ORINOIDa. 



57 



dial vessels (®) ending in the water-sacs (am) and ambulaoral feet. The 

 stone-canal begins at the outer and under side of the sieve-like niadre- 



poric body, passing directly forward 

 and downward in a sinuous course 

 to the underside of the circumoral 

 plates. The madreporic body (mJ) 

 is externally seen to be perforated 

 by linear apertures radiating and 

 subdividing toward the peripliery. 

 The sea-water in part enters tlie 

 body-cavity through the fissures in 

 the madreporic body, while most of 

 it enters the stone-canal, which is a 

 slender tube scarcely one fourth tlie 

 diameter of the entire madreporic 

 body. The water entering the stone- 

 canal (Fig. 60, i) passes directly into 

 the water- vascular ring (Fig. 60) and 

 then into the ten Polian vesicles and 

 the five radial canals, whence it is 

 conveyed to each water-sac or am- 

 pulla (Fig. 61, c; compare also Fig. 

 60). These pear-shaped water-sacs, 

 when contracted, are supposed to 

 press the water into tlie long slender 

 suckers or ambulacral feet, which are 

 distended, elongated, and by a suck- 

 er-like arrangement at the end of the 

 prehensile foot act in conjunction 

 with the otliers to warp or pull the 

 star-fish along. Besides locomotion, the ambulacral feet serve for 

 respiration and perception. The star-fish has the sense of smell. 



It will thus be seen that the water-vascular system in the star-fish 

 is in its functions partly respiratory and partly locomotive, while it 

 is in connection with the vascular system, and thus partly aids in 

 circulating the blood and chyle. There is, besides, a complicated 

 system of true blood-vessels, which are, however, ditBcuU to dis- 

 cover. 



Class I.— Crikoidea {Stone-UUea, E7icrinites, etc.). 



General Characters of Crinoids. — These are stalked star- 

 fishes, the stalk or stem being jointed. Most Crinoids are 

 extinct or fossil; such is the Stone-lily (Fig. 02). 



Fig. 62.-Encrinus or Stone-lily. 



