Types.—The types of the varieties mollis, cava and lobosa are in the collection of the Indian Museum. I regard as the type of the species the specimen sent by Carter to Bowerbank and by him named S. carteri, although, owing to some confusion, Carter's description under this name appeared some years before Bowerbank's. This specimen is in the British Museum, with a fragment in the Indian Museum.
Geographical Distribution.—The range of the species extends westwards to Hungary, southwards to Mauritius and eastwards to the island of Madura in the Malay Archipelago; a specimen from Lake Victoria Nyanza in Central Africa has been referred to it by Kirkpatrick (P. Zool. Soc. London, 1906 (i), p. 219), but I doubt whether the identification is correct. In India S. carteri is by far the most universally distributed and usually much the commonest freshwater sponge; it is one of the only two species as yet found in Ceylon. Specimens are known from the following localities:—Punjab, Lahore (J. Stephenson): Bombay Presidency, island of Bombay (Carter, Kirkpatrick, Annandale); Igatpuri, W. Ghats (alt. ca. 2,000 ft.) (Annandale): United Provinces (plains), Agra (Kirkpatrick); Lucknow: Himalayas, Bhim Tal, Kumaon (alt. 4,500 ft.) (Annandale); Tribeni, Nepal (Hodgart): Bengal, Calcutta and neighbourhood; Rajshahi (Rampur Bhulia) on the R. Ganges about 150 miles N. of Calcutta (Annandale); Berhampur, Murshidabad district (R. E. Lloyd); Pusa, Darbbhanga district (Bainbrigge Fletcher); Siripur, Saran district, Tirhut (M. Mackenzie); Puri and the Sur Lake, Orissa (Annandale): Madras Presidency, near Madras town (J. R. Henderson); Madura district (R. Bruce Foote); Bangalore (Annandale) and Worgaum, Mysore State (2,500-3,000 ft.); Ernakulam and Trichur, Cochin (G. Mathai); Trivandrum and the neighbourhood of C. Comorin, Travancore (var. lobosa) (R. S. N. Pillay): Burma, Kawkareik, interior of Amherst district, Tenasserim (Annandale); Rangoon (Annandale); Bhamo, Upper Burma (J. Coggin Brown): Ceylon, Peradeniya (E. E. Green); outlet of the Maha Rambaikulam between Vavuniya and Mamadu, Northern Province (Willey); Horowapotanana, between Trincomalee and Anuradihapura, North-Central Province (Willey).
Biology.—S. carteri usually grows in ponds and lakes; I have never seen it in running water. Mr. Mackenzie found it on the walls of old indigo wells in Tirhut.
The exact form of the sponge depends to some extent on the forces acting on it during life. At Igatpuri, for instance, I found that specimens attached to the stems of shrubs growing in the lake and constantly swayed by the wind had their surface irregularly reticulated with high undulating ridges, while those growing on stones at the bottom of a neighbouring pond were smooth and rounded.
Sponges of this species do not shun the light.
In Calcutta S. carteri flourishes during the cold weather (November to March). By the end of March many specimens that have attached themselves to delicate stems such as those of the leaves of Limnanthemum, or to the roots of Pistia stratiotes, have grown too heavy for their support and have sunk down into the mud at the bottom of the ponds, in which they are quickly smothered. Others fixed to the end of branches overhanging the water or to bricks at the edge have completely dried up. A large proportion, however, still remain under water; but even these begin to show signs of decay at this period. Their cells migrate to the extremities of the sponge, leaving a mass of gemmules in the centre, and finally perish.
Few sponges exist in an active condition throughout the hot weather. The majority of those that do so exhibit a curious phenomenon. Their surface becomes smoothly rounded and they have a slightly pinkish colour; the majority of the cells of their parenchyma, if viewed under a high power of the microscope, can be seen to be gorged with very minute drops of liquid. This liquid is colourless in its natural condition, but if the sponge is plunged into alcohol the liquid turns of a dark brown colour which stains both the alcohol and the sponge almost instantaneously. Probably the liquid represents some kind of reserve food-material. Even in the hot weather a few living sponges of the species may be found that have not this peculiarity, but, in some ponds at any rate, the majority that survive assume the peculiar summer form, which I have also found at Lucknow.
Reproduction takes place in S. carteri in three distinct ways, two of which may be regarded as normal, while the third is apparently the result of accident. If a healthy sponge is torn into small pieces and these pieces are kept in a bowl of water, little masses of cells congregate at the tips of the radiating fibres of the skeleton and assume a globular form. At first these cells are homogeneous, having clear protoplasm full of minute globules of liquid. The masses differ considerably in size but never exceed a few millimetres in diameter. In about two days differentiation commences among the cells; then spicules are secreted, a central cavity and an external membrane formed, and an aperture, the first osculum, appears in the membrane. In about ten days a complete young sponge is produced, but the details of development have not been worked out.
The most common normal form of reproduction is by means of gemmules, which are produced in great numbers towards the end of the cold weather. If small sponges are kept alive in an aquarium even at the beginning of the cold weather, they begin to produce gemmules almost immediately, but these gemmules although otherwise perfect, possess few or no gemmule-spicules. If the sponge becomes desiccated at the end of the cold weather and is protected in a sheltered place, some or all of the gemmules contained in the meshes of its skeleton germinate in situ as soon as the water reaches it again during the "rains." It is by a continuous or rather periodical growth of this kind, reassumed season after season, that large masses of sponge are formed. In such masses it is often possible to distinguish the growth of the several years, but as a rule the layers become more or less intimately fused together, for no limiting membrane separates them. A large proportion of the gemmules are, however, set free and either float on the surface of the water that remains in the ponds or are dried up and carried about by the wind. In these circumstances they do not germinate until the succeeding cold weather, even if circumstances other than temperature are favourable; but as soon as the cold weather commences they begin to produce new sponges with great energy.
Sexual reproduction, the second normal form, takes place in S. carteri mainly if not only at the approach of a change of season, that is to say about March, just before the hot weather commences, and about November, just as the average temperature begins to sink to a temperate level. At these seasons healthy sponges may often be found full of eggs and embryos, which lie in the natural cavities of the sponge without protecting membrane.