SCYPHOZOA 



175 



directions (Fig. 132). The water drawn in by the mouth passes first 

 into the gastric cavity and then the gastric pouches ; thence by the 

 adradial canals to the circular canal. It returns thence by the branched 

 interradial and perradial canals to exhalant grooves on the oral arms. 

 The whole circulation takes about twenty minutes, and it serves to 

 maintain a constant supply of food to all parts of the body. Food 



Fig. 131. Aurelia aurita. Somewhat reduced. From Shipley and MacBride. 

 M. mouth; oa. oral arm; tn. tentacles on the edge of the umbrella; p.cn. one 

 of the branching perradial canals ; there are four of these, and four similar 

 interradial canals ; the perradial canals correspond to the primary stomach 

 pouches of the hydratuba, the interradial to the pouches of the medusa; 

 a.cn. one of the unbranched adradial canals; c.cn. the circular canal; 

 tct. marginal lappets hiding tentaculocysts ; g.fil. gastral filaments, just outside 

 these are the genital organs. 



undergoes its preparatory digestion in the stomach : the half-digested 

 fragments are swept by the cilia on the round described above and 

 may be ingested by any of the endodermal cells of the canal system 

 and become available for local needs. The gastrovascular system thus 

 at once fulfils the functions of the digestive and circulatory systems 

 of higher animals. 



