OB ELI A. 117 



II._OBELIA. 



Phylum Ccelenterata. 



Class Hydrozoa. 



Order Hydromedus^. 



Obelia genicui>ata is a small marine organism, usually covering 

 seaweeds, such as the brown la7?tinaria, 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 theca, 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 perisarc^ consisting of thin 

 translucent chitin. It forms the hollow axis terminating in the thecoe 

 or cups. Inside the perisarc is a central protoplasmic axis, called the 

 caiiosarc, which runs up to the thecDe and here terminates in small 

 round bodies, having a ring of tentacles. These are the polypes 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 Hydra in having the aboral end of 

 the body produced into a long central axis or co^nosarc, and sections 

 show that this coenosarc is similarly formed of two layers with a central 

 canal, the coenosarcal canal, which communicates with the ccelenteron 

 of all the polypes. 



