Only the medusa, which was taken in the R. Yang-tze-kiang, is as yet known. (6) _Limnocnida tanganyikæ_, Bohm (1889). R. T. Günther, Ann. Nat. Hist. (6) xi, p. 269, pls. xiii, xiv (1893). Only the medusa, which is found in Lake Tanganyika, Lake Victoria Nyanza and the R. Niger, has been found and it is doubtful whether a hydroid generation exists. (7) _Polypodium hydriforme_, Ussow (1885). Morph. Jahrb. xii, p. 137 (1887). Two stages in this peculiar hydroid, which is found in the R. Volga, are known, (_a_) a spiral ribbon-like form parasitic on the eggs of the sterlet (_Acipenser ruthenus_), and (_b_) a small _Hydra_-like form with both filamentous and club-shaped tentacles. The life-history has not yet been worked out[AP]. [Footnote AP: Since this was written, Lippen has described a third stage in the life-history of _Polypodium_ (Zool. Anz. Leipzig, xxxvii, Nr. 5, p. 97 (1911)).] II. HISTORY OF THE STUDY OF HYDRA. Hydra was discovered by Leeuwenhoek at the beginning of the eighteenth century and had attracted the attention of several skilful and accurate observers before that century was half accomplished. Among them the chief was Trembley, whose "Mémoires pour servir à l'histoire d'un genre de Polype d'eau douce"* was published at Paris 1744, and is remarkable not only for the extent and accuracy of the observations it enshrines but also for the beauty of its plates. Baker in his work entitled "An attempt towards a natural history of the Polyp"* (London, 1743) and Rösel von Rosenhof in the third part of his "Insecten-Belustigung" (Nurenberg, 1755) also made important contributions to the study of the physiology and structure of _Hydra_ about the same period. Linné invented the name _Hydra_, and in his "Fauna Sueica" and in the various editions of his "Systema Naturæ" described several forms in a manner that permits some of them to be recognized; but Linné did not distinguish between the true _Hydra_ and other soft sessile Coelenterates, and it is to Pallas ("Elenchus Zoophytorum," 1766) that the credit properly belongs of reducing the genus to order. It is a tribute to his insight that three of the four species he described are still accepted as "good" by practically all students of the Coelenterates, while the fourth was a form that he had not himself seen. In the nineteenth century the freshwater polyp became a favourite object of biological observation and was watched and examined by a host of observers, among the more noteworthy of whom were Kleinenberg, Nussbaum, and Brauer, who has since the beginning of the present century made an important contribution to the taxonomy of the genus. BIBLIOGRAPHY OF HYDRA.