OF RESPIRATION. 125 



the hard shell, whilst in polyps they perforate the walls of 

 the general cavity of the body, which they constantly fill 

 with water. 



of Vertebrates. In those Radiates which are provided with distinct 

 respiratory organs, such as the Echinoderms, we find still another 

 typical structure, their gills forming bunches of fringes around the 

 mouth, or rows of minute vesicles along the radiating segments of 

 the body. 



11* 



