II. Gemmules surrounded in several layers by distinct polygonal air-s.p.a.ces with chitinous walls. (Subgenus _Eunapius_, p. 86.) A. Gemmules single. Skeleton- and gemmule-spicules smooth, pointed, not very stout _carteri_, p. 87.

B. Gemmules bound together in pairs.

Skeleton friable; skeleton-spicules slender _gemina_, nov., p. 97.

C. Gemmules bound together in free groups of more than two or forming a "pavement-layer" at the base of the sponge.

_c._ Skeleton friable; skeleton-spicules slender _fragilis_, p. 95.

_c'._ Skeleton very hard and resistant; skeleton-spicules stout _cra.s.sissima_, p. 98.

III. Gemmules without or with irregular pneumatic coat, covered by a chitinous membrane or membranes in which the gemmule-spicules lie parallel to the surface. (Subgenus _Stratospongilla_, p. 100.)

A. Skeleton spicules spiny or irregular in outline.

_a._ Skeleton-spicules blunt; gemmules covered by a single chitinous membrane _indica_, p. 100.

_a'._ Skeleton-spicules sharp; gemmules covered by two chitinous membranes _bombayensis_, p. 102.

B. Skeleton-spicules smooth.

Skeleton-spicules sharp; gemmule spicules very irregular in form _ultima_, p. 104.

Subgenus A. EUSPONGILLA, _Vejdovsky_.

_Euspongilla_, Vejdovsky, Abh. Bohm. Ges. xii, p. 15 (1883).

_Euspongilla_, _id._, in Potts's "Fresh-Water Sponges," P. Ac. Philad.

1887, p. 172.

_Euspongilla_, Weltner, in Zacharias's Tier- und Pflanzenwelt des Susswa.s.sers, i, p. 210 (1891).

TYPE, _Spongilla lacustris_, auctorum.

Spongillae in which the gemmules are covered with a thick, apparently granular pneumatic coat. A delicate membrane often occurs outside this coat, but it is never thick or h.o.r.n.y. The gemmules usually lie free in the sponge but sometimes adhere to its support; rarely they are fastened together in groups (_e. g._ in _S. aspinosa_, Potts). The skeleton-spicules are never very stout and the skeleton is always delicate.

The species in this subgenus are closely allied and must be distinguished rather by the sum of their peculiarities than by any one character. They occur in all countries in which Spongillidae are found.

Seven Indian species may be recognized.

1. Spongilla lacustris, _auctorum_.

_Spongilla lacustris_, Bowerbank, P. Zool. Soc. London, 1863, p. 441, pl. x.x.xviii, fig. 14.

_Spongilla lacustris_, Carter, Ann. Nat. Hist. (5) vii, p. 87 (1881).

_Euspongilla lacustris_, Vejdovsky, in Potts's "Fresh-Water Sponges,"

P. Ac. Philad. 1887, p. 172.

_Spongilla lacustris_, Potts, _ibid._, p. 186, pl. v, fig. 1, pl. vii, figs. 1-6.

_Euspongilla lacustris_, Weltner, in Zacharias's Tier- und Pflanzenwelt des Susswa.s.sers, i, p. 211, figs. 36-38 (1891).

_Spongilla lacustris_, _id._, Arch. Naturg. lxi (i), pp. 118, 133-135 (1895).

_Spongilla lacustris_, Annandale, J. Linn. Soc., Zool., x.x.x, p. 245 (1908).

[I have not attempted to give a detailed synonymy of this common species. There is no means of telling whether many of the earlier names given to forms or allies of _S. lacustris_ are actual synonyms, and it would serve no useful purpose, so far as the fauna of India is concerned, to complicate matters by referring to obscure descriptions or possible descriptions of a species only represented in India, so far as we know, by a specialized local race, to which separate references are given.]

_Sponge_ soft and easily compressed, very brittle when dry, usually consisting of a flat or rounded basal portion of no great depth and of long free cylindrical branches, which droop when removed from the water; branches occasionally absent. Colour bright green when the sponge is growing in a strong light, dirty flesh-colour when it is growing in the shade. (Even in the latter case traces of the "green corpuscles" can be detected in the cells of the parenchyma.) Oscula star-shaped, of moderate size, as a rule rendered conspicuous by the furrows that radiate from them over the outer surface of the parenchyma below the external membrane; oscular collars well developed.

_Skeleton_ reticulate, loose, with definite radiating and transverse fibres held together by a small quant.i.ty of spongin; the fibres slender but not extremely so.

_Spicules._ Skeleton-spicules smooth, sharply pointed, long, slender.

Flesh-spicules slender, covered with small spines, sharply pointed, nearly straight. Gemmule-spicules resembling the flesh-spicules but shorter and as a rule more strongly curved, sometimes bent so as to form semicircular figures, usually pointed somewhat abruptly; their spines relatively longer than those of the flesh-spicules, often curved backwards, especially near the ends of the spicules, at which points they are often longer than elsewhere.

_Gemmules_ usually numerous in autumn, lying free in the sponge, spherical, variable in size but usually rather large, as a rule covered with a thick granular coat in which the spicules are arranged tangentially; a horizontal layer of spicules often present in the external membrane; the granular coat and its spicules occasionally deficient. No foraminal tubule; its place sometimes taken by an open, bowl-shaped chitinous structure the base of which is in continuity with the inner chitinous coat of the gemmule.

_S. lacustris_ is an extremely variable species, varying in the size, proportions and shape of its spicules, in its external form and in the size and structure of the gemmule. A considerable number of varieties have been described from different parts of Europe and N. America, but some of these may represent distinct but closely-allied species; descriptions of most of them will be found in Potts's "Fresh-Water Sponges." The embryology and the earlier stages of the development from the egg have been described in great detail by Evans (Quart. J. Micr.

Sci. (n. s.) xlii, p. 363 (1899)), while the anatomy and physiology are discussed by most authors who have written on these features in the Spongillidae.

TYPE.--It is impossible to say who was the first authority to use the name _Spongilla lacustris_ in the sense in which it is used by recent authors. No type can therefore be recognized.

GEOGRAPHICAL DISTRIBUTION.--_S. lacustris_ occurs all over Europe and N.

America and is probably the commonest species in most parts of both continents. It has also been found in Northern Asia and may occur in the Himalayan lakes and in the north-west of India.

1 _a._ Subspecies reticulata*, _Annandale_.

_Spongilla reticulata_, Annandale, Rec. Ind. Mus. i, p. 387, pl. xiv, fig. 1 (1907).

_Spongilla lacustris_ subspecies _reticulata_, _id._, P.

U.S. Mus. x.x.xvii, p. 401 (1909).

This race differs from the typical _S. lacustris_ in the following particulars:--

(1) The branches are always compressed and anastomose freely when well developed (fig. 5, p. 37);

(2) the skeleton-fibres are finer;

(3) the skeleton-spicules are longer;

(4) the gemmule-spicules are longer and more slender and are never strongly bent.

[Ill.u.s.tration: Fig. 8.

A=gemmule-spicules of _Spongilla lacustris_ subsp. _reticulata_ (from type); B=gemmule-spicules of _S. alba_ from Calcutta: both highly magnified.]

As regards the form of the skeleton- and gemmule-spicules and also that of the branches the subspecies _reticulata_ resembles _S. alba_ rather than _S. lacustris_, but owing to the fact that it agrees with _S.

lacustris_ in its profuse production of branches, in possessing green corpuscles and in its fragility, I think it should be a.s.sociated with that species.

The branches are sometimes broad (fig. 5, p. 37), sometimes very slender. In the latter condition they resemble blades of gra.s.s growing in the water.

TYPE in the Indian Museum; a co-type in the British Museum.

GEOGRAPHICAL DISTRIBUTION.--All over Eastern India and Burma; also in the Bombay Presidency. _Localities:_--BENGAL, Port Canning, Ganges delta; Rajshahi (Rampur Bhulia) on the Ganges, 150 miles N. of Calcutta (_Annandale_); Puri district, Orissa (_Annandale_); R. Jharai, Siripur, Saran district, Tirhut (_M. Mackenzie_): MADRAS PRESIDENCY, Madras (town) (_J. R. Henderson_): BOMBAY PRESIDENCY, Igatpuri, W. Ghats (_Annandale_).

BIOLOGY.--This subspecies is usually found in small ma.s.ses of water, especially in pools of rain-water, but Mr. Mackenzie found it growing luxuriantly in the Jharai at a time of flood in September. It is very abundant in small pools among the sand-dunes that skirt the greater part of the east coast of India. Here it grows with great rapidity during the "rains," and often becomes desiccated even more rapidly as soon as the rain ceases. As early in the autumn as October I have seen ma.s.ses of the sponge attached, perfectly dry, to gra.s.s growing in the sand near the Sur Lake in Orissa. They were, of course, dead but preserved a life-like appearance. Some of them measured about six inches in diameter. At Port Canning the sponge grows during the rains on the brickwork of bridges over ditches of brackish water that dry up at the beginning of winter, while at Rajshahi and at Igatpuri I found it at the edges of small ponds, at the latter place in November, at the former in February.

Specimens taken at Madras by Dr. Henderson during the rains in small ponds in the sand contained no gemmules, but these structures are very numerous in sponges examined in autumn or winter.

Numerous larvae of _Sisyra indica_ (p. 92) were found in this sponge at Rajshahi. Unlike those obtained from _S. alba_, they had a green colour owing to the green matter sucked from the sponge in their stomachs. The _coralloides_ phase of _Plumatella fruticosa_ (p. 219) was also found in _S. lacustris_ subsp. _reticulata_ at Rajshahi.