to another, and arrested at all stages of development. Sometimes, although completely formed, they resign their freedom and end their existence in the colony where they were born.
In one group of Polyps the Medusæ associate themselves with the reproductive individuals to form a new unit—a small, distinct colony, that might be taken for a peculiar organ curiously analogous to a
Fig. 4.—Generative Buds or Gonophores of the Hydrozoa diagrammatically represented. a, simple gonophore; c, gonophore which has the structure of a Medusa (medusoid), but is not detached; d, free medusiform gonophore.
flower—with a separate chamber, and called the gonangium (Figs, 3 and 4), A step further and these strongly individualized Medusæ are seen descending to the rank of organs in more complex colonies.
All the colonies of Hydras are not fixed to submarine objects. Some of them lead a vagabond existence. They are often taken, not without reason, for simple animals analogous to the Medusæ, and called
Fig. 5.—Oceanic Hydrozoa, showing the specialization of Parts. 2, Siphonophore; n, Swimming-bells; p, alimentary region; t, tentacles; 3, diagram of the composite body of one of the Siphonifera; a a, swimming-bells; d, spines or defensive individuals; f, digester.
Siphonophores (Fig, 5), They sometimes attain a large size; and the variety and profusion of the parts which compose them, as well as the brilliancy of color and incomparable beauty of their forms, have made them subjects of the profound admiration of naturalists as well as sailors (Figs. 6 and 7). Each one of these parts is the equivalent of a
