The reproduction of many of these arborescent or compound Hydrozoa is one of the most unexpected and extraordinary phenomena in the life-history of the animal creation. For besides the system of consecutive budding from a single polype which builds up the compound animal, peculiar buds are formed and developed, which bear no resemblance whatever to the polype buds: on the contrary, when mature, they assume an organization exactly the same as that of the common jelly-fish or Medusoid Acalephæ, and swim freely away from their fixed parent as soon as they are detached. These medusiform zooids, which are extremely small, consist of a cup or umbrella-shaped bell of colourless transparent matter, which is their swimming apparatus; it is contracted and expanded by a muscular band under the rim, the water is alternately imbibed and forcibly ejected, and by its reaction the zooid is impelled in a contrary direction. From the centre of the bell a stomach hangs down in the form of a proboscis, with a mouth at its extremity, either with or without tentacles and sting-cells. Four canals, or a greater number, which begin in the stomach, radiate through the transparent matter of the bell, and are united by a circular canal round the rim; they convey the nutritious liquid from the stomach throughout the system. This general structure may be traced in the zooids of the three great families of the oceanic hydraform-zoophytes, in a greater or less degree, from deciduous perfect medusæ to such as are imperfect and fixed.
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.