SCYPHOZOA 



153 



canals to the circular canal. It returns thence by the branched inter- 

 radial 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 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 



Fig. 128. Aiirelia aurita. Somewhat reduced. From Shipley and MacBride. 

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

 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 alternate with these; a. en. one 

 of the unbranched adradial canals; e.en. the circular canal; tet. marginal 

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

 are the genital organs. 



available for local needs. The gastrovascular system thus at once 

 fulfils the functions of the digestive and circulatory systems of higher 

 animals. 



The neuromuscular system is further developed than in even the 

 medusoid individuals of the Hydrozoa. The muscles are ecto- 

 dermal, and each cell is almost entirely converted into contractile 



