OF RESPIRATION. 125 



i.he hard shell, whilst in polyps they perforate the walls of 

 the ge.ieral cavity of the body, which they constantly fili 

 with water. 



of Vertebrates. In those Radiates which are provided with distinct 

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

 typical structure, their gills forming bunches of fringes around the 

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

 ttib body. 



11* 



