The most abundant and possibly the most important organisms that may be considered as benefactors to the Spongillidæ are the green corpuscles that live in the cells of certain species (fig. 2, p. 31), notably _Spongilla lacustris_, _S. proliferens_, and _Dosilia plumosa_. I have already said that these bodies are in all probability algæ which live free in the water and move actively at one stage of their existence, but some of them are handed on directly from a sponge to its descendants in the cells of the gemmule. In their quiescent stage they have been studied by several zoologists, notably by Sir Ray Lankester[Q] and Dr. W. Weltner[R], but the strongest light that has been cast on their origin is given by the researches of Dr. F. W. Gamble and Mr. F. Keeble (Q. J. Microsc. Sci. London, xlvii, p. 363, 1904, and li, p. 167, 1907). These researches do not refer directly to the Spongillidæ but to a little flat-worm that lives in the sea, _Convoluta roscoffiensis_. The green corpuscles of this worm so closely resemble those of _Spongilla_ that we are justified in supposing a similarity of origin. It has been shown by the authors cited that the green corpuscles of the worm are at one stage minute free-living organisms provided at one end with four flagella and at the other with a red pigment spot. The investigators are of the opinion that these organisms exhibit the essential characters of the algæ known as Chlamydomonadæ, and that after they have entered the worm they play for it the part of an excretory system. [Footnote Q: Q. J. Microsc. Sci. London, xxii. p. 229 (1882).] [Footnote R: Arch. Naturg. Berlin, lix (i), p. 260 (1893).] 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). [Footnote S: Journ. As. Soc. Beng. n. s. ii, 1906, p. 189.]