PHYLUM ECHINODERMATA 



193 



perimsceral part of the ccelom (Fig. 133, IQ) surrounds the ali- 

 mentary canal and extends into the arms. It is lined with 

 peritoneum (Fig. 133, 4) and filled with sea- water containing 

 some albuminous matter. Oxygen is taken into the coelomic 

 fluid and carbon dioxide given off through outpushings of the 

 body-wall known as papula or^dermal branchia (Fig. 133, 5). 

 The coelom also has an ex- 

 cretory function, since cells 

 from the peritoneum are 

 budded off into the coelomic 

 fluid, where they move about 

 as amcebocytes gathering 

 waste matters. These cells 

 make their way into the der- 

 mal branchiae, through the 

 walls of which they pass to 

 the outside, where they dis- 

 integrate. 



The Water-vascular Sys- 

 tem. The water- vascular 

 system (Fig. 134) is a divi- 

 sion of the ccelom peculiar to 

 echinoderms. Beginning with 

 the madreporite (m) the fol- FlG I34 . _ WatCT -vascular system of 



lowing Structures are enCOUn- a starfish. a, ampullae ; ap, Polian 



tered: the stone-canal (') j^^ttc^ M 



running downwards enters r, radial canals; r', branches to am- 

 4.1 7 / \ i i pullae. (From Parker and Haswell, after 



the ring-canal (c), which Qegenbaur.) 

 encircles the mouth ; from 



this canal five radial canals (Fig. 134, r; Fig. 133, jj), one in 

 each arm, pass outward just above the ambulacral grooves. The 

 radial canals give off side branches from which arise the tube-feet 

 (Fig. 134, /; Fig. 133, if) and ampulla (Fig. 134, a; Fig. 133, 16.) 

 The ampullae are bulb-like sacs extending into the ccelom; they 

 are connected directly with the tube-feet, which pass through tiny 



