_a._ Medusid seen in profile. _b._ The same viewed from below. _c._ Its polypite. _d._ Part of its marginal canal, and other structures in connection therewith. ν. Disk or swimming organ. π. Polypite. ψ. Veil. τ. Tentacle. χ. Radiating canal. χ′. Marginal canal. ω. Reproductive organ. ο′. Coloured spot. ο″. Marginal vesicle. ] The Medusidæ are distinguished by their globular or bell-shaped disc, which by its alternate contractions and expansions forces them forward through the water. By contracting the whole or only part of its disc, the medusa has it in its power to direct its movements, and while thus swimming along with the convex side of the disc directed forwards, and its oral lobes and tentacles following behind like "streamers long and gay," it may well rank among the most elegant children of the sea. From the roof of the disc a single polypite is suspended, whose mouth, generally produced into four lobes, though in some forms it is much more divided, passes into the central cavity (stomach) of the swimming organ, from which canals (either four in number, or multiples of four) radiate to join a circular vessel surrounding the margin of the bell. A shelf-like membrane or veil, extending around the margin, and highly contractile, assists locomotion by narrowing more or less the aperture of the bell, and thus concentrating its efforts upon a narrower space. More or less numerous tentacles generally depend from the margin, and around it are disposed two kinds of remarkable bodies--"vesicles" and "pigment spots," or "eye-specks"--which are supposed to be able to communicate the impressions of light and sound. This complexity of organisation in creatures which Réaumur contemptuously styled mere lumps of animated jelly is all the more wonderful when we consider that they consist almost entirely of water, and shrink to a mere nothing when abandoned by their vital power. Thus of a medusa originally weighing many pounds but few traces remain after death; the ground is covered with a light varnish; all the rest has been absorbed by the thirsty sands. [Illustration: Various forms of Medusidæ. _a._ _Aequorea formosa_, seen in profile. _b._ The same, viewed from above. _c._ Upper view of _Willsia stellata_. _d._ _Slabberia conica._ _e._ Portion of the marginal canal of _Tiaropsis Pattersonii_. _f._ Polypite of _Bougainvillea dinema_. _g._ Part of its marginal canal. _h._ _Steenstrupia Owenii._ (_a_, _b_, and _d_ are about the natural size; the others are magnified.) ]