[AL] The statement is not strictly accurate as regards the Calcutta phase of H. vulgaris, for the summer brood apparently does not lay eggs but reproduces its species by means of buds only. This state of affairs, however, is probably an abnormality directly due to environment.
[AM] Bot. Zeitung, xlviii (1890): see p. 49, antea.
[AN] Zool. Anz. xxxvi, pp. 271-279, figs., Oct. 1910.
[AO] Mr. F. H. Gravely tells me that this is also the case as regards H. viridis in England, at any rate if freshly captured specimens are placed overnight in a bottle in a window in such a position that the early morning sunlight falls upon one side of the bottle.
[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 Cœlenterates, 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 Cœlenterates, 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.