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PHYLUM CCELENTERA 



Many points in the behaviour of the Ccelenterates may be 

 deduced from the structure of the nervous system and 

 the absence of any co-ordinating centre ; they are " reflex 

 repubhcs," in which any excited portion may acquire 

 temporary dominance over the rest. Nerve-networks 

 of a similar nature are found in the walls of the viscera of 



Vertebrates, e.g. 



Auerbach's plexus. 



Types of Ccelentera 



First Type — Hydra, a simple representative of the 



Class Hydrozoa 



General life. — The genus Hydra — cosmopolitan, like 

 many other small fresh-water animals — is represented by 

 several species, e.g. the green Hydra viridis, the brownish 

 H. oligactis or fusca, and the orange H. vulgaris or grisea, 

 widely distributed in fresh water. They are among the 

 simplest of Ccelentera, for the body 

 is but a two-layered tube, with a 

 crown of (6-10) hollow tentacles 

 around the mouth, and with no 

 organs except those concerned in re- 

 production. The body is usually 

 fixed by its base to some aquatic 

 plant, often to the lower surface of 

 a duckweed. It may measure J- J 

 inch in length, but it is as thin as 

 a needle, and contracts into a minute 

 knob. 



The animal sways its body and 

 tentacles in the water, and it can also 

 loosen its base, lift itself up by its 

 tentacles, stand on its head, or creep 

 by looping movements. According 



to some observers, its movements are helped by fine 

 pointed pseudopodia protruded from the ectoderm cells 

 of the tentacles and base, and by threads ejected from 

 large cylindrical stinging cells. Usually, however, the 

 Hydra prefers a quiet life. It feeds on small animals, 

 which are paralysed or killed by stinging cells on the 



Fig. 87. — Hydra hang- 

 ing from water-weed. 

 — After Greene. 



ov., Ovary ; t., testes. 



