CHAPTER IV 



POLYPS (CUPLIKE ANIMALS) 



Fig. 33.- 

 A Hydra. 



The Hydra, or Fresh - Water Polyp 



SUGGESTIONS. — Except in the drier regions of North America 

 the hydra can usually be found by careful search in fresh-water ponds 

 not too stagnant. It is found attached to stones, sticks, or leaves, 

 and has a slender, cylindrical body from a quarter to half an 

 inch long, varying in thickness from that of a fine 

 needle to that of a common pin. The green hydra 

 and the brown hydra, both very small, are common 

 species, though hydras are often white or colourless. 

 They should be kept in a large glass dish filled with 

 water. They may be distinguished by the naked 

 eye but are not studied satisfactorily without a 

 magnifying glass or microscope. Place a living specimen attached 

 to a bit of wood in a watch crystal filled with water, or on a hol- 

 lowed slip, or on a slip with a bit of weed to support the cover 

 glass, and examine with hand lens or lowest power of microscope. 

 Prepared microscopical sections, both transverse and longitudinal, 

 may be bought 

 of dealers in mi- 

 croscopic sup- 

 plies. One is 

 shown in Fig. 39. ^ 



Is the hy- 

 dra's body 

 round or two- 

 sided ? (Fig. 

 35.) What is 



its general shape ? Does one individual keep the same 

 shape ? (Fig. 34.) How does the length of the thread- 



22 



Fig. 34. 



• Forms assumed by Hydra. 



