Of animals living in more or less intimate relations with the polyp, I have found two very distinct species of protozoa, neither of which is identical with either of the two commonly found in association with _Hydra_ in Europe, _Trichodina pediculus_ and _Kerona polyporum_. On two occasions, one in January and the other at the beginning of February, I have seen a minute colourless flagellate on the tentacles of the Calcutta polyp. On the first occasion the tentacles were completely covered with this protozoon, so that they appeared at first sight as though encased in flagellated epithelium. The minute organism was colourless, transparent, considerably larger than the spermatozoa of _Hydra_, slightly constricted in the middle and rounded at each end. It bore a long flagellum at the end furthest from its point of attachment, the method of which I could not ascertain. When separated from the polyp little groups clung together in rosettes and gyrated in the water. On the other occasion only a few individuals were observed. Possibly this flagellate was a parasite rather than a commensal, as the individual on which it swarmed was unusually emaciated and colourless, and bore neither gonads nor buds. The larger stinging cells were completely covered by groups of the organism, and possibly this may have interfered with the discharge of stinging threads. The other protozoon was _Vorticella monilata_, Tatem, which has been found, not in association with _Hydra_, in Europe and S. America. In Calcutta I have only seen it attached to the column of the polyp, but probably it would also be found, if carefully looked for, attached to water-weeds. Especially in the four-rayed stage, the polyp not infrequently attaches itself to shells of _Vivipara_, and, more rarely, to those of other molluscs. It is doubtful whether this temporary association between _Hydra_ and the mollusc is of any importance to the latter. Even when the polyp settles on its body and not on its shell (as is sometimes the case) the _Vivipara_ appears to suffer no inconvenience, and makes no attempt to get rid of its burden. It is possible, on the other hand, that the _Hydra_ may protect it by devouring would-be parasites; but of this there is no evidence[AU].