Page:A Text-book of Animal Physiology.djvu/53

 be fitly undertaken at this stage. The Polyps are easily obtainable from ponds in which they are found attached to various kinds of weeds. To the naked eye, they resemble translucent masses of jelly with a greenish or reddish tinge. They range in size from one quarter to one half an inch; are of an elongated cylindrical form; provided at the oral extremity with thread-like tentacles of considerable length, which are slowly moved about in all directions; but they and the entire body may shorten rapidly into a globular mass. They are usually attached at the opposite (aboral) pole to some object, but may float free, or slowly crawl from place to place. It may be observed, under the microscope, that the tentacles now and then embrace some living object, convey it toward an opening (mouth) near their base, from which, from time to time, refuse material is cast out. It may be noticed, too, that a living object within the touch of these tentacles soon loses the power to struggle, which is owing to the peculiar cells (nettle-cells, urticating capsules, nemato-cysts) with which they are abundantly provided, and which secrete a poisonous fluid that paralyzes prey.

The mouth leads into a simple cavity (cœlom) in which digestion proceeds. The green color in Hydra viridis, and the red color of Hydra fusca, is owing to the presence of chlorophyl, the function of which is not known. Hydra is structurally a sac, made up of two layers of cells, an outer (ectoderm) and an inner (endoderm); the tentacles being repetitions of the structure of the main body of the animal, and so hollow and composed of two cell layers. Speaking generally, the outer layer is devoted to obtaining information of the surroundings; the inner to the work of preparing nutriment, and probably, also, discharging waste matters, in which latter assistance is also received from the outer layer. As digestion takes place largely within the cells themselves, or is intracellular, we are reminded of Vorticella and still more of Amœba. There is in Hydra a general advance in development, but not very much individual cell specialization. That of the urticating capsules is one of the best examples of such specialization in this creature. A Polyp is like a colony of Amœbæ in which some division of labor (function) has taken place; a sort of biological state in which every individual is nearly equal to his neighbor, but somewhat more advanced than those neighbors not members of the organization.

But in one respect the Polyps show an enormous advance. Ordinarily when nourishment is abundant hydra multiplies by