COELENTERATA 



151 



radiating from the gastric cavity. The oral cone becomes the manu- 

 brium ; the rim which bears the original tentacles of the polyp is now 

 separated widely from the mouth by differential growth and drawn 

 downwards in the formation of the bell. Very often a secondary set 

 of oral tentacles are developed on the manubrium. The radial sym- 

 metry of the polyp is more strongly emphasized in the medusa by the 



can.c 



—ten. 



Fig. III. Diagram to illustrate the relation between polyp and medusa. 

 A, Polyp. B, An imaginary intermediate form. C, Medusa. Ectoderm black, 

 endoderm cross-hatched, mesogloea stippled, can.c. circular canal ; can.r. radial 

 canal; end.l. endoderm lamella; ent. enteron; M. mouth; mb. manubrium; 

 or.c. oral cone ; ten. tentacle ; vm. velum. The velum, present in many medusae, 

 is absent in Obelia. 



radial development of the canal system. The muscular system of the 

 bell is greatly developed by the substitution of a type of cell in which 

 the muscular processes form a long striated fibre while the epithelial 

 part is greatly reduced; such a cell is capable of rapid rhythmical 

 contraction. The nervous system may be partially concentrated to 

 form a nerve ring and well-defined sense organs occur in connection 

 with this. In this phylum, the lowest of the Metazoa, the gametes are 



