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 space; 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.
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 mass 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 assume 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.