The Corynidæ form the first of the three families of the oceanic hydra zoophytes. They comprise six genera, and many species of compound animals of various forms, each derived from a single animal by budding; and although they possess a thin flexible coat, the polypes are sheathed either in a thin membrane or bone. Their club-shaped tentacles form either a single or double circlet round the base of their conical mouth, and are also scattered over their bodies when bare. [Illustration: Fig. 111. Syncoryna Sarsii with Medusa-buds.] The zooids are developed at once in the Syncoryna Sarsii, which is a long, thinly branched, and horny zoophyte, with a single naked, spindle-shaped polype at the extremity of each branch, as in fig. 111, A. The bodies of the polypes are studded with numerous tentacles, among which buds appear (fig. 111, _a_, _b_); these gradually expand into bell-shaped medusa-zooids (fig. 111, _c_), some being masculine and others feminine. They drop off their parent, swim away by the contraction of their bell, and their fertilized eggs are developed into single hydræ, which become arborescent like their parent by budding. The family of the Sertulariidæ take branching forms, sometimes of perfect symmetry: they have a firm, horny coat, which not only covers the stem and branches, but becomes a cup for the protection of the polype. The most common form of the family of the Tubularia has no branches: it has an erect, hollow stem like a straw, sometimes a foot high, coated by a horny sheath. The polype which terminates each plant has a mouth surrounded by alternately long and short tentacles. The stomach of the polype is connected with the hollow in the stem by a muscular ring, by whose alternate dilatation and contraction, at intervals of eighty seconds, the fluid is forced up from below, enters the stomach, and is again expelled. Another liquid carrying solid particles circulates in a spiral through the whole length of the stem. Some of this family are propagated by perfect deciduous medusæ, others by imperfect fixed ones; both are developed on the polypes or among their tentacles. Like the fresh-water Hydræ, these creatures can restore any part of their bodies that is injured. Numerous instances might be given to show that the minute medusiform zooids are only a stage or phase in the life of an oceanic hydra: conversely it will now be shown, that the single simple hydra is but a stage in the life-history of the highly organized medusa, jelly-fish, or sea-nettle of sailors, the Acalepha of Cuvier.