524 



ZOOLOGY. 



its direction periodically, so that in the space of some minutes 

 the same canal performs the function of an artery and a vein. 



Fig. 551. Biphore (Biphora.)* 



In this class are arranged the biphores (Fig. 551), the pyro- 

 soma, and the ascidise (Fig. 180), distinguished into simple 



* 5, mouth ; a, anus ; m, muscular bands surrounding 'the great pharyn- 

 geal or respiratory cavity; br, gills ; e t visceral mass, including the stomach, 

 fiver, &c.; c, the heart. 



