The external form of the Hydromedusae varies from that of a deep bell or thimble, characteristic of the Anthomedusae, to the shallow saucer-like form characteristic of the Leptomedusae. It is usual for the umbrella to have an even, circular, uninterrupted margin; but in the order Narcomedusae secondary down-growths between the tentacles produce a lobed, indented margin to the umbrella. The marginal tentacles are rarely absent in non-parasitic forms, and are typically four in number, corresponding to the four perradii marked by the radial canals. Interradial tentacles may be also developed, so that the total number present may be increased to eight or to an indefinitely large number. In _Willia_, _Geryonia_, &c., however, the tentacles and radial canals are on the plan of six instead of four (figs. 11 and 26). On the other hand, in some cases the tentacles are less in number than the perradii; in _Corymorpha_ (figs. 3 and 22) there is but a single tentacle, while two are found in _Amphinema_ and _Gemmaria_ (Anthomedusae), and in _Solmundella bitentaculata_ (fig. 67) and _Aeginopsis hensenii_ (fig. 23) (Narcomedusae). The tentacles also vary considerably in other ways than in number: first, in form, being usually simple, with a basal bulb, but in _Cladonemidae_ they are branched, often in complicated fashion; secondly, in grouping, being usually given off singly, and at regular intervals from the margin of the umbrella, but in _Margelidae_ and in some Trachomedusae they are given off in tufts or bunches (fig. 24); thirdly, in position and origin, being usually implanted on the extreme edge of the umbrella, but in Narcomedusae they become secondarily shifted and are given off high up on the ex-umbrella (figs. 23 and 25); and, fourthly, in structure, being hollow or solid, as in the polyp. In some medusae, for instance, the remarkable deep-sea family _Pectyllidae_, the tentacles may bear suckers, by which the animal may attach itself temporarily. It should be mentioned finally that the tentacles are very contractile and extensible, and may therefore present themselves, in one and the same individual, as long, drawn-out threads, or in the form of short corkscrew-like ringlets; they may stream downwards from the sub-umbrella, or be held out horizontally, or be directed upwards over the ex-umbrella (fig. 23). Each species of medusa usually has a characteristic method of carrying its tentacles. [Illustration: After O. Maas, _Die craspedoten Medusen der Plankton Expedition_, by permission of Lipsius and Tischer. FIG. 23.--_Aeginopsis hensenii_, slightly magnified, showing the manner in which the tentacles are carried in life.] [Illustration: After O. Maas, _Craspedoten Medusen der Siboga-Expedition_, by permission of E. S. Brill & Co. FIG. 24.--_Rathkea octonemalis._]