2o6 ECHINODERMATA. 



body, and is like a complex calcareous filter. It is called 

 the stone-canal. 



The stone-canal leads into a water-ring round about the 

 mouth. From this circumoral ring are given off nine trans- 

 parent vesicles, and five radial tubes, one for each of the 

 arms. Considerations of symmetry suggest that there should 

 be ten transparent vesicles, but the stone-canal has taken 

 the place of one. 



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 oiff 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, but into the ampuUse. 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. 



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 bases 

 of the tube-foot communicate with the ampullae. 



This water-vascular system is in many ways diificult to 

 understand, but the questions involved are hardly within the 

 province of an elementary manual. It develops from one 

 of the body-cavity pouches (see p. 221), but to what in other 

 animals does it correspond? May it be an excretory 

 system which has taken on other functions, and has it still 

 some excretory significance ? It is certainly for the most 

 part .locomotor in Starfishes, but may it not help also in 

 respiration, which seems to be its function in Brittle-Stars 

 and Crinoids ? 



Vascular System. — By the side of the stone-canal, enclosed 

 in a space which is probably part of the body-cavity, lie 

 numerous blood-vessels. They form an imperfectly under- 

 stood structure called the "heart" or "plexiform organ," 

 and are connected dorsally with a vascular ring round the 

 anus, and ventrally with another round the mouth. From 



