40 



ZOOLOGY. 



dial vessels (v) ending in the water-sacs (am) and ambulacral 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 (jnb) 

 is externally seen to be perforated 

 by linear apertures radiating and 

 subdividing toward the periphery. 

 The sea-water in part enters the 

 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 the 

 diameter of the entire madreporic 

 body. The water entering the stone- 

 canal (Fig. 37, t) passes directly into 

 the water-vascular ring (Fig. 37) and 

 then into the ten Polian vesicles and 

 the five radial canals, whence it is 

 conveyed to each water-sac or am- 

 pulla (Fig. 38, c; compare also Fig. 

 37). These pear-shaped water-sacs, 

 when contracted, are supposed to 

 press the water into the 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 others 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, :\nd thus parti}' aids in 

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

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

 cover. 



Fig. 39— Encrinus or Stone lily. 



Class I. — Crinoidea (Stone-lilies, Encrinites, 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. 39). 



