Thus regarded, the Polyps comprehend a great variety of animals, the bodies of which are generally soft or gelatinous substances. The principal and smaller divisions, to the number of more than two, are arranged round an imaginary axis, represented by the central part of the body. These divisions of the body have in their ensemble the appearance of a regular cylinder, of a truncated cone, or of a disk. They are invested with a skin or envelope of calcareous or siliceous corpuscles, and even a portion of the deepest-lying tissues may be invaded by a calcareous deposit, the mass of which belongs sometimes to an individual; sometimes it is common to many, constituting what Dr. Johnston calls the Polypidom, of which Professor Grant says, "there is but one life and one plan of development in the whole mass, and this depends, not on the Polypi, which are but secondary, and often deciduous parts, but on the general fleshy substance of the body;"[5] "the ramifications," says Dr. Johnston, "being disposed in a variety of elegant plant-like forms. The stem and branches are alike in texture: slender, horny, fistular, and almost always jointed at short and regular intervals, the joint being a mere break in the continuity of the sheath, without any character of a proper hinge, and formed by regular periodical interruptions in the growth of the polypidoms. Along the sides of these, or at their extremities, we find the denticles or cup-like cells of the polypi arranged in a determinate order, either sessile or elevated on a stalk." Near the base of each of these there is a partition or diaphragm, on which the body of the polyps rests, with a plain or tubulous perforation in the centre, through which the connection between the individual polyps and the common medullary pulp is retained. Besides the cells, there are found at certain seasons a larger sort of vesicle, readily distinguished from the others by their size, and the irregularity of their distribution, which are destined to contain and maturate the ovules. With these animals the digestive tube is very simple, and presents only one distinct orifice; the same opening serving at once for receiving the food and the expulsion of the residuum of digestion. This is one of Nature's economies, which it is not for us to dispute: we must record it without further remark. In nearly all the Polyps the sexes are separate; the generation is sometimes sexual; but these beings multiply also by what the zoologists call gemmation, or buds. They are provided with organs of the senses; nearly all of them have eyes--an immense progress in organization as compared with the animals which have hitherto engaged our attention. Their respiration is effected by the skin--another instance of the economy of Nature. The apparatus of their circulation is indistinct, but they have a nourishing fluid analogous to the blood in vertebrated animals. Vibratile cilia and stinging hairs often cover the entire surface of the Polyps.