely circular, having a sort of slit or gap that interrupts the circle. The edge of this opening is furnished with a great number of fibrillÆ, which by their brisk and continual motions excite a current of water; the small bodies that float or swim near this current, are forced by it into the mouth of the little animal. Trembley says, that he has often seen a number of very small animalcula fall one after another into the mouth, some of which were afterwards let out again at another opening, which he was not able to describe.

They can fashion their mouths into several different forms. If any thing touch them, they shrink back and contract themselves. They live independent of each other, swimming freely through the water in search of their prey, and fix to any thing they meet with.

These animals multiply by dividing themselves, not longitudinally, nor transversely, but sloping and diagonal wise; the proceedings in nature continually varying in every new form of life. Of the two polypes produced by the division of one, the first has the old head and a new tail; the other, the old tail and a new head.

To make the description more clear, Trembley called that with the old head the superior polype, that with the new head the inferior one. The first particular that is observable in these polypes, when they are going to divide, is the lips of the inferior one; a transverse and oblique stripe indicates the part where it is going to divide; the new lips are formed at about two-thirds of the length of the polype, reckoning from the head; the division is made in a sloping line, that goes about half way round the parent animal; these lips are at first discerned by a slow motion, which engages the attention of the observer. They then insensibly approach each other and close, whereby a swelling is formed on the side of the polype, which is soon found to be a new head. When the swelling is considerably increased, the two polypes may be plainly distinguished. The superior one being now connected with the inferior one only by its lower extremity, is soon detached from it, and swims away to fix itself on some convenient substance; the inferior one remains fastened to the place where the original polype was fixed before the division.

From the various modes by which different species of polypes are multiplied, we are led to form more exalted ideas of nature, and to see that the little we discover is but an exceeding small part of her contents; we learn also to be more cautious in reasoning from analogy, and laying down the known for a model to the unknown, because we find that the operations in nature are varied ad infinitum.

The growth of the hydra fusca is very quick, but that of the hydra stentorea is much more so. The progress of the foetus is always more rapid than that of the infant and adult animal; but in these organized atoms the evolution is so rapid, as to appear almost like an immediate creation.

Fig. 28 represents the hydrÆ stentoreÆ, or funnel-polypes, fixed to the under side of a piece of some vegetable substance; they are in this figure of their natural size.

Fig. 27, the same polypes magnified; the different forms they assume are also seen here, sometimes short and thick, as at m m; long, as at n; nearly globular, as at o; extended to the full size, as at k; seen as contracted at i. The fibrillÆ or little hairs may be seen in most of the attitudes except those of l.

OF THE HYDRA SOCIALIS.

Plate XXI. Fig. 11.

Hydra socialis mutica torosa rugosa.[117]

Social hydra, bearded thick and wrinkled.

This species of hydra has been described by many writers. It is the vorticella socialis of MÜller, who defines it as vort cella caudata, aggregata, clavata; disco obliquo. MÜller Animalcula Infusoria, p. 304. Pallas makes it a brachionus, Pall. Zooph. 53.

In Fig. 11, these animals are represented as considerably magnified; they appear like a circle, surrounded with crowns, or ciliated heads, tied by small thin tails to a common center, from whence they advance towards the circumference, where they turn like a wheel, with a great deal of vivacity and swiftness, till they occasion a kind of whirlpool, which brings into its sphere the proper food for the polype. When one of them has been in motion for a time, it stops, and another begins; sometimes two or three may be perceived in motion together. They are often to be found separate, with the tail sticking in the mud. The body contracts and dilates very much, so as sometimes to have the appearance of a cudgel; at others, to assume almost a globular form. The young polypes of this species have been sometimes taken for the hydra stentorea.

OF THE VORTICELLÆ.

We now come to another division of these animals, to which later writers have given the name of vorticellÆ; this term I shall therefore adopt, being of opinion that it behoves every man to maintain that order in scientific arrangement which is not inconsistent with truth, except he can produce another arrangement more expressive of the nature of the objects it is designed to discriminate; a process requiring no small degree of attention.

The variety that may be observed in these minute animals confirms a principle, which, the more it is inquired into, the more it will be found to accord with the general operations in nature, namely, that there is always a pre-existent principle of life necessary to the organization both of animals and vegetables; that the alimentary and other particles which are added to, or apparently belong to them, produce nothing of themselves; they are incapable of forming the least fibre, but they are able to become constituent parts of one organical whole, together with the instruments whereby the forming principle is manifested, and rendered capable of acting upon certain orders of creatures.

VORTICELLA.

Animal calyce vasculoso; ore contractili ciliato, terminali. Stirps fixa.

A small animal, with a vascular cup; the mouth is at one end ciliated, and capable of being contracted, the stem fixed.

VORTICELLA ANASTATICA.

Plate XXI. Fig. 13, 14, 15, and 16*.

Vorticella anastatica, composita, floribus campanulatis, stirpe multiflora rigescente.

Vorticella anastatica, compound, with bell-shaped flowers, and a rigid stem.

Cluster polype, second species. Trembley, Philos. Trans. vol. xliv. part. 2. p. 643.

These polypes form a group resembling a cluster, or more properly an open flower; this flower or cluster is supported by a stem, which is fixed by its lower extremity to some of the aquatic plants or extraneous bodies that are found in the water; the upper extremity forms itself into eight or nine lateral branches, perfectly similar to each other; these have also subordinate branches, whose collective form much resembles that of a leaf. Every one of these assemblages is composed of one principal branch or nerve, which makes with the main stem of the cluster an angle somewhat greater than a right one; from both sides of this nerve the smaller lateral branches proceed; these are shorter the nearer their origin is to the principal branch.

At the extremity of the principal branch, and also of all the lateral ones, there is a polype or vorticella. There are others on both sides of the lateral twigs, but at different distances from their extremity. These polypes are all exceeding small, and of a bell-like figure; near their mouth a quick motion may be discerned, though not with sufficient distinctness to convey an adequate idea of its cause; upon the branches of these clusters are round bodies, which will be more particularly described presently.

Every cluster has eight or nine of these branches or leaves; they do not all proceed from the same point, but the points from whence they set out are not far asunder; each of these branches is bent a little inwards, so that all of them taken together form a kind of shallow cup. If the eye be placed right over the base of this cup, the appearance of the whole eight or nine branches is like unto that of a star, with so many rays proceeding from the center. If the cluster be slightly touched, all the branches instantly fold up, and form a small round mass. The stem which supports the cluster contracts also at the same time, folding up like a workman’s measuring rule, that consists of three or four joints. This extraordinary assemblage constitutes one organized whole, formed of a multitude of similar and particular ones. A new species of society, in which all the individuals are members of each other in the strictest sense, and all participate of the same life.

A few days after one of these clusters is formed, small round bodies or bulbs may be perceived to protrude in several places from the body of the branch; these grow very fast, and arrive at their greatest growth in two or three days. The bulbs detach themselves from the branches out of which they spring, and go away, swimming till they can settle upon some substance which they meet with in the water, and to which they fix themselves by a short pedicle; the bulbs are then round, only a little flatted on the under side, the pedicle continues to lengthen gradually for about twenty-four hours, during the same time the bulbs also change their figure, and become nearly oval. There are in a cluster but few of these bulbs, compared with the number of the vorticellÆ, neither do all the bulbs come out at the same time. The bulb then divides lengthways into two smaller ones, but which are still much larger than the vorticellÆ themselves. It is not long before these are separated like the first, and thus form four bulbs on the same stalk; these again divide themselves, and form eight; which again subdivide, and consequently make sixteen. They are all connected with the stalk by a proper pedicle, but they are not all of an equal size; the largest continue to divide, and the smallest begin to open, and take the bell-formed shape. Trembley observed from one round bulb, in about twenty-four hours, by repeated divisions, one-hundred and ten vorticellÆ to be formed.

It has been asked with propriety, what plant or what animal could have led us to expect an existence and mode of propagation similar to that of the vorticella anastatica?

Fig. 13 represents one branch of the vorticella anastatica; on this branch, besides the vorticellÆ which are of a bell-like form, some of those round bodies from which they first spring, and by which they are so remarkably distinguished from any other species, may be seen.

Fig. 14 represents one of the globular bodies after it has parted from the cluster, and has fixed itself to some other body, and after the globule itself and its pedicle have begun to lengthen.

Fig. 15 represents the two bodies that were formed by the parting of that which is shewn at Fig. 14.

Fig. 16* represents four that were formed by the separation of the two bulbs, exhibited in the foregoing figure.

VORTICELLA PYRARIA.

Plate XXII. Fig. 25, 26.

Vorticella composita, floribus muticis obovatis; tentaculis bigeminis, stirpe ramosa. Compound, with beardless oval florets, two double arms, the stem branched.

It is somewhat of a pear shape, the base is pellucid, the top truncated, the lateral arms, which are a pair on each side, cannot be distinguished without some attention; they are sometimes to be seen disengaged from the pedicle, and rolling swiftly in a kind of circle.

VORTICELLA CRATÆGARIA.

Plate XXII. Fig. 40.

Vorticella composita, floribus muticis globosis; tentaculis binis, stirpe ramosa. Compound, with globous naked florets, two tentacules, and a branched stem.

These vorticellÆ are to be found in the month of April, both in the mud, and upon the tail of the monoculus quadricornis; they are generally heaped together in the manner in which they are represented in the figure; they are of a spherical form, and united to one common stalk. They are also often to be found without any pedicle. The body is rather contracted; the aperture is circular, and surrounded with a marked margin; it has two small arms. With a deep magnifier, a vehement rotatory motion may be seen. They sometimes separate from the community, and go forwards in a kind of spiral line, and then in a little time come back again to the rest.

The figure represents a parcel of these vorticellÆ united together.

Among the other authorities for this animal, LinnÆus refers to Baker’s description of the mulberry insect, “Employment for the Microscope,” p. 348, which, as it differs a little from the preceding account, we shall insert here. That from which his drawing was made, and which he has described, was found in a ditch near Norwich; he called it the mulberry insect, from the resemblance it bore to that fruit; though the protuberances that stand out round it are more globular than those of a mulberry. It is to be seen rolling about from one place to another, and is probably a congeries of animalcula; they are to be met with in different numbers of knobs or protuberances, some having fifty or sixty, others more or less, down to four or five. The manner of moving is the same in all. They are generally of a pale yellow.

VORTICELLA OPERCULARIA.

Plate XXII. Fig. 29.

Vorticella composita floribus muticis ovalibus, stirpe ramosa. Compound, with naked oval florets, and a branched stem.

These vorticellÆ are of a lemon shape, and are generally found in clusters, branching out from a stem, which mostly adheres to some convenient substance.

That species of them which is described by Baker had a very short pedicle, and the animals were much longer than those which are represented at Fig. 29. There was no main stem, but all the pedicles were joined in one center, round which the animals extended themselves as so many radii, forming a very pleasing figure.

The mouths of these animalcula are not ciliated, but they are furnished with a round operculum or cover, connected by a long ligament or muscle, which extends downwards through the body, and is affixed withinside of it, near the tail. This ligament may be contracted or dilated, so that the cover can be removed to some distance from the mouth; in this situation several short hairs maybe found to radiate from it; these have a vibratory motion, by which they excite a current of water, most probably to draw in the proper nourishment, after which they shut or pull down the cover, which they again extend at pleasure: when the cover is pulled close down, the mouth contracts, and no hairs are to be seen.

Fig. 29 represents the vorticella opercularia; ?, the operculum removed at some distance from the mouth, at t; it is nearly close at r, the mouth contracted, the cover drawn in, and no hairs to be seen; u, a part of the stalk, from which some of the animalcula are separated.

VORTICELLA UMBELLARIA.

Plate XXII. Fig. 30.

Vorticella composita, floribus ciliatis globosis muticis, stirpe umbellata. Compound, with ciliated globous naked florets and an umbellated stem.

Vorticella acinosa, simplex, globosa, granis nigricantibus, pedunculo rigido. MÜller Animal. Infus. p. 319.

We frequently find in divers places, upon water-plants, and other bodies in the water, a whitish substance that looks like mould; plants, pieces of wood, snail shells, &c. are often entirely covered over with this substance. If we examine any of these minute bodies by the microscope, we shall find such motions as will induce us to think them an assemblage of living animals, severally fixed to the extremities of small stems or pedicles, many of which are often so united as to form together a sort of branches or clusters, from whence they have been termed clustering polypes, or des polypes en bouquet.

These clusters are larger or smaller, according to the species of the vorticellÆ which form them, as well as owing to the concurrence of many other circumstances. To obtain a clear idea of the figure of these animals, it is best to observe the smaller clusters, as in the larger they are often rendered less distinct on account of the number.

The length of those which are represented at Fig. 30, is about the 240th of an inch; they are of a bell-shape. The anterior part a c generally appears open, the posterior part is fixed to a stem or pedicle, b e; it is by the extremity of this pedicle that the vorticella fastens itself to any substance. It appears in the microscope of a brownish colour, excepting at the smaller end b, where it is transparent, as well as the whole pedicle b e. When the anterior part a c is open, a very lively motion may be perceived about its edges; and when it presents itself in a particular manner, something very much resembling the little wheels of a mill, moving with great velocity, may be discovered on both sides of the edges of this anterior part.

These vorticellÆ are able to contract themselves suddenly. They may be made to do this, either by touching them, or moving the substance to which they are fixed. When they contract, the edges of the anterior parts are drawn quite into the body; on resuming their former posture, the edges may be seen to come forth, and put themselves in motion as before. Minute substances that float in the water are often forced down into these openings, and sometimes are thrown out again.

They are capable of swimming about singly, but their form is in that case considerably different from that which they have when they are fixed. To see regularly in what manner the clusters are formed, and in what way these little creatures multiply, it is best to observe one that is fixed by itself.

The pedicle of a single vorticella is at first short, but it soon grows longer, and then begins to multiply, that is, to divide or split itself into two lengthways. To effect this, the lips are first drawn into the body, the anterior part closes and becomes round, and loses its bell shape, the motion about the lips ceases, though a small degree of motion may be perceived within the body. The anterior end flattens gradually, and spreads wider in proportion as it grows smaller. It then gradually splits down the middle, that is, from the middle of the head to the pedicle, so that in a little time two separate round bodies appear to be joined to the end of the pedicle that before supported but one.

The mouth or anterior part of each of these bodies now opens by degrees; and in proportion as they open, the lips of the new vorticella begin to display themselves. The motion before spoken of may then also be perceived. Indeed it is the best time of observing it; it is at first slow, but more rapid in proportion as the mouth opens, when it is as swift as that of the vorticella before it began to divide, and we may now look upon it as completely formed. A vorticella is generally about one hour in dividing itself.

The lower of the three drawings, Fig. 30, represents two vorticellÆ joined by their posterior extremity to one pedicle; soon after the division, each vorticella begins to shew a pedicle of its own.

Fig. 30 represents a cluster of eight vorticellÆ; by this figure we may form some idea in what manner the pedicles are disposed as their number increases. There were at first only two at b, whose branches lengthened to d, and then each of them was divided into two, now forming four; these again lengthened and reached i; when they were again subdivided, as in the figure.

The reader will join with Bonnet in admiring the group of wonders afforded by a single spot of mouldiness. What unforeseen, varied, and interesting scenes are presented within so small a compass! what a theatre is exhibited to a thinking mind! But our abode is so recluse, that we have but a glimmering view of it: how great would our astonishment be, if the whole spectacle was disclosed to us at once, and we were enabled to penetrate into the interior structure of this wonderful assemblage of living atoms! Our eyes see only the gross parts of the decorations, whilst the machines that execute them remain in impenetrable darkness! Who shall enlighten this profound obscurity, or dive into an abyss where reason is lost; or draw from thence the treasures of wisdom concealed within it? Let us learn to be content with the small portion that is communicated to us, and contemplate with gratitude the first traces of human understanding that are imparted to us in these discoveries.

VORTICELLA BERBERINA.

Vorticella composita, floribus ovalibus muticis, stirpe ramosa. Compound, with oval beardless florets.

This is a species of the vorticellÆ, which resembles the preceding one in many respects, particularly in being multiplied in the same manner, that is, by dividing or splitting, according to its length.

They are more slender than the vorticella umbellaria; the branches of the clusters are transparent. When many of them are together, they appear of a changeable violet colour; the clusters are not unlike a sprig of spun glass. The motion of the lips is not so easily distinguished as in the foregoing species, though it may be observed in these whilst they are opening and completing their formation. For at these times the motion is but slow, whereas it becomes afterwards very quick in those that are arrived at a state of perfection.

All the cluster vorticellÆ detach themselves from time to time from the stem, and from these they swim about till they fix again upon some convenient substance; the branches, when deserted, bear no more vorticellÆ.

VORTICELLA DIGITALIS.

Plate XXII. Fig. 31.

Vorticella composita, floribus cylindricis, unisulcatis semiclausis, stirpe ramosa. Compound, with cylindrical florets.

Vorticella composita, cylindrica, crystallina, apice truncata et fissa, pedunculo fistuloso ramosa. MÜller Animal. Infus. p. 327.

This species of the vorticella is very scarce, it seems only to have been seen by RÖsel, who found it on the monoculus quadricornis, till it was discovered in the year 1784 by MÜller, who had sought for it several years before, but in vain.

The body is cylindrical, crystalline, and appears almost empty; it has three pellucid points disposed lengthways, the apex is truncated in an oblique direction, the margin bent back. The upper part contracts itself, and the margin then assumes a conical shape, with a convex surface; there are in general but few branches from the principal stem, and these are short and thick. It excites an undulatory motion, but no hairs, nor any rotatory motion, have been discovered. Fig. 31, o and n, represents the vorticella adhering to the monoculus quadricornis.

VORTICELLA CONVALLARIA.

Plate XXII. Fig. 39.

Vorticella simplex, gregaria, flore campanulata mutico; tentaculis bigeminis, stirpe fixa. Simple, but gregarious, the florets bell-shaped, with two pair of little arms, and a fixed stem.

Vorticella simplex, campanulata, pedunculo rotortili. MÜller Animal. Infus.

These vorticellÆ, or bell-animals, as they are termed by Baker, are generally found adhering to some substance in the water; they are represented here as found by RÖsel, fixed to a curious cornu ammonis, with points projecting from the back. To the naked eye they appear only as so many little white points, but under the microscope, as little bells, agitating the water to a considerable distance. The stems of these have a particular motion, they draw themselves up and shorten all at once, taking the form of a spiral wire or screw; in a moment after they again resume their former shape, stretching themselves out straight as before. Many of them may be seen at times adhering to each other by their tails; the cilia, which are two on each side of the mouths, are very seldom to be perceived.

VORTICELLA URCEOLARIS.

Plate XXII. Fig. 33, 34, 35, 36, 37, 38.

Vorticella simplex, pedunculata, ore dentato. Single, with a short tail, and toothed mouth.

Brachionus capsularis testa ovata apice sexdentata basi incisa, cauda longa bicuspi. MÜller Animal. Infus. p. 356.

To the naked eye it appears as a white moveable point; but when examined by the microscope, a tail projecting from the lower part is discovered, and a double rotatory instrument is seen, which it can conceal or expose at pleasure. It has been seen and described by most microscopical writers; but as Baker’s seems to be the most perfect description, I shall principally follow his account of it.

He discovered three species of them, two of which are included under the vorticella urceolaris. Fig. 33, 34, 35, are of the first species; Fig. 36, 37, 38, are of the second kind. The first sort, when extended, is about twice as long as it is broad. It is contained in a shell; the fore part of this is armed with four sharp teeth or points; the opposite side has no teeth, but is waved or bent in two places, like the form of a Turkish bow. At the bottom there is a hole, through which it pushes the tail. It fastens itself by this tail to any convenient substance when it intends to use its rotatory organs; but when it is floating in the water, and at all other times when not adhering to any body, it wags the tail backwards and forwards something like a dog.

We may consider it as divided into a head, thorax, and abdomen; each of which may be extended and contracted considerably: it can, by dilating all three, protrude the head beyond the shell, or by contracting them, draw the whole body within the same.

The head, when extended, divides itself into two branches, between which, another part, a kind of proboscis, is pushed out; at the end of this are two fibrils, that appear when they are at rest like a broad point, but which can be moved to and from each other very briskly with a vibratory motion, see Fig. 33.

The form and situation of the two branches are sometimes changed, the ends thereof becoming more round, and the vibratory motion is altered to a rotatory one: this alteration is represented at Fig. 34: the head also appears in this figure. The thorax is annexed to the lower part of the head; it is muscular: within it there is a moving intestine, which has been supposed to be either the lungs or the heart of the little creature, see b, Fig. 33 and 34.

A communication is formed between the thorax and the abdomen by means of a short vessel c, whose alternate contractions and dilatations occasion the abdomen to rise and fall alternately, having at the same time a sort of peristaltic motion. The food is conveyed through this vessel into the abdomen, where it is digested; it is then discharged by the anus, which is placed near the tail.

The tail has three joints, and is cleft or divided at the extremity, by which means it can better fasten itself to suitable objects. It is in general projected from the lower end of the shell, moving nimbly to and fro, serving the animal as a rudder when it is swimming, to direct its course.

When the water in which the little animal is placed is nearly dried away, or when it has a mind to compose itself to rest, it contracts the head and fore-part of the body, brings them down into the shell, and pulls the tail upwards, so that the whole of this minute creature is contained within the shell, see Fig. 35. The shell is so transparent that the terminations cannot be easily distinguished when the animal is extended; but whatever is transacted within the shell, is as plain as if there was no substance between the eye and the interior parts.

Fig. 36, 37, 38, exhibit the appearance of another species of these animals, which differs from the foregoing kind. This has also a head, a thorax, and abdomen, but then they are not separated by a gut or intermediate vessel, as in the former, but are joined immediately together, and at the place where in the first kind a moveable intestine was seen; in this a muscle, most probably the heart, may be discovered; it has a regular systole and diastole: this part is intended to be shewn at a, Fig. 36, 37, 38. Like the other, it draws the head and tail within the shell, which then appears to have six teeth or spikes on one side, and two on the other. It very seldom protrudes its head so far out as the other; sometimes the fibrillÆ may be seen within the margin of the shell.

Both species carry their young in an oval integument or bag, fastened externally to the lower part of the shell, somewhere about the tail; these bags are sometimes opake at one end, and seemingly empty at the other, see d, Fig. 34: sometimes the middle is opake, with a transparent margin, see b, Fig. 36.

It is highly entertaining to see a young one burst its integument, and gradually force its way out; in performing this operation, it is much assisted by the motion of the tail of the parent. The head part comes out first, it then sets its rotatory organ in motion, by which it is completely disengaged, leaving the integument behind, which the vorticella freed itself from by repeated strokes with its tail. A young one almost disengaged is seen at b, Fig. 38; another embryo, c, was left adhering to the shell.

There are four more species of the vorticellÆ mentioned by LinnÆus, which are, the vorticella encrinus, the vorticella polypina, the vorticella stellata, and the vorticella ovifera; which, being marine animals, do not come properly within our plan. The vorticella polypina will be described hereafter.

TUBULARIA CAMPANULATA.

Plate XXII. Fig. 32.

Tubularia reptans, tubis campanulatis. Creeping, with campanulated tubes.

It is called by Baker the bell-flowered, or plumed animal.

These little creatures dwell in colonies together, from ten to fifteen in number, living in a kind of slimy mucilaginous case, which, when expanded in the water, has some resemblance to a bell with its mouth upwards. These bells or colonies are to be found adhering to the large leaves of duck-weed and other aquatic plants.

The bell or case which these animals inhabit, being very transparent, all the motions of its inhabitants may be discerned distinctly through it. There are several ramifications or smaller bells proceeding from the larger one; in each of these there is an inhabitant. The opening at the top of these bells is just large enough for the creature’s head, and a small part of its body to be thrust out from it, the rest remaining in the case, into which it also draws the head on the least alarm.

Besides the particular and separate motions which each of these creatures is able to exert within its case, and independent of the rest, the whole colony has a power of altering the position of the bell, and removing it from one place to another. These animalcula seem not to like to dwell in societies, whose number exceeds fifteen; when the colony happens to increase in number, the bell may be observed to split gradually, beginning from about the middle of the upper extremity, and proceeding downwards towards the bottom, till they at last separate and become two colonies, independent of each other.

The arms are very near each other; sixty may often be counted in one plume, having each the figure of an Italic ?, one of whose hooked ends is fastened to the head; and altogether, when expanded, compose a figure somewhat like a horseshoe, convex on the side next the body, but gradually opening and turning outwards, so as to leave a considerable distance within the outer extremities of the arms.

The plumed polype is of a very voracious disposition, devouring a great number of small animals. If the arms, when extended, be observed attentively with the microscope, they will be found to have a constant vibratory motion; alternately bending withinside of the plume, and then rising up again. When one arm ceases its motion, the same is performed by another; thus by the perpetual agitation of the several arms, such a strong current is produced in the water, as brings the animalcula, and other minute bodies, that are floating near the polype, into its mouth, which is situated between the arms. The food, if agreeable to the creature, is swallowed; if otherwise, it is rejected by a contrary motion.

The animal may be seen very plain when it has retired within the tube. The body is about one-eighth of an inch long, without reckoning the plume, which is about the same length. It is cylindrical, and the skin is very transparent. The plume is only a continuation of this transparent skin, it is very broad in proportion to the body, and of a remarkable figure; the base is of the shape of a horseshoe; from this base the arms project, they bend rather outwards. The plume which they form, gives them a resemblance to some flowers. The arms may be compared, from their fineness and transparency, to very fine threads of glass. The base of the plume is grooved, and is fixed to the animal by the middle of the horseshoe which it forms, and it is here that there is an opening which serves as a mouth to the animal. The intestines are easily distinguished through its transparent skin; when it has just been eating, they are of a deep brown colour. Three principal parts are very visible, the oesophagus, the stomach, and the rectum.

In the inside of these animals a small oblong whitish body is formed, which is carried to the outside, and remains fixed in a perpendicular direction to the body; many of these are formed daily, and of these oval bodies new animals are produced, exactly similar to the parent.

If these minute bodies be eggs, they are of a singular kind, being destitute of any covering, and are neither membranaceous nor crustaceous; we cannot with propriety say the young ones are hatched from them; we can, however, perceive these oviform bodies to unfold themselves gradually. The developement is accomplished in a few minutes, and an animalculum appears like the parent.

Trembley amassed a great number of these eggs, and carried them from England with him, keeping them quite dry; on putting them into water, they gradually developed, and became as perfect as the tubularia from which they proceeded.

There is a very great similarity in the construction of this little creature and many of the marine polypes, who, like it, exist in tubes of the same growth with themselves.

Fig. 32 represents three tubulariÆ campanulatÆ or plumed polypes very much magnified, namely, one, b f a c d d e h g i, which is out of its cell; e h, the oesophagus; f g, the stomach; a f, the rectum; a c d d e, the plume, consisting of the base a e, which is but little seen, and the arms c d d, which proceed from the edges of this base; a second polype, A B I, which is within its cell, and in which the skin containing the plume is reversed. The third polype, s t u u, is a young one exhibited out of its cell; g o o, threads which are fixed at one end to the intestines of the animal, by the other to the bottom of the cell, l k.