a species of fresh-water insects, belong- ing to the genus of hydra, of the order of zoophytes, and clasps of vermes. (See Animalcule, no. 243, &c.) The name of hydra was given them by Linnæus on account of the property they have of reproducing themselves when cut in pieces, every part soon becoming a perfect animal. Dr Hill called them biota, on account of the strong principle of life with which every part of them is endowed.

These animals were first discovered by Leeuwenhoek, who gave some account of them in the Philosophical Transactions for 1703; but their wonderful properties were not thoroughly known till the year 1740, when Mr Trembley began to investigate them. Previous to his discoveries, indeed, Leibnitz and Boerhaave, by rea- sonings a priori, had concluded that animals might be found which would propagate by flips like plants; and their conjectures were soon verified by the observations of the gentleman above-mentioned. At first, however, Mr Trembley was uncertain whether he should reckon these creatures animals or plants; and while thus uncer- tain, he wrote a letter on the subject to Mr Bonnet in Ja- nuary 1741; but in March the same year he had satisfied himself that they were real animals. The surprize of Mr Trembley, and of others, on discovering the true nature of these animals, was very great. When Mr Reaumur saw for the first time two polypes formed from one which he had divided into two parts, he could hardly believe his own eyes; and even after having repeated the operation a hundred times, he said that the sight was by no means familiar to him. On the 18th of July 1741, M. Buffon wrote to Martin Folkes, Esq; presi- dent of the Royal Society, acquainting him with "the discovery of a small insect called a polypus, which is found 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 middle- most also 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." species."—In September the same year, a letter was communicated from C. Bentink, Esq.; at the Hague, describing the insects discovered by Mr Trembley, adding, that he himself had seen them; and in November that year, a letter was read before the Society from Gronovius at Leyden, giving an account of a water-insect, which, says he, if cut into five or six pieces, in a few hours there will be as many animals exactly similar to their parent. These accounts, however, were all deemed to extraordinary, that they were not credited, until professors Albinus and Mulchienbroeck provided themselves with them, and found every thing related concerning them to be exactly true. In March 1742, Mr Folkes gave an account of them to the Royal Society, from some observations made on several polypes which Mr Trembley had sent from Holland. They were soon after found in England, and the observations made upon them were published by several persons; so that no doubt remained concerning the reality of what had been related concerning them.

The general character of the polype is, that it fixes itself by its base; is gelatinous, linear, naked, contractile; and can change its place. The mouth, which is placed at one end, is surrounded by hair-like feelers. The young ones grow out from its sides; but in autumn it produces eggs from its sides. There are fix varieties.

1. The viridis, or green polype, has commonly ten short arms.

2. The fusca has frequently eight arms several times longer than the body.

3. The grifeca is of a yellowish colour, small towards the bottom, and has long arms, generally about seven in number.

4. The pallens has generally about six arms of a moderate length.

5. The hydatula has a vesicular body, and four obsolete arms. It is found in the abdomen of sheep, swine, &c.

6. The fenestra has been called the tunnel-shaped, and has a mouth surrounded with a row of hairs.

7. The socialis is bearded, thick, and wrinkled.

The three first species are those on which the greatest number of experiments have been made; and their shapes are so various, that it is by no means easy to describe them. They are generally found in ditches. Whoever has carefully examined these when the sun is very powerful, will find many little transparent lumps of the appearance of a jelly, and size of a pea, and flattened upon one side. The same kind of substances are likewise to be met with on the under side of the leaves of plants which grow in such places. These are the polypes in a quiescent state, and apparently inanimate. They are generally fixed by one end to some solid substance, with a large opening, which is the mouth, at the other; having several arms fixed round it, projecting as rays from the centre. They are slender, pellucid, and formed of a tender substance like the horns of a snail, and capable of contracting themselves into a very small compass, or of extending to a considerable length. The arms are capable of the same contraction and expansion as the body; and with these they lay hold of minute worms and other insects, bringing them to the mouth and swallowing them; the indigestible parts are again thrown out by the mouth.

The green polype was that first discovered by Mr Trembley; and the first appearances of spontaneous motion were perceived in its arms, which it can contract, extend, and twist about in various directions. On the first appearance of danger they contract to such a degree that they appear little bigger than a grain of sand, of a fine green colour, the arms disappearing entirely. Soon after he found the grifeca, and afterwards the fusca.

The bodies of the viridis and grifeca diminish almost infinibily from the anterior to the posterior extremity; but the fusca is for the most part of an equal size for two-thirds of its length from the anterior to the posterior extremity, from which it becomes abruptly smaller, and then continues of a regular size to the end. These three kinds have at least six, and at most 12 or 13 arms, though sometimes the grifeca is met with having 18 arms. They can contract themselves till their bodies do not exceed one-tenth of an inch in length, and they can stop at any intermediate degree of contraction or extension. They are of various sizes, from half an inch to an inch and a half long; their arms are seldom longer than their bodies, though some have them an inch, and some even eight inches, long. The thickness of their bodies decreases as they extend themselves, and vice versa; and they may be made to contract themselves either by agitating the water in which they are contained, or by touching the animals themselves. When taken out of the water, they all contract so much that they appear only like a little lump of jelly. The arms have the same power of contraction or expansion that the body has; and they can contract or expand one arm, or any number of arms, independent of the rest; and they can likewise bend their bodies or arms in all possible directions. They can also dilate or contract their bodies in various places, and sometimes appear thick set with folds, which, when carelessly viewed, appear like rings. Their progressive motion is performed by that power which they have of contracting and dilating their bodies. When about to move, they bend down their head and arms, lay hold by means of them on some other substance to which they design to fasten themselves; then they loosen their tail, and draw it towards the head; then either fix it in that place, or stretching forward their head as before, repeat the same operation. They ascend or descend at pleasure in this manner upon aquatic plants, or upon the sides of the vessel in which they are kept; they sometimes hang by the tail from the surface of the water, or sometimes by one of the arms; and they can walk with ease upon the surface of the water. On examining the tail with a microscope, a small part of it will be found to be dry 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. When a polype, therefore, 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 is to be supported, and to give it time to dry, which it always does upon these occasions; and they attach themselves so firmly by the tail to aquatic plants, stones, &c. that they cannot be easily disengaged; they often further strengthen these attachments by means means of one or two of their arms, which serve as a kind of anchors for fixing them to the adjacent substances.

The stomach of the polype is a kind of bag or gut into which the mouth opens, and goes from the head to the tail. This, in a strong light, is visible to the naked eye, especially if the animal be placed between the eye and a candle, for these animals are quite transparent whatever their colour may be. The stomach, however, appears to more advantage through a powerful magnifier. Mr Trembley, by cutting one of these animals transversely into three parts, satisfied himself that they were perforated throughout. Each piece immediately contracted itself, and the perforation was very visible through a microscope. The skin which incloses the stomach is that of the polype itself; so that the whole animal, properly speaking, consists only of one skin, in the form of a tube, and open at both ends. No vessels of any kind are to be distinguished.

The mouth is situated at the anterior end in the middle between the shooting forth of the arms, and assumes different appearances according to circumstances; being sometimes lengthened out in the form of a nipple, at others appearing truncated; sometimes the aperture is quite closed, at others there is a hollow; though at all times a small aperture may be discovered by a powerful magnifier.

The skin of a polype, when examined with a microscope, appears like flagreen, or as if covered with little grains, more or less separated from each other, according to the degree of contraction of the body. 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 grains, and the interior part is found to be more flagreened than the exterior one; but they are not strongly united to each other, and may be separated without much trouble. They even separate of themselves, though in no great numbers, in the most healthy animals of this kind; for where they are observed to separate in large quantities, it is a symptom of a very dangerous disorder. In the progress of this disorder, the surface of the polype becomes gradually more and more rough and unequal, and no longer well defined or terminated as before. The grains fall off on all sides; the body and arms contract and dilate, and assume a white shining colour; and at last the whole dissolves into an heap of grains, which is more particularly observed in the green polype. By a careful examination we find, that the skin of the polype is entirely composed of grains, cemented by means of a kind of gummy substance; but it is to the grains entirely that the polype owes its colour. The structure of the arms is analogous to that of the body; and they appear flagreened when examined by the microscope, whether they be in a state of contraction or extension; but if very much contracted, they appear more flagreened than the body, though almost quite smooth when in their utmost state of extension. In the green polype the appearance of the arm is continually varying; and these variations are more sensible towards the extremity of the arm than at its origin, but more scattered in the parts further on. The extremity is often terminated by a knob, the hairs of which cannot be observed without a very powerful magnifier. They have a remarkable inclination of turning towards the light; so that if that part of the glass on which they are be turned from the light, they will quickly remove to the other.

That species named the *fusca* has the longest arms, and makes use of the most curious manoeuvres to seize its prey. They are best viewed in a glass seven or eight inches deep, when their arms commonly hang down to the bottom. When this, or any other kind, is hungry, it spreads its arms in a kind of circle to a considerable extent, inclosing in this, as in a net, every insect which has the misfortune to come within the circumference. (See Animalcule, n° 27.) While the animal is contracted by seizing its prey, the arms are observed to swell like the muscles of the human body when in action. Though no appearance of eyes can be observed in the polype, they certainly have some knowledge of the approach of their prey, and show the greatest attention to it as soon as it comes near them. It seizes a worm the moment it is touched by one of the arms; and in conveying it to the mouth, it frequently twists the arm into a spiral like a corkscrew; by which means the insect is brought to the mouth in a much shorter time than otherwise it would be; and so soon are the insects on which the polypes feed killed by them, that M. Fontana thinks they must contain the most powerful kind of poison; for the lips scarce touch the animal when it expires, though there cannot be any wound perceived on it when dead. The worm, when swallowed, appears sometimes fling, sometimes double, according to circumstances. When full, the polype contracts itself, hangs down as in a kind of stupor, but extends again in proportion as the food is digested and the excrementitious part is discharged. The bodies of the insects, when swallowed, are first macerated in the stomach, then reduced into fragments, and driven backward and forward from one end of the stomach to the other, and even into the arms, however fine they may be; whence it appears that the arms, as well as the other parts of this remarkable creature, are a kind of hollow guts or stomachs. In order to observe this motion, it is best to feed the polypes with such food as will give a lively colour; such, for instance, as those worms which are furnished with a red juice. Some bits of a small black snail being given to a polype, the substance of the skin was soon dissolved into a pulp consisting of small black fragments; and on examining the polype with a microscope, it was found that the particles were driven about in the stomach, and that they passed into the arms, from thence back into the stomach, then to the tail; from whence they passed again into the arms, and so on. The grains of which the body of the polype consist take their colour from the food with which it is nourished, and become red or black as the food happens to afford the one colour or the other. They are likewise more or less tinged with these colours in proportion to the strength of the nutritive juices; and it is observable that they lose their colour if fed with aliments of a colour different from themselves. They feed on most insects found in fresh water; and will also be supported with worms, the larvae of gnats, &c. and even with snails, large aquatic insects, and fish or flesh, if cut into small bits. Sometimes two polypes lay hold of the same worm, and each begins... begins to swallow its own end till their mouths meet and the worm breaks. But should this happen not to be the case, the one polype will sometimes devour the other along with its portion. It appears, however, that the stomach of one polype is not fitted for diffusing the substance of another; for the one which is swallowed always gets clear again after being imprisoned for an hour or two.

The manner in which the polypes generate is most perceptible in the grifea and fufca, as being considerably larger than the viridis. If we examine one of them in summer, when the animals are most active, and prepared for propagation, some small tubercles will be found proceeding from its sides, which constantly increase in bulk, until at last in two or three days they assume the figure of small polypes. When they first begin to shoot, the excrescence becomes pointed, assuming a conical figure, and deeper colour than the rest of the body. In a short time it becomes truncated, and then cylindrical, after which the arms begin to shoot from the anterior end. The tail adheres to the body of the parent-animal, but gradually grows smaller, until at last it adheres only by a point, and is then ready to be separated. When this is the case, both the mother and young ones fix themselves to the sides of the glass, and are separated from each other by a sudden jerk. The time requisite for the formation of the young ones is very different, according to the warmth of the weather and the nature of the food eaten by the mother. Sometimes they are fully formed, and ready to drop off, in 24 hours; in other cases, when the weather is cold, 15 days have been requisite for bringing them to perfection.

It is remarkable, that there is a reciprocal communication of food betwixt the young and old before they be separated. The young ones, as soon as they are furnished with arms, catch prey for themselves, and communicate the digested food to the old ones, who on the other hand do the same to the young ones. This was fully verified by the following experiment: One of the large polypes of the fufca kind being placed on a slip of paper in a little water, the middle of the body of a young one growing out from it was cut open; when the superior part of that end which remained fixed to the parent was found to be open also. By cutting over the parent polype on each side of the shoot, a short cylinder was obtained, open at both ends; which being viewed through a microscope, the light was observed to come through the young one into the stomach of the old one. On cutting open the cylindrical portion lengthwise, not only the hole of communication was observed, but one might see through the end of the young one also. On changing the situation of the two pieces, the light was seen through the hole of communication. This may be seen between the parent polype and its young ones after feeding them; for, after the parents have eaten, the bodies of the young ones swell as if they themselves had been eating.

The polypes produce young ones indiscriminately from all parts of their bodies, and five or six young ones have frequently been produced at once; nay, Mr Trembley has observed nine or ten produced at the same time.

Nothing like copulation among these creatures was ever observed by Mr Trembley, though for two years he had thousands of them under his inspection. To be more certain on this subject, he took two young ones the moment they came from their parent, and placed them in separate glasses. Both of them multiplied, not only themselves, but also their offspring, which were separated and watched in the same manner to the seventh generation; they have even the same power of generation while adhering to their parent. In this state the parent, with its children and grandchildren, exhibits a singular appearance, looking like a shrub thick set with branches. Thus several generations sometimes are attached to one another, and all of them to one parent. Mr Adams gives a figure of one polype with nineteen young ones hanging at it; the whole group being about an inch broad, and an inch and an half in length: the old polype eat about twelve monoculi per day, and the young ones about 20 among them.

When a polype is cut transversely or longitudinally into two or three parts, each part in a short time becomes a perfect animal; and so great is this prolific power, that a new animal will be produced even from a small portion of the skin of the old one. If the young ones be mutilated while they grow upon the parent, the parts so cut off will be reproduced, and the same property belongs to the parent. A truncated portion will send forth young ones before it has acquired a new head and tail of its own, and sometimes the head of the young one supplies the place of that which should have grown out of the old one. If we slit a polype longitudinally through the head to the middle of the body, we shall have one formed with two heads; and by flitting these again in the same manner we may form one with as many heads as we please.

A still more surprising property of these animals is, that they may be grafted together. If the truncated portions of a polype be placed end to end, and gently pushed together, they will unite into a single one. The two portions are first joined together by a slender neck, which gradually fills up and disappears, the food passing from the one part into the other; and thus we may form polypes not only from portions of the same, but of different animals; we may fix the head of one to the body of another, and the compound animal will grow, eat, and multiply, as if it had never been divided. By pushing the body of one into the mouth of another, so far that their heads may be brought into contact, and kept there for some time, they will at last unite into one animal, only having double the number of arms which it would otherwise have had. The hydra fufca may be turned inside out like a glove, at the same time that it continues to live and act as before. The lining of the stomach now forms the outer skin, and the former epidermis constitutes the lining of the stomach. If previous to this operation the polype have young ones attached to it, such as are but newly beginning to vegetate turn themselves inside out, while the larger ones continue to increase in size till they reach beyond the mouth of the parent, and are then separated in the usual manner from the body. When this turned the polype combines itself in many different ways. The fore part frequently closes and becomes a supernumerary tail. The animal, 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 placed as it were at the joint of the compasses, but loses its power of action; to supply which, a new one is formed. formed in its neighbourhood; and in a little time there is a new species of hydra formed with several mouths.

The sides of a polype, which has been cut through in a longitudinal direction, begin to roll themselves up, commonly from one of the extremities, with the outside of the skin inwards; but in a little time they unroll themselves, and the two cut edges join together, sometimes beginning at one extremity, and sometimes approaching throughout their whole length. As soon as the edges join, they unite so closely that no fear can be perceived. If a polype be partly turned back, the open part closes, and new mouths are formed in different places. Every portion of a polype is capable of devouring insects almost as soon as it is cut off; and the voracity of the whole genus is astonishing; for Mr Adams observes, that most of the insects on which they feed bear the same proportion to the mouth of a polype that an apple of the size of a man's head bears to his mouth.

The *hydra pallens* is very rarely met with, and is described only by M. Roesel. It is of a pale yellow colour, growing gradually smaller from the bottom; the tail is round or knobbed; the arms are about the length of the body, of a white colour, generally seven in number, and are apparently composed of a chain of globules. The young are brought forth from all parts of its body.

The *hydratula* is mentioned by many medical writers. Dr Tylon, in dissecting an antelope, found several hydroids or films, about the size of a pigeon's egg, filled with water, and of an oval form, attached to the omentum; and some in the pelvis between the bladder of urine and rectum. He suspected them to be animals for the following reasons: 1. Because they were included in a membrane like a matrix, so loosely, that by opening it with the finger or a knife, the internal bladder, containing the serum or lymph, seemed nowhere to have any connection with it, but would very readily drop out, still retaining its liquor without spilling any. 2. This internal bladder had a neck or white body, more opaque than the rest, 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. On bringing this neck near the candle, it moved and shortened itself. It is found in the abdomen of sheep, swine, mice, &c., lying between the peritoneum and the intestines.

The *flutea*, or funnel-like polype, is of three colours, green, blue, and white; but the last is the most common. They do not form clutters, but adhere singly by the tail to whatever comes in their way; the anterior end is wider than the posterior; and, being round, gives the animal somewhat of a funnel form, though the circle is interrupted by a kind of slit or gap. The edge of this gap is surrounded with a great number of little lamellae, which by their motions excite a current of water, that forces into the mouth of the animal the small bodies that come within its reach. Mr Trembley says, that he has often seen a great number of animalcules fall into the mouths of these creatures; some of which were let out again at an opening which he could not describe. They can fashion their mouths into several different forms; and they multiply by dividing neither transversely nor longitudinally, but diagonally.

The *socialis* is described by Muller under the title of *vorticella*. They are found in clutters; and when viewed by a microscope, appear like a circle surrounded with crowns or ciliated heads, tied by small thin tails to a common centre, from whence they advance towards the circumference, and then turn like a wheel, occasioning a vortex which brings along with it the food proper for them.

The *angulata*, or clustering polypes, form a group resembling a clutter, or rather an open flower, 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 is formed into eight or nine lateral branches, perfectly similar to each other, which have also subordinate branches, whose collective form much resembles that of a leaf. Every one of these affinities is composed of one principal branch or nerve, which makes the main stem of the clutter an angle somewhat larger than a right one; the smaller lateral branches proceed from both sides of this nerve, and these are shorter the nearer their origin is to the principal branch. There is a polype at the extremity, and others on both sides of the lateral twigs, but at different distances from their extremities. They are all exceedingly small, and bell-shaped, with a quick motion about the mouth, though it is impossible to discern the cause of it. See *Animalcule*, n° 24, 26, *Pulex*, and *Vorticella*.

The several strange properties recorded of this animal, though very surprising, are, however, none of them peculiar to it alone. The Surinam toad is well known to produce its young, not in the ordinary way, but in cells upon its back. Mr Sherwood has very lately discovered the smallest eels in four puffs to be without exception full of living young ones. And as to the most amazing of all its properties, the reproduction of its parts, we know the crab and lobster, if a leg be broken off, always produce a new one; and Mons. Bonet, Mons. Lyonet, Mons. de Reaumur, and Mr Folkes, have all found, by experiment, that several earth and water worms have the same property, some of them even when cut into thirty pieces. The *urtica marina*, or sea-nettle, has been also found to have the same; and the sea star-fish, of which the polype is truly a species, though it had long escaped the searches of the naturalists, was always well known by the fishermen to have it also.

Marine *Polypus*, is different in form from the freshwater polype already described; but is nourished, increases, and may be propagated, after the same manner; Mr Ellis having often found, in his inquiries, that small pieces cut off from the living parent, in order to view the several parts more accurately, soon gave indications that they contained not only the principles of life, but likewise the faculty of increasing and multiplying into a numerous issue. It has been lately discovered and sufficiently proved by Peyronel, Ellis, Joffieu, Reaumur, Donati, &c., that many of those substances which had formerly been considered by naturalists as marine vegetables or sea-plants, are in reality animal-productions; and that they are formed by polypes of different shapes and sizes, for their habitation, defence, and propagation. To this class may be referred the corals, corallines, keratophyta, echinaria, sponges, and alcyonium: nor is it improbable, that the more compact bodies, known by