546 The Animal Kingdom 



that great flexibility is given to the entire body. Beneath the skeleton 

 is the large coelom containing the viscera. The coelom is lined with 

 ciliated epithelium and filled with a fluid containing many amoebocytes. 

 This fluid circulates into the dermal branchiae, providing a means for 

 respiratory exchange. The amoebocytes pick up waste materials and 

 also eliminate them through the dermal branchiae. 



A portion of the larval coelom develops into the water-vascular 

 system of the adult. The water enters through the madreporite on the 

 aboral surface and passes through the stone canal to the ring canal 

 which surrounds the mouth. From the ring canal, five radial canals 

 extend into the arms along the ambulacral grooves. From the radial 

 canal of each arm many lateral canals extend into the tube feet. One 

 canal goes to each tube foot where it ends in the ampulla. Projecting 

 inward from the ring canal are nine very small vesicles, Tiedemann's 

 bodies, which apparently produce the amoeboid cells of the water-vascular 

 system. 



LATERAL CANAL 



Fig. 179. — The water-vascular system of the starfish, diagrammatic. (From 

 Coe: Echinoderms of Connecticut, Connecticut Geological and Natural History 

 Survey.) 



Another part of the coelom gives rise to the much reduced circula- 

 tory system which consists only of small vessels around the mouth and 

 minute vessels radiating to the arms, canals going to the nerve ring, 

 a pentagonal sinus on the aboral disc, and five pairs of genital sinuses. 



