(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. [Footnote T: See Potts, Proc. Ac. Philad. 1884, p. 28.] INDIAN SPONGILLIDÆ COMPARED WITH THOSE OF OTHER COUNTRIES.