CHAPTER V 



HYDRA : OBELIA SYMBIOSIS ALTERNATION OF GENERA- 

 TIONS CHARACTERS OF THE PHYLUM CGELENTERATA 



A CAREFUL search in ponds will often result in the 

 capture of some small organisms known as " fresh- 

 water polypes/' belonging to the genus Hydra. 



Although far from uncommon in pond-water, Hydra 

 is not always easy to find, being rarely abundant and by 

 no means conspicuous. In looking for it the best plan 

 is to fill either a clear glass bottle or beaker, or a saucer, 

 with weeds and water from a pond, and to let it 

 remain undisturbed for a short time. If the gathering 

 is successful there will be seen adhering to the sides 

 of the glass, the bottom of the saucer, or the weeds, 

 little white, tawny, or green bodies, about as thick as 

 fine sewing-cotton, and 2-10 mm. in length. They adhere 

 pretty firmly by one end, and examination with a 

 pocket lens shows that from the free extremity a 

 number of very delicate filaments, barely visible to the 

 naked -eye, are given off. 



Under the low power of a compound microscope a 

 Hydra (Fig. 75) is seen to have a cylindrical body 

 attached by a flattened base to a weed or other 

 aquatic object, and bearing at its opposite or distal 

 end a conical structure, the hypostome (hyp), at the 



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