ANNULOIDA ECHItfODERMATA. 



convey tlie blood back to the heart. The details of the 

 circulation in the star-fishes are similar in all essential 

 features. 



125. Respiration. There are no distinct organs of re- 

 spiration. The aeration of the blood is believed to be 

 accomplished partly by the sea water with which the peri- 

 visceral cavity is filled, and partly by the water- vascular 

 system. 



Fio. 38. LARVA OF SEA-URCHIN (Enlarged 160 times). 



a, mouth ; b, stomach ; c d, intestine ; e, calcareous plate ; /, ciliated organs, 

 assisting in swimming and respiration perhaps the future ambulacra! feet ; 

 09'9!.'9'"! calcareous stalks, serving as a frame-work for the body of the animal. 



126. Nervous System. The gullet is surrounded by a 

 nervous cord, which connects a series of ganglia. From 

 this, filaments proceed along the ambulacral areas. 



127. Development. The sexes are always distinct. 



