. Elements of Comparative Anatomy. WATEE-VESSELS OF ECHINODEEMA, 223 these parts have a different relation to that which they have in the rest of the Echinoderma. The walls of the stone-canal, which hangs freely into the coelom, are more or less calcified; when they •are more so they form a firm capsnle. The porous parts of the canal are usually distinguished by calcification, and so repeat within the body the arrangements of a madreporic plate. The ends of each branch carry a porous piece when the stone-canal T ^ a ^ ^ T is broken up into branches; this repetition of parts leads to the format
. Elements of Comparative Anatomy. WATEE-VESSELS OF ECHINODEEMA, 223 these parts have a different relation to that which they have in the rest of the Echinoderma. The walls of the stone-canal, which hangs freely into the coelom, are more or less calcified; when they •are more so they form a firm capsnle. The porous parts of the canal are usually distinguished by calcification, and so repeat within the body the arrangements of a madreporic plate. The ends of each branch carry a porous piece when the stone-canal T ^ a ^ ^ T is broken up into branches; this repetition of parts leads to the formation of race- mose structures, which are only functionally similar to a number of madreporic plates grouped around the stone-canal. The stone- canals vary in number, as well as in arrangement. Often only one is present; in other cases, and notably iu the SynaptEe, there are several arranged around the circular canal. The number too of the Polian vesicles (Fig. 113,p),which are present in these forms, varies; in Holothuria and Molpadia there is one, in Synapta Beselii about fifty, and in Cladolabes about a hundred. The canals from the circular canal (0) run for- wards inside the calca- reous ring (i?.), and give off branches to the oral tenta- cles (T); a c^cal elongated tube, corresponding to the ampullae of the suckers, is connected with each of them. These tubes are of some size in the Holothurida, and lie on the outer side of the calcareous ring; they are only feebly developed in the Synaptidee. The radial ti-unks going to the ambulacra are placed, in Holothm-ia, in the bundles of longitudinal muscles, which are thus divided into two halves. In Cucumaria they are placed on the outer side of these muscles. The branches of these canals, as in other forms, go to the feet. When the feet are atrophied the vascular branches, which go to them, are atrophied also; iDut the principal trunks appear to persist even in the Apodia, for they have been observed in Synapta, although, indee
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