436 



GENERAL ZOOLOGY 



madreporite, through which water enters a stone canal, leading to a 



ring canal encircling the mouth 

 (Fig. 256). From the ring 

 canal, five radial canals extend 

 into the arms, and from each of 

 these are given off at right 

 angles paired ambulacr at canals. 

 Each of these, in turn, joins a 

 tubular muscular sac, the 

 ambulacrum, expanded at one 

 end into an ampulla, and ter- 

 minating in a sucker disk at its 

 longer free end, which can be 

 extended or retracted through 

 system of the ambulacral groove on the 



the starfish, diagrammatic. A, ampulla; oral Surface of each arm. The 

 AM, ambulacra; C, ring canal; M. , _ • £ii„j „*j.u „ a *j 



madreporite; P, Polian bodies; R, s y stem 1S filled Wlth a flmd > 



radial canal; S, stone canal; T, Tiede- mostly water, but containing 



mann bodies. lymph an( j Uood corpuscles 



supplied by glands (Polian vesicles, Tiedemann bodies) attached 

 to the system at various points. The ambulacra are used in loco- 



Fig. 257. — Asterias feeding on a clam. The valves of the clam are pulled 

 apart by the ambulacra and the stomach of the starfish is then everted about the 

 soft parts of the clam. The stomach juices of the starfish may aid in causing 

 the valves of the clams to open. 



motion by extending and attaching the sucking disks to the sub- 

 stratum, a muscular contraction of the appendage then pulling 



