WATER-VASCULAR SYSTEM. 



called on account of the calcareous deposits in its walls, either 



hangs within the body cavity, 



whence it takes up fluid through 



the pores in its walls (HolotJtn- 



rians), or ends in a porous calca- 

 reous plate, the maclreporic plate, 



which is inserted in the external 



covering of the body, and through 



the pores of which the sea water 



percolates into the lumen of the 



canal system. The position of 



the madreporic plate varies con- 

 siderably. In the Clypeastridea 



it is at the apical pole ; in the 



Cidaridea and Spatangidea it is 



interradial, and falls in the an- 

 terior right interraclius near the 



apex ; in the Astericlea it is also 



interradial and dorsal ; in the 



Euryalidce and the Ophiuridce it 



lies on one of the five buccal 



plates. Some Echinoderms, e.g., 



species of Ophidiaster and Echi- 



naster echinites, possess several 



stone canals and madreporic 



plates. 



On the lateral branches of the five or more radial trunks are found 



the appendages known as the 

 ambulacral feet (fig. 216). 

 These are extensible tubes or 

 sacs, which pass through pores 

 and openings in the dermal 

 skeleton and project on the 

 surface of the body. They 

 are capable of being swollen 

 out, and are frequently pro- 



Fit?. 215. Diagramatic representation of 

 the water-vascular sj stem of a. Star-fish. 

 Me, Circular vessel ; Ap, ampulloe or Polian 

 vesicles ; Sic, stone canal ; A% madreporic 

 plate ; P, ambulacral feet connected with 

 the side twigs of the radial canals ; Ap', 

 the ampullae of the same. 



FIG. 216. Diagrammatic section through one of 

 the arms of Axteracanthion (after VV. Lunge). 

 JV, Nervous system; P, ambulacral fjet ;A, 

 calcareous portions of integument ; T, dermal 

 branchia. 



vided with a sucking disc at 

 their free extremity. Con- 

 tractile ampullae are placed at 

 the point of junction of the 



Lube feet with the side branch of the radial vessel 



they force the 

 18 



