Spongillae in the gemmules of which the pneumatic layer is absent or irregularly developed, its place being sometimes taken by air-s.p.a.ces between the stout chitinous membranes that cover the gemmule. At least one of these membranes is always present.

The gemmule-spicules lie in the membrane or membranes parallel to the surface of the gemmule, and are often so arranged as to resemble a mosaic. The gemmules themselves are usually adherent to the support of the sponge. The chitinous membrane or membranes are often in continuity with a membrane that underlies the base of the sponge. The skeleton is usually stout, though often almost amorphous, and the skeleton-spicules are sometimes sausage-shaped.

Sponges of this subgenus form crusts or sheets on solid submerged objects.

_Stratospongilla_ is essentially a tropical subgenus, having its head-quarters in Central Africa and Western India. One of its species, however, (_S. sumatrana_*, Weber) occurs both in Africa and the Malay Archipelago, while another has only been found in S. America (_S.

navicella_, Carter).

Aberrant species occur in China (_S. sinensis_*, _S. coggini_*) and the Philippines (_S. clementis_*). Three species have been found in the Bombay Presidency and Travancore, one of which (_S. bombayensis_*) extends its range eastwards to Mysore and westwards across the Indian Ocean to Natal.

12. Spongilla indica*, _Annandale_.

_Spongilla indica_, Annandale, Rec. Ind. Mus. ii, p. 25, figs. 1, 2 (1908).

_Sponge_ forming a very thin layer, of a bright green or pale grey colour; surface smooth, minutely hispid; pores and oscula inconspicuous, the latter approached in some instances by radiating furrows; subdermal cavity small; texture compact, rather hard.

_Skeleton_ incoherent, somewhat ma.s.sive owing to the large number of spicules present. Spicules forming triangular meshes and occasionally arranged in vertical lines several spicules broad but without spongin.

_Spicules._ Skeleton-spicules straight or nearly straight, slender, cylindrical, amphistrongylous, uniformly covered with minute, sharp spines; flesh-spicules slender, sharply pointed, straight or curved, irregularly covered with relatively long, straight sharp spines, abundant in the dermal membrane, scarce in the substance of the sponge.

Gemmule-spicules short, stout, sausage-shaped, covered with minute spines, which are sometimes absent from the extremities.

_Gemmules_ spherical, somewhat variable in size, with a single aperture, which is provided with a trumpet-shaped foraminal tubule and is situated at one side of the gemmule in its natural position; the inner chitinous coat devoid of spicules, closely covered by an outer coat composed of a darkly coloured chitinoid substance in which the gemmule-spicules are embedded, lying parallel or almost parallel to the inner coat. The outer coat forms a kind of mantle by means of the skirts of which the gemmule is fastened to the support of the sponge. This coat is pierced by the foraminal tubule. The gemmules are distinct from one another.

[Ill.u.s.tration: Fig. 17.--Gemmule of _Spongilla indica_ seen from the side (from type specimen), magnified.]

Average length of skeleton-spicules 0.2046 mm.

" breadth of skeleton-spicules 0.0172 "

" length of flesh-spicules 0.053 "

" breadth of flesh-spicules 0.0053 "

" length of gemmule-spicules 0.044 "

" breadth of gemmule-spicules 0.0079 "

_S. indica_ is closely allied to _S. sumatrana_*, Weber, which has been found both in the Malay Archipelago and in East Africa. It may be distinguished by its blunt, almost truncated megascleres and comparatively slender gemmule-spicules.

TYPE in the Indian Museum.

HABITAT, etc.--Growing, together with _S. cinerea_ and _Corvospongilla lapidosa_, on the stone sides of an artificial conduit in the R.

G.o.daveri at Nasik on the eastern side of the Western Ghats in the Bombay Presidency. The water was extremely dirty and was used for bathing purposes. The sponge was green where the light fell upon it, grey where it was in the shadow of the bridge under which the conduit ran. The only specimens I have seen were taken in November, 1907.

13. Spongilla bombayensis*, _Carter_. (Plate II, fig. 2.)

_Spongilla bombayensis_, Carter, Ann. Nat. Hist. (5) x, p.

369, pl. xvi, figs. 1-6 (1882).

_Spongilla bombayensis_, Annandale, Zool. Jahrb., Syst.

xxvii, p. 562, figs. B, C (1909).

_Sponge_ hard but friable, forming thin layers or cushions; its surface often irregular but without a trace of branches; its area never very great; oscula inconspicuous; external membrane adhering closely to the sponge; colour brownish or greyish.

[Ill.u.s.tration: Fig. 18.--Gemmule of _Spongilla bombayensis_ as seen from above (from type specimen), magnified.]

_Skeleton_ almost amorphous, very dense, consisting of large numbers of spicules arranged irregularly; radiating fibres occasionally visible in sections, but almost devoid of spongin; a more or less definite reticulation of horizontal spicules lying immediately under the external membrane.

_Spicules._ Skeleton-spicules slender, pointed, feebly curved, irregularly roughened or minutely spined all over the surface.

Flesh-spicules straight, narrowly rhomboidal in outline, sharply pointed, slender, covered with minute, irregular, straight spines, scanty in the parenchyma, abundant in the external membrane.

Gemmule-spicules sausage-shaped or bluntly pointed, variable in length but usually rather stout, covered with minute spines, as a rule distinctly curved.

_Gemmules_ round or oval, firmly adherent[AH] to the base of the sponge, as a rule rather shallowly dome-shaped, covered by two thick chitinous membranes, in each of which there is a dense horizontal layer of spicules; no granular or cellular covering; the two chitinous coats separated by an empty s.p.a.ce; the aperture or apertures on the side of the gemmule in its natural position, provided with foraminal tubules, which may be either straight or curved, project through the outer chitinous membrane and often bend down towards the base of the gemmule.

The spicules of the outer layer often more irregular in outline and less blunt than those of the inner layer.

[Footnote AH: The outer covering by means of which the gemmule is fixed is not formed until the other structures are complete. In young sponges, therefore, free gemmules may often be found.]

This sponge is allied to _S. indica_, but is distinguished among other characters by its sharp skeleton-spicules and by the fact that the gemmule is covered by two chitinous membranes instead of one.

TYPE in the British Museum; a fragment in the Indian Museum.

GEOGRAPHICAL DISTRIBUTION.--S. and W. India and S. Africa. Carter's type was found in the island of Bombay, my own specimens in Igatpuri Lake in the Western Ghats. I have recently (October 1910) found sponges and bare gemmules attached to stones at the end of a tank about 10 miles from Bangalore (Mysore State) in the centre of the Madras Presidency. Prof.

Max Weber obtained specimens in Natal.

BIOLOGY.--The specimens collected by Prof. Weber in Natal and those collected by myself in the Bombay Presidency were both obtained in the month of November. It is therefore very interesting to compare them from a biological point of view. In so doing, it must be remembered that while in S. Africa November is near the beginning of summer, in India it is at the beginning of the "cold weather," that is to say, both the coolest and the driest season of the year. The lake in which my specimens were obtained had, at the time when they were collected, already sunk some inches below its highest level, leaving bare a gently sloping bank of small stones. Adhering to the lower surface of these stones I found many small patches of _Spongilla bombayensis_, quite dry but complete so far as their harder parts were concerned and with the gemmules fully formed at their base. From the shallow water at the edge of the lake I took many similar stones which still remained submerged.

It was evident that the sponge had been just as abundant on their lower surface as on that of the stones which were now dry; but only the gemmules remained, sometimes with a few skeleton-spicules adhering to them (Pl. II, fig. 2). The bulk of the skeleton had fallen away and the parenchyma had wholly perished. In a few instances a small sponge, one or two millimetres in diameter, had already been formed among the gemmules; but these young sponges appeared to belong to some other species, possibly _Spongilla indica_, which was also common in the lake.

Carter's specimen of _S. bombayensis_, which was evidently in much the same condition as those I found still submerged a month later, was taken in October in a disused quarry. It was surrounded by a ma.s.s of _S.

carteri_ three inches in diameter, and was attached to a herbaceous annual. The point on the edge of the quarry at which this plant grew was not reached by the water until July. It is therefore necessary to a.s.sume that the gemmules of _S. bombayensis_ had been formed between July and October. Probably the larva of the sponge had settled down on the plant during the "rains"--which commence in Bombay about the beginning of June--and had grown rapidly. The production of gemmules may have been brought about owing to the sponge being choked by the more vigorous growth of _S. carteri_, a species which grows to a considerable size in a comparatively short time, while _S. bombayensis_ apparently never reaches a thickness of more than a few millimetres.

The manner in which the gemmules of _S. bombayensis_ are fastened to the solid support of the sponge must be particularly useful in enabling them to sprout in a convenient environment as soon as the water reaches them.

The fact that the gemmules remained fixed without support renders it unnecessary for the skeleton to persist as a cage containing them (or at any rate a proportion of them) during the period of rest.

Prof. Weber's specimens of _S. bombayensis_ were collected in a river, apparently on stones or rocks, towards the beginning of the S. African summer. They contain comparatively few gemmules and were evidently in a vigorous condition as regards vegetative growth. Unfortunately we know nothing of the seasonal changes which take place in freshwater sponges in S. Africa, but the difference between these changes in Europe and in India shows that they are dependent on environment as well as the idiosyncrasy of the species. It is very interesting, therefore, to see that the condition of sponges taken in S. Africa differs so widely from that of other individuals of the same species taken in India at the same season.

In Prof. Weber's specimens I have found numerous small tubules of inorganic debris. These appear to be the work of Chironomid larvae, of which there are several specimens loose in the bottle containing the sponges. Other tubules of a very similar appearance but with a delicate chitinoid foundation appear to be the remains of a species of _Plumatella_ of which they occasionally contain a statoblast.

14. Spongilla ultima*, _Annandale_. (Plate II, fig. 3.)

_Spongilla ultima_, Annandale, Rec. Ind. Mus. v, p. 31 (1910).

_Sponge_ hard and strong, forming a thin layer on solid objects, of a pale green colour (dry); the oscula small but rendered conspicuous by the deep radiating furrows that surround them; external surface of the sponge rough but not spiny.

_Skeleton_ forming a compact but somewhat irregular reticulation in which the radiating fibres are not very much more distinct than the transverse ones; a considerable amount of almost colourless spongin present.

_Spicules._ Skeleton-spicules smooth, stout, amphioxous, as a rule straight or nearly straight, not infrequently inflated in the middle or otherwise irregular. No flesh-spicules. Gemmule-spicules variable in size, belonging to practically every type and exhibiting practically every abnormality possible in the genus, the majority being more or less sausage-shaped and having a roughened surface, but others being cruciform, spherical, subspherical, rosette-like, needle-like, bifid or even trifid at one extremity.

[Ill.u.s.tration: Fig. 19.--Spicules of _Spongilla ultima_ (from type specimen), 120.]

_Gemmules_ adherent, spherical, large, each covered by two distinct layers of horizontal spicules; the outer layer intermixed with skeleton-spicules and often containing relatively large siliceous spheres, a large proportion of the spicules being irregular in shape; the spicules of the inner layer much more regular and as a rule sausage-shaped. The outer layer is contained in a chitinous membrane which spreads out over the base of the sponge. The foraminal tubules are short and straight.