ECHINODEEMATA 



217 



250. Excretion. — It is impossible to name any organs known 

 to be solely excretory in function. As in respiration many 

 organs may take part in the work. The gaseous and soluble 

 excreta are eliminated through the general body surface, the 

 papulae, the respiratory tree, or the ambulacral organs. The 

 skeletal ossicles in the mesoderm represent, in part at least, 



Fig. zoo. Diagram of a portion of the water- vascular (ambulacral) system of the Starfish, a 

 ampullse;/, ambulacral feet; m, madreporic body; p, Folian vesicles; r.c, ring canal, with the upper 

 portion removed at the right of the figure; r,t., radial water tubes (in r.t/ the upper portion is re- 

 moved at the distal end and the proximal portion is represented entire) ; 5, stone canal. 



Questions on the figure. — Where does the water enter this system of vessels? 

 At what points in the system is it of use? By comparing all illustrations at your 

 disposal, describe the mode of using this system of organs for locomotion. How 

 may it be used in respiration? 



the elimination of certain inorganic salts which can not be used 

 in the vital activities and are therefore excretions. 



251. The Muscular System. — The degree of the develop- 

 ment of the muscular system varies. In those forms which 

 have a well-developed skeleton the body muscles are not of 



