[Footnote 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.] The association, however, is undoubtedly useful to _Hydra_. The mud on the shells of _Vivipara_ taken on floating objects shows that in cool weather the snail comes up from the bottom to the surface, and it probably goes in the opposite direction in hot weather. Moreover, the common Calcutta species (_V. bengalensis_) feeds very largely, if not exclusively, on minute green algæ. It therefore naturally moves towards spots where smaller forms of animal and vegetable life abound and conditions are favourable for the polyp. The polyp's means of progression are limited, and the use of a beast of burden is most advantageous to it, for it can detach itself when it arrives at a favourable habitat. If specimens are kept in water which is allowed to become foul, a very large proportion of them will attach themselves to any snails confined with them. Under natural conditions they would thus in all probability be rapidly conveyed to a more suitable environment. In the tanks it is far commoner to find young four-rayed polyps on _Vivipara_ than individuals with five or six rays; but the adults of the species are far less prone to change their position than are the young. The Calcutta _Hydra_, especially in spring, exhibits a distinct tendency to frequent the neighbourhood of sponges and polyzoa, such as _Spongilla carteri_ and the denser forms of _Plumatella_. Possibly this is owing to the shade these organisms provide. 25. Hydra oligactis, _Pallas_. Polypes de la troisième espèce, Trembley, Mém. hist. Polypes,* pl. i, figs. 3, 4, 6; pl. ii, figs. 1-4; pl. iii, fig. 11; pl. v, figs. 1-4; pl. vi, figs. 3-7, 9, 10; pl. viii, figs. 8, 11; pl. ix (1744). Rösel von Rosenhof, Insekt.-Belustigung, iii, Hist. Polyp., pls. lxxxiv-lxxxvi (1755).