As regards habit of life the vast majority of Hydromedusae are pelagic organisms, floating on the surface of the open sea, propelling themselves feebly by the pumping movements of the umbrella produced by contraction of the sub-umbral musculature, and capturing their prey with their tentacles. The genera _Cladonema_ (fig. 20) and _Clavatella_ (fig. 21), however, are ambulatory, creeping forms, living in rock-pools and walking, as it were, on the tips of the proximal branches of each of the tentacles, while the remaining branches serve for capture of food. _Cladonema_ still has the typical medusan structure, and is able to swim about, but in _Clavatella_ the umbrella is so much reduced, that swimming is no longer possible. The remarkable medusa _Mnestra parasites_ is ecto-parasitic throughout life on the pelagic mollusc _Phyllirrhoe_, attached to it by the sub-umbral surface, and its tentacles have become rudimentary or absent. It is interesting to note that _Mnestra_ has been shown by J. W. Fewkes [15] and R. T. Günther [19] to belong to the same family (_Cladonemidae_) as _Cladonema_ and _Clavatella_, and it is reasonable to suppose that the non-parasitic ancestor of _Mnestra_ was, like the other two genera, an ambulatory medusa which acquired louse-like habits. In some species of the genus _Cunina_ (Narcomedusae) the youngest individuals (actinulae) are parasitic on other medusae (see below), but in later life the parasitic habit is abandoned. No other instances are known of sessile habit in Hydromedusae. [Illustration: After E. T. Browne, from _Proc. Zool. Soc. of London_. FIG. 22.--_Corymorpha nutans_, adult female Medusa. Magnified 10 diameters.]