POLYPUS, 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 Linnaeus, 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 Leuwenhoeck, 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 à priori, had concluded that animals might be found which would propagate by slips like plants. Their conjectures have been verified.