. The Bible and science. we havea distinct advance on the Hydrozoa, inasmuch asthe aiiimals contained in it have a digestive canalshut off from the body-cavity. This separation pre-vents the sea-water and the oxygen it contains fromcirculating freely through the body, and-thus necessi-tates a separate provision for the supply of oxygenand removal of carbonic acid, or, in other words, forrespiration. This is effected in the annuloida by theso-called water-vascular system, a system whichreminds us of the vacuoles in the amoeba, althoughmuch more complicated — a set of canals commu-nicating with
. The Bible and science. we havea distinct advance on the Hydrozoa, inasmuch asthe aiiimals contained in it have a digestive canalshut off from the body-cavity. This separation pre-vents the sea-water and the oxygen it contains fromcirculating freely through the body, and-thus necessi-tates a separate provision for the supply of oxygenand removal of carbonic acid, or, in other words, forrespiration. This is effected in the annuloida by theso-called water-vascular system, a system whichreminds us of the vacuoles in the amoeba, althoughmuch more complicated — a set of canals commu-nicating with the exterior of the body, and inwhich fluid is kept circulating. In some members ofthis sub-kingdom, such as the sea-urchins and star- 144 WATER-VASCULAR SYSTEM. fishes, this water-vascular system is much developed,and sustains the progression of the animaL In theechinus, little rows of tentacles project from the sur-face of the body in several bands. These are connectedwith the water-vascular system, and can be filled. Fig. 60.—Pingrammatic representation of the water-vascular system of a star-fish,c, circular canal. « 75, Polian vesicles, to, madreporic phite. ?», stone \ radically arranged principal trunks (anibulacral canals), r, lateral suckers. «, their ampull*. Part only of their ambulacral canals and theirappendages are figured. with water or emptied at the pleasure of the animaLWhen full they project far enough to touch the ground,and by means of them the animals crawl along. Inthe star-fish the same sort of tentacles are found ANNULOIDA—ECHINODERMATA. 145 running along the surface of the arms. In some ofthe allies of the star-fish the arms, instead of beingshort and thick, are long and thin, but the animal.
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