HYDRA AND OBELI A. POLYPS AND MEDUSAL 177 



spaces which form the gullet, stomach, and canals. The 

 whole outside of the body and tentacles is covered with 

 ectoderm. Between the ectoderm and the endoderm is a 



Fig. hi. — A medusa of Obelia, magnified. 



layer of jelly, which is very thick, especially on the exum- 

 brella side. The medusa may be compared to a polyp 

 which is greatly widened and shortened, the walls of the 

 wide, flat enteron coming together in places, as we have 



FlG. 112. — A diagram to illustrate the relation between polyp 

 and medusa. 



A, The polyp ; B, an imaginary intermediate form ; C, the medusa. 



can.c, Circular canal ; can.r., radial canal ; ect., ectoderm ; end., endoderm ; ent., 

 enteron ; m. t ,mouth ; mb., manubrium ; or.c, oral cone ; ten., tentacle ; vm., 

 velum. The dotted line represents the velum as it is found in many medusae 

 but not in Obelia.. 



seen, and the structureless lamella increasing in thickness 

 to form the jelly. The manubrium represents the oral cone 

 and the tentacles stand around it at a greater distance 

 owing to the widening of the body. The arrangement of 



