40 
ZOOLOOt, 
dial vessels ending in the water-sacs {am) and ambiilacral feet. The 
stone-canal begins at the outer and under side of the sieve-like madre- 
poric body, passing directly forward 
and downward in a sinuous course 
to the underside of the circumoral 
plates. The madreporic body {rnh) 
is externally seen to be perforated 
by linear apertures radiating and 
subdividing toward the periphery. 
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 scr.rcely 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, and thus partly 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. — Ckikoidea {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). 
