22 



BIOLOGY OF THE PROTOZOA 



For such colonies of protozoa, as for analogous colonies of hydroids, 

 the expression "individual of a second order" has been applied. 



Between the limits of the simplest and the most complex of uni- 

 cellular organisms are the great majority of the (estimated) 15,000 

 or more known Protozoa. In each of the main subdivisions sim- 

 plicity as well as extreme complexity of organization is represented, 

 each subdivision including a series of representative forms ranging 

 from one extreme to the other. Differentiation in the different 

 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 



Fig. 3. — Types of Protozoa. A, Amoeba proteus, a rhizopod; B, Peranema tricho- 

 phora, a flagellate; C, Stylonychia mytilis, a ciliate; D, a polycystic! gregarine; E, 

 Tokophrya quadripartita, a suctorian. (A, after Calkins, B, C, E, after Butsehli; 

 D, after Wasielewsky.) 



foulest ditch, and adaptations to environments varying in charac- 

 ter from a mountain stream to the semifluid substance of an epithe- 

 lial, 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 organ- 

 isms which are invariably parasitic in mode of life and are essentially 

 without motile organs (Fig. 3). 



