The colour of Indian examples of H. oligactis apparently resembles that of the Calcutta winter brood of H. vulgaris so far as visual effect is concerned, but I have noticed in specimens from Lahore and the neighbourhood that very minute spherical bodies of a dark green colour are present in the endoderm cells.
[AQ] A small form of H. viridis (var. bakeri, Marshall) is found in brackish water in England.
[AR] Richard, Mém. Soc. zool. France, vii, p. 237 (1894).
[AS] See Zykoff, Biol. Centralbl. xviii, p. 272 (1898), and Annandale, Rec. Ind. Mus. i, p. 67 (1907).
[AT] Further particulars regarding the life-history of this larva will be found on pp. 114 and 115, J. Asiat. Soc. Bengal, ii (n. s.) 1906.
[AU] In the Calcutta tanks operculate molluscs such as Vivipara are certainly more free from visible attack than non-operculate species. This is the case for instance, as regards the common aquatic glowworm (Luciola sp.), which destroys large numbers of individuals of Limnophysa, Limnæus, &c. If it has been starved for several days in an aquarium it will attack an operculate form, but rarely with success. Similarly Chætogaster bengalensis attaches itself exclusively to non-operculate forms. In the one case the polyp could do very little against an adversary with so stout an integument as the insect, while, in the other, it is doubtful whether the worm does any harm to its host. The polyp would afford very little protection against the snail's vertebrate enemies or against what appears to be its chief foe, namely, drought. As the water sinks in the tank non-operculate species migrate to the deeper parts, but Vivipara and Ampullaria close their shells, remain where they are, and so often perish, being left high and dry, exposed to the heat of the sun.
[AV] Pallas writes as regards this "pulcherrime vegetantem varietatem" with his usual critical insight, "Vix tamen peculiaris speciei nomine salutanda videtur." It is probably the Hydra socialis of Linné.