OBELId. 117 
II._OBELIA. 
PHYLUM C@:LENTERATA. 
Cxass HypDROZzOA. 
ORDER HypROMEDUSA. 
OBELIA GENICULATA is « small marine organism, usually covering 
seaweeds, such as the brown /amznaria, between tidemarks. It has 
the appearance of a small plant and is hence often called a zoophyte. It 
has a creeping basal portion from which there grow up main branches. 
Fig. 50.—CoLoNY OF OBELIA GENICULATA. 
(Natural size.) 
Main Branch with Polypes. 
Basal part of Colony. 
Seaweed. 
The main branch appears a zigzag, from each corner of which is pro- 
duced a small branch. With a lens it can be seen that each branch 
terminates in a swollen cup-shaped head or ¢heca, and, if the zoophyte 
be alive and undisturbed, a ring of delicate tentacles will be seen 
protruding from the theca. 
Further examination shows that there are two separate structures— 
the outer, hard and non-living part, and the inner, soft and living portion 
of the zoophyte. The outer part is called the Zerzsarc, consisting of thin 
translucent chitin. It forms the hollow axis terminating in the thecz 
or cups. Inside the perisarc is a central protoplasmic axis, called the 
cenosarc, which runs up to the thecze and here terminates in small 
round bodies, having a ring of tentacles. These are the Aolypes which 
conform closely in structure to Hydra. Each has a terminal mouth 
inside the tentacles; each has a two-layered body-wall with nematocysts 
and ccelenteron. They differ from AMydra in having the aboral end of 
the body produced into a long central axis or cenosarc, and sections 
show that this ccenosarc is similarly formed of two layers with a central 
canal, the ccenosarcal canal, which communicates with the ccelenteron 
of all the polypes. 
