250 BIOLOGY OF THE PROTOZOA 



value of sub-phyla, each with its own peculiar type of structural 

 and functional diflerentiations. Of these we regard the Mastigo- 

 phora as the central group from which the present day Sarcodina, 

 Infusoria and Sporozoa have been derived, either directly or indi- 

 rectly, evidence of which has been given in the preceding chapters. 

 The Spirocha^tida are not included here, as their main characteris- 

 tics place them much closer to the bacteria than to Protozoa. Their 

 transverse division and spore formation through coccoid bodies 

 are not duplicated amongst flagellated Protozoa, but are distinctly 

 Spirillum-like. The columella of Spirochceta and the crista of 

 CVistispira (Fig. 12.3) are not homologous with the undulating 

 membrane of Trypanosoma as was earlier assmned. In short, 

 there is nothing in their morphology or life histories that justifies 

 their inclusion in the group of Protozoa. 



SUB-PHYLUM MASTIGOPHORA. 



Classification of the flagellated Protozoa involves an old problem 

 of distinguishing between animals and plants, and while the diffi- 

 culty is more or less of an academic character, there are some prac- 

 tical problems connected with it. A large number of the flagellated 

 Protozoa possess chloroph\'ll through which, by energy of the sun- 

 light, they are enabled to manufacture their own food. Such auto- 

 trophic nutrition is the most characteristic feature distinguishing 

 plants and animals, but the real difficulty lies in the fact that many 

 such chlorophyll-bearing forms are heterotrophic in nutrition. To 

 classify such forms as either animal or plant is unsatisfactory and 

 it seems best to frankly admit that they lie on the boundary between 

 the two great groups. Another difficulty in classification of the 

 flagellates is the difficulty of drawing the line between bacteria and 

 Protozoa, or between unicellular Protozoa and multicellular plants 

 and animals. Here again, certain organisms lie on the boundary line. 

 Volvox and other phytomastigida, for example, closely approach the 

 many-celled plants. There is no difficulty, however, in distinguish- 

 ing between j\Ietazoa and Protozoa. 



The sub-phylum, as a whole, manifests a high potential of evolu- 

 tion with highly developed powers of adaptation, resulting in the 

 most diverse modes of life from intracellular parasitism to life in 

 the purest drinking water. The typical form is elongate and 

 ovoidal, variable, however, in many cases of pseudopodia-bearing 

 forms, or variable by virtue of the highly metabolic plasm. The 

 cortex, however, in many cases may be firm and rigid, resulting in 

 a permanent form of the cell. Highly sculptured membranes of 

 pseudochitin or cellulose are frequently present; gelatinous secre- 

 tions are prevalent. These may be of pseudochitin or cellulose 

 and frequently form a matrix in which the indi\idual cells lie. 



