120 C@LENTERATA. 
and endoderm, but the mesogloea is much thicker and forms the bulk 
of the body. The radial and ring-canals_are produced from a continu- 
ous ccelenteron 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 dicecious, 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.) 
Mesogleea. Stomach, 
‘un A 
Sense-Organ. 
Manubrium. 
Tentacle. 
Ring-Canal. 
bottom, fixes itself and grows into a young hydroid polype. Thus 
Obelia is an illustration of me¢ageneszs or alternation of generations, 
the hydroid giving rise to a number of other hydroids, some of which 
grow into medusz 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 Zoothamnium). The following 
differences of Obelta from Hydra should 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. 
