xxn 



Introduction 



in one jar, get more of the material from the same place 

 and treat it in the same way. The hydras may be trans- 

 ferred to covered aquaria containing Vaucheria, Nitella, or 

 similar plants, where under favorable conditions they 

 will multiply and be available when wanted. 



With a dozen or more aquaria containing water plants 



in good condition in the 

 laboratory, hydras are 

 pretty sure to be present 

 in some of them. Water 

 snails should not be al- 

 lowed in hydra jars. Cy- 

 clops or other minute 

 Crustacea are also likely 

 to be found in some of 

 these jars. 



Marine Animals. — The 

 various marine forms 

 treated of in this book 

 may be purchased in 

 good condition for study, 

 at small expense, from 

 the Supply Department, 

 Marine Biological Labora- 

 tory, Woods HoU, Mass., 

 as well as from various 

 other dealers in such ma- 

 terials. 



Cyclops. Magnified. 



INSECTS 



It is of course necessary that insects should be collected 

 during the season when they are abundant. Some of them, 



