406 PHYSIOLOGY AND MORPHOLOGY OK ANIMALS. 



the mouth below (cv, Fig. 285). From these rings go 

 vessels to the end of each arm, which distribute blood 

 to all the viscera. This blood is aerated on the spot by 

 the water in the perivisceral cavity. Moreover, a por- 

 tion of the blood is specially aerated by distribution on 

 the ambulacral vesicles. 



In addition to these two methods the highest echino- 

 derms, such as the Echinus, have tufted external gills 

 attached around the mouth. 



COELKNTERATA. 



Thus far we have a distinct blood system. The three 

 systems — food system, blood system, and respiratory 

 system — are well differentiated. But in the coelenterates 

 these three are not yet completely separated. The di- 

 gested food serves as blood, and the digestive system as 

 blood system. Neither is there any respiratory system 

 distinct from either, for the aeration of the tissues takes 

 place through the contact of water on the outside and 

 ciliary circulation of the food mixed with water on the 

 interior surface. 



Taking the polyp (actinia) as example, as already 

 shown (Fig. 232, page 344), food taken into the stomach, 

 s, is retained by pyloric contraction until digested, then 

 dropped into the general cavity, and mixed with fresh 

 sea water. The mixture is stirred about by ciliary cur- 

 rents and directly aerates the tissues. Also the tissues 

 are similarly aerated by contact with sea water on the 

 whole exterior surface. 



PROTOZOA. 



Finally, in the protozoa there is no circulation of any 

 kind, nor is there any interior aeration, but the living 

 protoplasm is directly aerated only by contact of water 

 with the external surface. 



