130 



THE INVERTEBRATA 



polyp in the great widening of the body, especially along the oral 

 surface, and the restriction of the enteron by the increase in thickness 

 of the structureless lamella on the aboral side of the endoderm, so that 

 while a central gastric cavity remains, the two endodermal surfaces 

 have come together peripherally to form a solid two-layered endoderm 

 lamella except along certain lines, where the canal system is developed 

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



can.c. 



Fig. 109. 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; mh. manubrium; 

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

 is absent in Ohelia. 



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. Ver^' 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 

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



