As they exist in the cells of Spongilla the corpuscles are minute oval bodies of a bright green colour and each containing a highly refractile colourless granule. A considerable number may be present in a single cell. It is found in European sponges that they lose their green colour if the sponge is not exposed to bright sunlight. In India, however, where the light is stronger, this is not always the case. Even when the colour goes, the corpuscles can still be distinguished as pale images of their green embodiment. They are called Chlorella by botanists, who have studied their life-history but have not yet discovered the full cycle. See Beyerinck in the Botan. Zeitung for 1890 (vol. xlviii, p. 730, pl. vii; Leipzig), and for further references West's 'British Freshwater Algæ,' p. 230 (1904).
The list of beneficent organisms less commonly present than the green corpuscles includes a Chironomus larva that builds parchment-like tubes in the substance of Spongilla carteri and so assists in supporting the sponge, and of a peculiar little worm (Chætogaster spongillæ[[S]]) that appears to assist in cleaning up the skeleton of the same sponge at the approach of the hot weather and in setting free the gemmules (p. 93).
(c) Organisms that take shelter in the Sponge or adhere to it externally.
There are many animals which take shelter in the cavities of the sponge without apparently assisting it in any way. Among these are the little fish Gobius alcockii, which lays its eggs inside the oscula of S. carteri, thus ensuring not only protection but also a proper supply of oxygen for them (p. 94); the molluscs (Corbula, spp.) found inside S. alba var. bengalensis (p. 78); and the Isopod (Tachæa spongillicola) that makes its way into the oscula of Spongilla carteri and S. crateriformis (pp. 86, 94).
In Europe a peculiar ciliated Protozoon (Trichodina spongillæ) is found attached to the external surface of freshwater sponges. I have noticed a similar species at Igatpuri on Spongilla crateriformis, but it has not yet been identified. It probably has no effect, good or bad, on the sponge.
Freshwater Sponges in relation to Man.
In dealing with Spongilla carteri I have suggested that sponges may be of some hygienic importance in absorbing putrid organic matter from water used both for ablutionary and for drinking purposes, as is so commonly the case with regard to ponds in India. Their bad odour has caused some species of Spongillidæ to be regarded as capable of polluting water, but a mere bad odour does not necessarily imply that they are insanitary.
Unless my suggestion that sponges purify water used for drinking purposes by absorbing putrid matter should prove to be supported by fact, the Spongillidæ cannot be said to be of any practical benefit to man. The only harm that has been imputed to them is that of polluting water[[T]], of blocking up water-pipes by their growth—a very rare occurrence,—and of causing irritation to the human skin by means of their spicules—a still rarer one. At least one instance is, however, reported in which men digging in a place where a pond had once been were attacked by a troublesome rash probably due to the presence of sponge-spicules in the earth, and students of the freshwater sponges should be careful not to rub their eyes after handling dried specimens.
Indian Spongillidæ Compared With Those of Other Countries.
In Weltner's catalogue of the freshwater sponges (1895) seventy-six recent species of Spongillidæ (excluding Lubosmirskia) are enumerated, and the number now known is well over a hundred. In India we have twenty-nine species, subspecies, and varieties, while from the whole of Europe only about a dozen are known. In the neighbourhood of Calcutta nine species, representing three genera and a subgenus, have been found; all of them occur in the Museum tank. The only other region of similar extent that can compare with India as regards the richness of its freshwater sponge fauna is that of the Amazon, from which about twenty species are known. From the whole of North America, which has probably been better explored than any other continent so far as Spongillidæ are concerned, only twenty-seven or twenty-eight species have been recorded.