The first two of these species belong to the order Gymn.o.blastea (Anthomedusae) and the third to the Calyptoblastea (Leptomedusae).

(b) _Actinozoa._

(4) _Sagartia schilleriana_, Stoliczka (1869).

_S. schilleriana_, Stoliczka, Journ. As. Soc. Beng. (2) x.x.xviii, p. 28, pls. x, xi (1869); _Metridium schillerianum_, Annandale, Rec. Ind. Mus. i, p. 47, pl. iii (1907).

This sea-anemone, which has only been found in the delta of the Ganges, offers a most remarkable instance of what appears to be rapid adaptation of a species to its environment. The typical form, which was described in 1869 by Stoliczka from specimens taken in tidal creeks and estuaries in the Gangetic area and in the ponds at Port Canning, is found attached to solid objects by its basal disk. The race (subsp. _exul_), however, that is now found in the same ponds has become elongate in form and has adopted a burrowing habit, apparently owing to the fact that the bottom of the ponds in which it lives is soft and muddy.

In addition to these four species a minute hydroid belonging to the order Gymn.o.blastea and now being described by Mr. J. Ritchie has been taken in the ponds at Port Canning. It is a very aberrant form.

FRESHWATER COELENTERATES OTHER THAN HYDRA.

_Hydra_ is the only genus of coelenterates as yet found in fresh water in India, but several others have been discovered in other countries.

They are:--

(1) _Cordylophora lacustris_, Allman (1843).

Hincks, Hist. Brit. Hydr. Zooph. p. 16, pl. iii, fig. 2 (1868).

This is a branching hydroid that does not produce free medusae. It forms bushy ma.s.ses somewhat resembling those formed by a luxuriant growth of _Plumatella fruticosa_ (pl. iii, fig. 1) in general appearance. _C.

lacustris_ is abundant in ca.n.a.ls, rivers, and estuaries in many parts of Europe and has recently been found in the isolated salt lake Birket-el-Qurun in the Faym of Egypt.

(2) _Cordylophora whiteleggei_, v. Lendenfeld (1887).

Zool. Jahrb. ii, p. 97 (1887).

A species or race of much feebler growth; as yet imperfectly known and only recorded from fresh water in Australia.

_Cordylophora_ is a normal genus of the cla.s.s Hydrozoa and the order Gymn.o.blastea; the next four genera are certainly Hydrozoa, but their affinities are very doubtful.

(3) _Microhydra ryderi_, Potts (1885).

Potts, Q. J. Micr. Sci. London, l, p. 623, pls. x.x.xv, x.x.xvi; Browne, _ibid._ p. 635, pl. x.x.xvii (1906).

This animal, which has been found in N. America and in Germany, possesses both an as.e.xual hydroid and a s.e.xual medusoid generation. The former reproduces its species by direct budding as well as by giving rise, also by a form of budding, to medusae that become s.e.xually mature.

The hydroid has no tentacles.

(4) _Limnocodium sowerbii_, Lankester (1880).

Lankester, Q. J. Micr. Sci. London, xx, p. 351, pls. x.x.x, x.x.xi (1880); Fowler, _ibid._ x.x.x, p. 507, pl. x.x.xii (1890).

There is some doubt as to the different stages in the life-cycle of this species. The medusa has been found in tanks in hot-houses in England, France and Germany, and a minute hydroid closely resembling that of _Microhydra ryderi_ has been a.s.sociated with it provisionally.

(5) _Limnocodium kawaii_, Oka (1907).

Oka, Annot. Zool. j.a.pon. vi, p. 219, pl. viii (1907).

Only the medusa, which was taken in the R. Yang-tze-kiang, is as yet known.

(6) _Limnocnida tanganyikae_, Bohm (1889).

R. T. Gunther, 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, x.x.xvii, 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 "Memoires pour servir a 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 ent.i.tled "An attempt towards a natural history of the Polyp"* (London, 1743) and Rosel von Rosenhof in the third part of his "Insecten-Bel.u.s.tigung"

(Nurenberg, 1755) also made important contributions to the study of the physiology and structure of _Hydra_ about the same period. Linne invented the name _Hydra_, and in his "Fauna Sueica" and in the various editions of his "Systema Naturae" described several forms in a manner that permits some of them to be recognized; but Linne 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.

_Hydra_ has been examined by thousands of students in biological laboratories all over the civilized world, and the literature upon it is hardly surpa.s.sed in magnitude by that on any other genus but _h.o.m.o_. The following is a list of a few of the more important general memoirs and of the papers that refer directly to Asiatic material. A systematic bibliography is given by Bedot in his "Materiaux pour servir a l'Histoire des Hydrodes," Rev. Suisse Zool. xviii, fasc. 2 (1910).

(a) _General._

1743. BAKER, "An attempt towards a natural history of the Polyp"*

(London).

1744. TREMBLEY, "Memoires pour servir a l'histoire d'un genre de polypes d'eau douce"* (Paris).

1755. RoSEL VON ROSENHOF, "Insecten-Bel.u.s.tigung: iii, Hist. Polyporum."

1766. PALLAS, "Elenchus Zoophytorum."

1844. LAURENT, "Rech. sur l'Hydre et l'Eponge d'eau douce" ("Voy. de la Bonite, Zoophytologie").

1847. JOHNSTON, "A History of the British Zoophytes" (2nd edition).

1868. HINCKS, "History of British Hydroid Zoophytes."