CHAPTER V. 



HYDRA : OBELIA SYMBIOSIS ALTERNATION OF GENERA- 

 TIONSCHARACTERS OF THE PHYLUM CCELENTERATA. 



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 class 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 6 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 bear- 

 ing at its opposite or distal end a conical structure, the 



