46 THE PROTOZOA 



podia consists in the simple protrusion and retraction of the change- 

 able processes. It becomes much more definite in forms provided 

 with flagella, where, in many cases, a steady progression with the 

 flagellum in advance is the characteristic motion. In one group of 

 the Mastigophora, however, the Choanoflagellida, the flagellum, like 

 the tail of a spermatozoon, is directed backward during motion. 

 Among the Ciliata, complex movements accompany the high organiza- 

 tion of the cell, and the change from one form to another, apparently 

 at the will of the organism, is extremely suggestive of conscious action. 

 Here, in addition to the normal and constant motion of the cilia, are 

 various forms of contractile movement varying from the simple 

 sarcode streaming, which is characteristic of the Suctoria, to the 

 definite contraction of distinct muscular elements in the myonemes of 

 Heterotrichida and Peritrichida. 



B. GENERAL PHYSIOLOGY 



In all Protozoa, as in higher animals, the functions of nutrition, 

 respiration, excretion, reproduction, and irritability the analogue of 

 nerve-response, are indispensable for the life of the organism. When 

 compared with the vital functions of the higher animals, all of these 

 processes appear simple ; yet the difference is one of degree only, and 

 among the unicellular animals, as among the multicellular, the func- 

 tions become more complicated and difficult to analyze as the cell- 

 structures become more complex. In the simpler forms, the naked 

 unmodified protoplasm contains the beginnings of the most compli- 

 cated functions, none of which can be regarded as having a par- 

 ticular time and place of birth in the series of animal forms ; all are 

 characteristic of the cell, and beyond that, of living protoplasm, of 

 which they are the distinguishing properties. The most primitive 

 Protozoa, entirely destitute of organs, feed without mouth or digestive 

 tract, move without appendages, react to external stimuli, excrete, 

 and reproduce. In the higher types the cell-organism becomes dif- 

 ferentiated^ into special parts for the performance of these various 

 functions, and the relative position of the organism in the scale of 

 Protozoa depends upon the degree of this differentiation. In no class 

 of animals is the connection between division of physiological labor 

 and regional differentiation so clearly marked as here. This is 

 especially noteworthy in the outer plasm, which, directly correlated 

 with the action of the environment, has apparently become progres- 

 sively modified into external coverings, into motile organs, and into 

 organs of sensation, while the endoplasm retains the same character 

 throughout the group. 



One function, that of encystment, is limited almost exclusively to 



