252 BIOLOGY OF THE PROTOZOA 



ments of flagella belong to the same category of kinetic elements 

 (p. 141). Filopodia or ray-like pseudopodia without axial filaments, 

 are present in some of the Chrysomonadida ; lobopodia are widely 

 distributed in the same group and in the Rhizomastigidse. 



A definite cytopharynx even in the chlorophyll-bearing forms is 

 frequently present, in which case the flagella arise from its walls. 

 The function in many cases is only conjectural, but in other cases 

 it acts as a cell mouth or cytostome for ingestion of solid food, and 

 in still other cases provides a receptive area for intake of food in 

 saprozoic forms. C'ontractile vacuoles usually empty into it either 

 directly or through tlie medium of reservoirs and canals. 



Contractile vacuoles are either simple or complex. The simple 

 vacuoles have no accessory structures; the complex vacuoles form 

 a vacuole system consisting of smaller contractile vesicles which 

 empty their contents into a common reservoir, the latter being con- 

 nected with the outside by a longer or shorter canal. Such complex 

 systems are characteristic of the Euglenida and the Chloromonadida. 

 Non-pulsating vesicles known as pusules, serving a hydrostatic 

 function, are characteristic of the Dinoflagellida. 



Nuclei are either vesicular or massive in type, the former pre- 

 dominating. A single nucleus is the rule but there may be two 

 (friardia) or many {Calonympha, Sphceronympha, etc.). A central 

 nuclear mass (endosome) is typical and may be a combination of 

 chromatin, })lastin, or chromatin and plastin, with a kinetic (endo- 

 basal) body. The division figure assumed by the nucleus during 

 reproduction may be one of an enormous number of variations. 



The classification of flagellated Protozoa is not yet on a satisfac- 

 tory basis and a "natural" system appears to be impossible with our 

 present limited knowledge. Any classification, however, should 

 be considered a means to an end and, except for a few specialists, 

 not an end in itself, and any system which furnishes a comprehensive 

 idea of the wealth of form and function in these Protozoa is adequate 

 until a better one is forthcoming. Biitschli's (1884) system based 

 upon the nature and number of the flagella was superseded by that 

 of Klebs (1893) who found that the nature of the anterior end or 

 mouth-bearing parts gives a better means of grouping than do the 

 variable flagella, especially when combined with the method of 

 nutrition. Senn (1900) adopted the same principle for his major 

 groups while the larger sub-groups were based upon the nature of 

 the contractile vacuole and the vacuole system. In his classifi- 

 cation the nature of the cortex and its secretions in the form of 

 gelatinous membranes, tests, shells, stalks, etc., formed the basis 

 <^)f generic distinctions, together with the number and structure of 

 the flagella, metaboly or rigidity, undulating membranes, etc. 

 This system was adopted on the whole, by the majority of protozo- 

 ologists including Minchin (1912) and Doflein (3d edition) until 



