INTRODUCTION 



23 



DISTRIBUTION OF PROTOZOA. 



Protoplasm is an aggregate of fluid colloidal substances in which 

 water plays a conspicuous part; exposed to the air it dries and desic- 

 cation is fatal to the majority of Protozoa, although it is possible 

 that some forms, like certain rotifers, may reabsorb moisture and 

 again become active. If after losing its water the protoplasm is 

 surrounded by impervious membranes, further evaporation is pre- 

 vented and within such capsules the protoplasm remains alive. This 

 is the condition of encystment and many kinds of Protozoa, protected 

 by their cyst membranes, may live for long periods in a dried state 

 (Fig. 4). Because of their light weight these cysts may be carried 

 in the air and blown by the winds with dust, until surrounded 



1 2 3 



Fig. 4.— Types of cysts. Eughjpha alveolata, testate rhizopod; Podophrya fixa, 

 suctorian; and Chilomastix mesnili, a parasitic flagellate, urn, undulating mem- 

 brane. (First and second, original; third, after Kofoid and Swezy, University of 

 California Publications in Zoology, 1920.) 



again by water the organisms emerge from their cysts and are 

 active once more for a few hours. Such encysted forms account 

 in part for the surprising protozoan fauna in uncovered sterilized 

 water in which food substances come from similarly protected germs 

 of Bacteria and minute plant forms. Similar encysted forms may 

 be present on the blades of dried grass, leaves and other vegeta- 

 tion. In the infusions formed by soaking such dried vegetation in 

 water various species of monads (Monas, Oicomonas, Bodo) and of 

 ciliates (Colpoda, Oxytricha, Stylonychia, Urostyla, Gastrostyla and 

 Vorticella) and the rhizopod Ameba make their appearance in the 

 order given (Woodruff, 1912). Puschkarew (1913) concluded that 

 air-borne cysts play only a minor role, however, in the spread of 

 Protozoa. It was found that, on the average, there are only 2| 

 protozoon cysts per cubic millimeter of air and that these are limited 



