First lesson in zoology : adapted for use in schools . egins at the outer and underside of the sieve-like madreporic body,passing directly forward and down-ward in a sinuous course to the edgeof the mouth. The sea-water in partenters the body-cavity through the As-sures in the madreporig body, whilemost of it enters the stone-canal, whichis a slender tube scarcely one fourththe diameter of the entire madreporicbody. The water entering the stone-canal (Fig. 44, b) passes directly intothe water-vascular ring (Fig. 44), andthen into the ten Polian vesicles andthe five radial canals, whence it is
First lesson in zoology : adapted for use in schools . egins at the outer and underside of the sieve-like madreporic body,passing directly forward and down-ward in a sinuous course to the edgeof the mouth. The sea-water in partenters the body-cavity through the As-sures in the madreporig body, whilemost of it enters the stone-canal, whichis a slender tube scarcely one fourththe diameter of the entire madreporicbody. The water entering the stone-canal (Fig. 44, b) passes directly intothe water-vascular ring (Fig. 44), andthen into the ten Polian vesicles andthe five radial canals, whence it is con-veyed to each water-sac or ampulla(Fig. 44, c; compare also Fig. 43).These pear-shaped water-sacs, whencontracted, are supposed to press thewater into the long slender suckersor ambulacral feet, which are dis-tended, elongated, and, by a sucker-like arrangement at the end, act inconjunction with the others to warp ^°- ^--Enerinus or pull the starfish along. Besides locomotion, the ambulacral feetserve for respiration and perception. 46 FIB8T LESSONS IN ZOOLOGY. It will thus be seen that the water-vascular system in the starfish isin its functions partly respiratory and partly locomotive, while it is inconnection with the vascular system, and thus partly aids in circulat-ing the blood and chyle. There is, besides, a complicated system oftrue blood-vessels, which are, however, difficult to discover. ^ By placing a living starfish in a tank of water one cansee how it moves by means of its suckers. When turnedupon its back, it can right itself in the following maimer:
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Keywords: ., bookcentury1800, bookdecade1890, booksubjectzoology, bookyear1894
