An inspection of that Case and the accompanying woodcuts will make clear the distinctive characters of the Echinoderma. Unlike that of a Crayfish or a Mussel, the body does not appear to be divided into two equal or symmetrical halves, though it really is; this is due to the possession of a number of rays, of which there are ordinarily five. The skin is strengthened by the deposition in it of carbonate of lime, which may be in the form of continuous plates or bars, or of separate scattered spicules. A series of tube-feet or suckers (podia) are generally developed along each ray, and these are supplied by a system of water-vessels peculiar to Starfish and their allies. These rays are often called “_ambulacra_.” [Illustration: Fig. 1. A. Anchor and plate of _Synapta_. B, C. Tables of _Holothuria impatiens_; and D. _Holothuria atra_: from various aspects. E. Spicule from sucker of _Stichopus variegatus_, magnified about 200 times. ] [Illustration: Fig. 2. Diagram of Water-vessels. _c.c._ Circular canal, with _p.v._, its Polian vesicles; from it a radial canal (_v.c._) is given off along the lower surface of each arm; this supplies, by side branches, the suckers, _s_; connected with each sucker is a contractile swelling or ampulla (_a_). The circular canal is in connection with the exterior by _s.c._, the stone-canal, and opens to it by the madreporite (_m_). ] [Illustration: Fig. 3. Figure of a Starfish (_Asterias rubens_). In the ray marked I. the skin has been removed from the upper surface, and the ambulacral ossicles (_ao_) and the podia (_s_) are seen _in situ_; the blind outgrowths (_c_) from the central stomach (_sp_) have been dissected out. In II. the gonads (_g_) are exposed; and in the centre above the stomach the rectal glands (_rg_) are to be seen. The anus (_a_) is seen to be subcentral in position. ] In the body of the Starfish (Fig. 3) the arms are seen to be continuous with the disk and to contain portions or prolongations of the chief organs. The middle of the arm is occupied by two rows of hard pieces (ambulacral ossicles), the fellows of which make an open angle with each other, and so form an open ambulacral groove; along this we find the suckers, the water-canal that supplies them, the blood-vessel of the arm, and a nerve-cord. At the centre of the disk is the mouth. The ossicles at the sides of the arms bear spines, which vary in different species; the surface of the back is supported by a network of hard pieces, and through the intervening spaces there project membranous pouches, which are respiratory in function. The modified plate on the upper surface opens into a tube by means of which the water-vessels communicate with the exterior; this plate is known as the madreporite (Fig. 2, _m_).