Introduction 



XXI 



are long and slender, and at other times Short and 

 thick. Their appearance when slightly magnified is 

 shown in the figure herewith. Two species are com- 

 monly found : in one the body is green ; in the other it 

 is light brown. 



Hydras are common in ponds and ditches, but are 

 difficult to obtain in quantities under natural conditions. 

 To collect them the following method is recommended : 



T1..J rr T T' r 1 1 -i. t ;i 



The Hydra. Magnified. 



Bring in from various situations in ponds or sluggish 

 streams small quantities of water-weeds, — as Nitella, 

 Spirogyra, or Vaucheria, — collecting them with a small 

 quantity of water in jars or cans, and labelling each so as 

 to know where it came from. Place the material of each 

 collection in a jar of clear water near a window, and let it 

 stand for a day or two. Then examine the jar carefully, 

 especially the sides near the window, for hydras. In this 

 a reading glass is very helpful. In case hydras are found 



