ECHINODERMATA 



133 



one into each arm; the radial canals give off side branches from which 

 come connecting canals to the tube feet and ampullae. The tube feet 

 furnish a means of adhering to smooth surfaces when a vacuum has 

 been created by the withdrawing of water into the ampullae. 

 Squeezing the ampullae causes the water to distend the tube feet 

 and they protrude through the pores. 



Madreporite 

 Stone Canol 



Tiedemonn Body 

 Circular Cona/ —■ 



/Radial Canol - 



Ampulla 



Connecting Cono/ 



Tube Foot 



Fig. 57. Water-vascular system of the starfish. (Drawn by W. J. Moore.) 



Respiration. — The dermal branchiae are thin-walled finger-like 

 sacs that protrude through little holes in the wall of the animal, 

 called dermal pores. Dermal branchiae, as the name indicates, are 

 for respiration. 



Circulation. — The presence of a well-developed digestive system 

 and of a quantity of coelomic fluid renders the blood vascular system 

 of less importance. The coelomic fluid contains a number of 

 ameboid corpuscles that collect wastes and pass them to the exterior 

 by passing along the walls of the dermal branchiae. It is also 

 worthy of note that the minute Tiedemann's vesicles on the circular 



