1 20 CCELENTERA TA. 



and endoderm, but the mesoglcea is much thicker and forms the bulk 

 of the body. The radial and ring-canals are produced from a continu- 

 ous coelenteron by the squeezing together of the two layers of endoderm 

 in the intermediate parts. 



The medusa moves through the water by contractions of the 

 *' umbrella" or bell, which force water out of the sub-umbrellar cavity. 

 After some time there appear four swellings of the ectoderm lining the 

 sub-umbrellar cavity, overlying the four radial canals. These are the 

 gonads or reproductive organs. The medusa is dioecious, the sexes 

 being separate. The egg develops into a larva which swims to the 



Fig. 53. — Lateral View of a Medusa of Obelia. 



Magnified. {Ad nat.) 



Mesogloea. 



Stomach. 



Gonad. 



Radial 

 Canal, 



Radial 

 Canal. 



Ring-Canal. 



bottom, fixes itself and grows into a young hydroid polype. Thus 

 Obelia is an illustration of metagenesis or alternation of generations, 

 the hydroid giving rise to a number of other hydroids, some of which 

 grow into medusas which in turn give rise to hydroid polypes by 

 sexual reproduction. 



Obelia is also a remarkable instance of physiological division of 

 labour between the individuals of a colony producing nutritive hydroid 

 polypes and reproductive medusoids {cf. Zoothamnitim). The following 

 differences of Obelia from Hydra ^q\\\& be noted : — 



1. Obelia is a compound animal or colony, produced by asexual 

 reproduction from a simple polype. 



2. It has two phases : a sedentary hydroid and a free-swimming 

 medusoid. 



3. It has a chitinous exoskeleton, the perisarc. 



