CHAPTER XLIV 

 PROTOZOA * 



1108. Introduction. At the outset great emphasis must be laid 

 on the necessity for examining all protozoa alive, and as nearly 

 as possible in their natural conditions, whether they be parasitic 

 or free-living forms. 



For this reason it is essential, when making preparations for 

 the microscope, to mount the protozoa in their natural medium, 

 or one isotonic with and approaching as nearly as possible to it. 

 Saline solutions, including Locke's and Ringer's mixtures (see 

 appendix), are varied slightl}- for use with parasites and tissues 

 of different animals. 



It is also most important for the medium to have the same 

 reaction (H-ion concentration or /?H), i.e., to be neither more 

 acid nor more alkaline than the natural medium. White of 

 egg added to a mounting medium for examining protozoa over 

 long periods has the advantages of preventing bacterial growth 

 to some extent as well as supplying a colloid. 



Much advance has recently been made in methods of main- 

 taining protozoa in artificial culture over long periods. Such 

 cultures facilitate enormously the study of life histories, besides 

 aflbrding the great convenience of having a supply of the organisms 

 at hand in the laboratory. Before dealing, therefore, with the 

 action of any special reagents on protozoa, some of the chief 

 methods by which they may be cultivated will be indicated. 



I. CULTIVATION OF FREE-LIVING PROTOZOA 



1109. Collection. Notwithstanding the wide distribution of 

 many protozoa, it may be a matter of considerable difficulty 

 to collect enough specimens for class or research purposes. 

 Pelagic forms may, of course, be collected by tow-netting, etc. 

 (See The Microscope and its Revelation, Carpenter and 

 Dallinger, 1901, or Fresh Water Biology, Ward and Whipple, 

 1918.) Sedentary and many other forms from below the sur- 

 face may have to be picked out by examining weed, mud, etc., 

 with a lens. Occasionally good temporary cultures of a required 

 form can be obtained by keeping tow-nettings in water from the 

 same source and supplying food ; e.g., we have had on several 

 occasions, from April to June, good su})plies of Stentor by leaving 



• By H. P. G. 



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