a species of fresh-water insects, belonging to the genus of hydra, of the order of zoophytes, and class of vermes. See Helminthology.

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 Leewenhoek, 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 reasonings *a priori*, had concluded that animals might be found which would propagate by slips like plants. Their conjectures have been verified.

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 Peyssonel, Ellis, Jussieu, 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, eschara, sponges, and alcyonium: nor is it improbable, that the more compact bodies, known by the common appellations of star-stones, brain-stones, petrified fungi, and the like, brought from various parts of the East and West Indies, are of the same origin. To this purpose Mr Ellis observes, that the ocean, in all the warmer latitudes, near the shore, and wherever it is possible to observe, abounds so much with animal life, that no inanimate body can long remain unoccupied by some species. In those regions, ships bottoms are soon covered with the habitations of thousands of animals: rocks, stones, and every thing lifeless, are covered with them instantly; and even the branches of living vegetables that hang into the water are immediately loaded with the spawn of different animals, shell-fish of various kinds: and shell-fish themselves, when they become impotent and old, are the basis of new colonies of animals, from whose attacks they can no longer defend themselves. See Corallina, Helminthology Index.

**Polypus of the Heart.** See Medicine, No 97, 98, 274, and 290.