INTRODUCTION 23 



subdivisions do not follow the same lines of development, however, 

 so that we are able to classify Protozoa according to a fairly natural 

 system. These diverse lines of development make it difficult to 

 treat this branch of the animal kingdom in any general way; the 

 wide range in habitat from the purest waters of lake or sea to the 

 foulest ditch, and adaptations to environments varying in char- 

 acter from a mountain stream to the semifluid substance of an epi- 

 thelial, nerve, or muscle cell, has brought about manifold varieties 

 of structure. To describe all of these modifications under a few 

 headings, or to attempt to formulate general laws from the different 

 and often highly complicated life histories, is out of the question. 

 The general trends of differentiation, however, permit of grouping 

 the different kinds of Protozoa in four types which were first out- 

 lined by the French microscopist Felix Dujardin in 1841, Three 

 of these types— Sarcodina, Mastigophora, and Infusoria— are based 

 upon the nature of the locomotor organs— pseudopodia, flagella, 

 and cilia respectively— while a fourth type— Sporozoa— includes 

 organisms which are invariably parasitic in mode of life and are 

 essentially without motile organs (Fig. 4). 



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 desiccation is fatal to the majority of Protozoa, although 

 it is possible that some forms, like certain rotifers, may reab- 

 sorb moisture and again become active. If the fluid protoplasm 

 is surroiuided by impervious membranes evaporation is prevented 

 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 

 out of water (Fig. 5). Because of their light weight these C3'sts 

 maA' be carried in the air and blown by the winds with dust, until 

 surrounded 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 vegetation. In the infusions formed by soaking such dried 

 vegetation in water various species of monads {Monas, Oicomonas, 

 Bodo) and of 'ciliates (Colpoda, Oxyiricha, Stylonychia, Urostyla, 

 Gastrosiyla , and Vorticella) and the rhizopod Amoeba 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 



