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TEXT -BOOK OF ZOOLOGY 





ii 



FRESH-WATER POLYPS 



of different ages, and therefore different sizes ( x about 

 4 times). They are attached to the roots of duck- 

 weed (also enlarged), to a plant-stalk, and to the 

 . bottom. 1, Hydra much retracted ; 2, less re- 

 l tracted, and with a bud ; 3, fully extended, with 

 an excrescence at the lower part of the cylindrical 

 body containing an egg ; within the body cavity 

 is seen a copepod crustacean (Cyclops) ; attached 

 to one of the tentacles is seen a water-flea (Daph- 

 nia) caught by one of the "thread cells," while 

 another arm embraces a small gnat larva ; 4, 

 hydra extended in the act of subduing a chretopod 

 worm (Nais), and with a bud "ripe" for detach- 

 ment which has captured a water-flea ; 5 (in back- 

 ground), much -con tracted hydra with a captured 

 worm in its body cavity ; 6, hydra creeping on its 

 tentacles ; 7, another creeping after the manner of 

 the looper caterpillars. 



at most f inch long, the two other species 

 may extend themselves to a length of 

 from 4 to 1 inch, for the body of these 

 animals is uncommonly extensile. Thus, 

 if a completely extended polyp be touched 

 ever so lightly, it instantly contracts to a 

 minute ball or pear-shaped body, in which 

 the arms, previously about an inch long, 

 appear now as mere tiny protuberances. 



C. Structural Characters and Habitat. 



If a polyp is taken out of the water, 

 it at once shrinks up, so that it resembles 

 a small lump of protoplasm. This re- 



