HYDRuE, JELLYFISIIES, SEA ANEMONES, CORAL POLYPS 53 



floating on the surface of the sea (Fig. 19). In the center 

 of the untler surface of the body is the mouth. Above 

 the moutli is the stomach, 

 and higher up, from tlie end 

 of the digestive cavity, canals 

 racUate Hke the ribs of an 

 umbrella. These canals extend 

 downward along the insiele of 

 the dome antl finally com- 

 municate with a circular canal 

 around the edge of the disk. 

 This system of tubes constitutes the water vascular 

 system. 



Aurelia has an interesting and complex hfe histoiy. The 

 adult produces eggs, each of which hatches into a cihated 



Fig. 20. — Embryo of jellyfish 

 (Aurelia). 



Pig. 21 . — Hydralike stage of Aurelia. After Agassiz. 



embryo (Fig. 20). The embryo soon attaches itself to a 

 rock and transforms to a hydralike animal about one half 



