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TWO-LAYERED ANIMALS— PHYLUM COELENTERATA 



Reproductive polyps are also formed when the colony grows older, 

 and they produce tiny jellyfish called medusae which bud off and swim 

 away. Obelia in the hydroid stage is surrounded by a delicate, transparent 

 shell, which has a series of joints at the base of each polyp. This gives the 

 animal a flexibility so that it can wave to and fro in the water in search 

 of food. Each feeding polyp can withdraw into a cup-shaped cavity in 

 the shell in case of danger. There is a common gastrovascular cavity 

 connecting all the polyps ; thus, when one gets a good meal, all the others 



Ciliated 

 larva 



Fig. 9.5. Obelia life cycle. 



share it. Since the reproductive polyps have no mouth or tentacles, they 

 are entirely dependent on the food that comes to them through this 

 common digestive and circulatory system. 



There are two kinds of medusae — females which produce eggs, and 

 males which produce sperms. After the egg is fertilized, it develops 

 into an embryo which swims around until it finds a suitable place to 

 settle down. Here it becomes attached and develops into a colony of 

 polyps. 



Thus we see that obelia has two distinct generations, a hydroid stage 

 which reproduces asexually by budding, and a medusa or jellyfish stage 

 which reproduces sexually. This sort of double life is known as alterna- 

 tion of generations. 



