Page:Popular Science Monthly Volume 8.djvu/34

24 around the mouth is a series or several series of tubular offsets, ranged radially about the stem. The shape of these blossom-like zoöids varies in the different species. In some varieties they are unprotected, while in others the tentacles may be withdrawn into a horny, cup-shaped sheath. The number of tentacles varies with the different species. The plates of Tubularia indivisa and Hydra vulgaris show the tentacles expanded. The other plates give, in the magnified portions, only the chitinous sheath, into which the polyp has withdrawn itself.

In the Plumularians, a branch of the Sertularian group, curious little cups of the horny sheath are developed. Unlike the cups which contain the living flower, these extensions are filled with the sarcode, or soft, gelatinous flesh of the animal. This sarcode, or protoplasmic flesh, acts like the flesh of the rhizopods and amœbæ; long filamentary processes are extended, just as the rhizopods improvise legs or arms when they need them, till sometimes the horny sheath is invested in this living gossamer. The function of these cups is not known. Allman considers them as special zoöids, whose morphological differentiation from the other zoöids is carried to an extreme. Hincks believes them to be a lower form of life, in organic union with the higher zoöids of the hydroid colony.

The horny sheath, which is described by earlier writers as an excretion, is by Allman considered to be rather the result of metamorphosis of tissue. In many varieties the stem and branches of the creature are slender, horny, and pipe-shaped, and the chitinous sheath is jointed at regular intervals, the joint being a mere break in the continuity of the chitine, not a movable hinge; while the living pulp within forms a continuous body, and is invested by its sheath as the pith of a plant is invested by its stalk.

The generative buds are cæcal offshoots from the body, the reproductive elements always developing between the inner and outer membrane (see Fig. 2, d). They sometimes, after development, free themselves from the parent stem, and lead a roving life as medusæ. In some cases the nutritive bud has its alimentary function suppressed, and, though not itself sexual, it is henceforth destined to produce sexual buds, either directly or through the medium of a non-sexual bud.

There is, it may almost be said, no differentiation of organs among the hydroids. In the adult form they possess no organs of sense, and have no circulatory, respiratory, nor nervous systems. All the functions of life are performed without the intervention of special organs. Voluntary motion takes place without muscles, sensibility is present without nerves, respiration is performed without lungs, and digestion goes on without a true stomach. The sea-water which flows within and about the creature bears to it the oxygen necessary to the maintenance of vital combustion, as well as the small living creatures and comminuted organic matter which form its food. Like the