Fresh-water Polyp {Hydra Sp.) 



Material. — It is seldom that one is so situated as to 

 have a supply of living hydras at hand whenever he 

 wishes, as they are very unreliable animals in their 

 habits ; they may be found in great abundance one 

 year and not at all the next. Yery httle can be given 

 in the way of definite directions for obtaining living 

 specimens, and, owing to the difficulty of killing them in 

 the expanded state, those preserved in alcohol are usu- 

 ally so shrunken as seldom to be suitable for beginners 

 to study. Perhaps the best method of obtaining mate- 

 rial is to get from stagnant ponds or marshy lakes a good 

 supply of submerged or floating water weeds, for exam- 

 ple Elodea Canadensis and duck-weed {Lemna), put that 

 gathered at different places into separate glass jars, with 

 a label on each to indicate the locality whence the ma- 

 terial came, and set the jars filled with clear, fresh wa- 

 ter on a table near a window, but not exposed to the 

 direct sunlight. In the course of a few hours carefully 

 examine the lighted side of each jar for hydras. Hav- 

 ing learned in this manner that specimens are to be 

 found in a certain locality, further supplies of water 

 weeds may be gathered. If the number of specimens 

 found should not be sufficiently large to supply the 

 class, work on this form may be delayed until addi- 

 tional specimens can be raised. This may easily be 



