These medusiform zooids are male and female, and when detached from their parent they are independent creatures, each of them being furnished with nutrient and locomotive organs of its own. They produce fertilized eggs, which are developed into ciliated locomotive larvæ; after a time these lose their cilia and acquire a rayed sucking disc, with which they fix themselves permanently to a solid object, and, after various changes, each gets a mouth and tentacles and becomes a perfect young hydra. Thus a brood of young hydræ is produced, each of which acquires the compound form of its parent by budding, and as each of these compound animals in its turn gives off medusa-buds, there is a cycle of the alternate forms of hydra and medusa or jelly-fish, showing a singular connection between two animals which seem to have nothing in common. The analogy which so often prevails between plants and animals obtains here also, for the medusa-buds bear the same relation to the hydra or polype-buds that the flower-buds of a tree do to the leaf-buds: the flower-buds contain the germs of future generations of the tree, while the leaf-buds contain only the undeveloped stems, stalks, and leaves of the individual plant on which they grow.
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.
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.
The medusæ vary in size, from microscopic specks that swim on the surface of the sea in a warm summer day to large umbrella-shaped jelly fish almost a yard in diameter. They abound in every part of the ocean and in all seas, often in such shoals that the surface of the water is like a sheet of jelly. Their substance is transparent, pure, and nearly colourless; chiefly consisting of water, with so little solid matter, that a newly caught medusa, weighing two pounds, dries into a film scarcely weighing thirty grains.
The Pulmograde Medusæ, which swim by the contractions of their umbrella-shaped respiratory disc, form two distinct groups, the naked-eyed medusæ and the covered-eyed group. Both are male and female; each has its own form of thread-cells; and the stinging power or strength of the poison is nearly in proportion to the size of the animal and the coarseness of its threads.
The disk, or umbrella-shaped swimming organ, in both groups consists of a large cavity included between two layers of gelatinous matter, which unite at the rim. The interior membrane, called the sub-umbrella, is encircled at its edge by a ring of highly contractile muscular fibre like the iris of our eyes, by which this swimming organ is expanded and contracted. From the centre of the sub-umbrella a stomach, in the form of a proboscis, is suspended, which is of a very different structure in the two groups.