COELENTERATA 



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animal serves for protection as well as movement and sensation, 

 certain cells being fitted for each of those different purposes. 



The tentacles are provided with thousands of minute darts or 

 stinging cells, by means of which prey is killed. The tentacles 

 then reach out like arms, grasp the food, bend over with it, and 

 pull it toward the mouth. Certain cells lining the hollow digestive 

 cavity pour out a fluid 

 which digests the food. 

 Other cells with long 

 cilia circulate the food, 

 while still other cells 

 lining the cavity put 

 out pseudopodia, which 

 surround and take in 

 the food particles. 

 The outer layer of the 

 animal does not digest 

 the food, but receives 

 some of it already di- 

 gested from the inner 

 layer. Oxygen is 

 passed through the 

 body wall, for there are 

 no special organs for 

 respiration. 



Reproduction. The 

 Hydra reproduces asex- 

 ually by budding, as is seen in the diagram. The bud appears on 

 the body as a little knob, the body cavity extending into it. After 

 a short time (usually a few days) the young hydra separates from 

 the old one and begins life alone. This is asexual reproduction. 



The Hydra also reproduces by sex cells. The sperms develop in 

 little groups near the free end of the animal, and the egg cells de- 

 velop near the base. The sperms, when ripe, are set free in the 

 water ; one of them unites with an egg, which is usually still at- 

 tached to the body of the Hydra, and development begins which 

 results in the growth of a new hydra. 



Colony of Obelia. 

 A jellyfish. 



Amer. Mus. of Nat,. Hist. 



Why are the Obelia and jellyfish classified as coelenter- 

 ates ? In what way are they similar ? In what ways are 

 they unlike each other ? 



