ECHINODERMATA. 209 



there are thin outpocketings of the body-wall, papulae or 

 branchiae (Fig. 97, d.b.), the cavity in which is continuous 

 with the body-cavity. The body fluids may thus be aerated 

 from the water outside. In some forms water is drawn into 

 special branching pockets (respiratory tree} in the wall of the 

 rectum, and later is forced out again. 



243. Circulation. The circulatory vessels are merely 

 partly differentiated portions of the ccelom or body cavity. 

 Our knowledge is by no means complete but it seems that in 

 none of the groups is there a complete separation of the blood 

 spaces from the ccelom. There are probably no contractile 

 hearts. The walls of the blood spaces may bear cilia, which 

 assist in securing the motion of the fluid. The blood contains 

 migratory cells, usually colorless, and is identical with the fluid 

 in the body cavity. The general body contractions are im- 

 portant in causing motion of the. fluids. It should be remem- 

 bered that the water vascular system is also partly circulatory 

 in function. The blood vessels of the various classes agree 

 in having a central circular portion consisting of one or more 

 rings, with radial tubes running into the arms, and in some 

 instances vessels which accompany the intestine. The vessels 

 of the oral surface are, throughout, in close connection with 

 the nervous epithelium (Fig. 96, r.b.}. 



244. 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 sur- 

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

 The skeletal ossicles in the mesoderm represent, in part at 

 least, the elimination of certain inorganic salts which can not 

 be used in the vital activities, and are therefore excretions. 



'245. 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 

 much significance. In the holothurians, on the contrary, the 

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