WATER VASCULAR SYSTEM. 



233 



The stone canal leads into a water ring round about the mouth. From 

 this circumoral ring are given off nine glandular bodies (Tiedemann's 

 bodies), and five radial tubes, one for each of the arms. Considerations 

 of symmetry suggest that there should be ten glandular bodies, but the 

 stone canal has taken the place of one. In many starfishes there are five 

 or ten little reservoirs (Polian vesicles) opening into the circumoral ring, 

 but in Asterias rtibens these are hardly distinguishable from the first 

 ampullae of the radial vessels. 



Along each arm, then, there runs a radial vessel. It lies in the 

 ambulacral groove beneath the shelter of the rafter-like ossicles. From 

 it branches are given off to the bases of the tube feet, but from each of 

 these bases a canal ascends between each pair of ambulacral ossicles, and 

 expands into an ampulla or reservoir on the dorsal or more internal side. 

 The fluid in the system may pass from the radial vessels into the tube 

 feet, and from the tube feet it can flow back, not into the radial vessel, 



FIG. 76. Diagrammatic cross section of starfish arm. 

 (After LUDWIG.) 



n., radial nerve ; b.v., radial blood vessel according to Ludwig, sep- 

 tum in blood vessel according to others iv.v., radial water vessel ; 

 am., ampulla ; tf., tube foot ; p.c., a pyloric caecum cut across ; s.p., 

 a calcareous spine ; g. , a skin gill ; lac., spaces in the skin ; go.^ ova 

 in ovary; a.o., ambulacral ossicle. 



but into the ampullae. There are muscles on the walls of the tube feet, 

 ampullae, and vessels. At the end of each arm, there is a long unpaired 

 tube foot, which seems to act as a tactile tentacle, and has also olfactory 

 significance. 



To recapitulate, the madreporic plate leads into the stone canal, this 

 passes into the ring round the mouth with its nine vesicles, from the ring 

 radial vessels run along the arms, they give off branches to the tube feet, 

 and the base of each tube foot communicates with an ampulla. 



