PROTOZOA. 71 



of cell; for example, movement is performed by the muscle cells, digestion 

 is partially provided for by the cells of the alimentary canal, and urine is 

 excreted by the kidney cells. A protozoon is a unicellular animal and each 

 of these functions must be performed within the single cell of which it 

 consists. Consequently, areas in every protozoon are differentiated so 

 as to form portions which are each devoted to special cell functions. 

 These portions are called organella, and by means of them all the ac- 

 tivities of a large animal go on in an organism which consists of a single 

 cell. 



The functions of the nucleus are not completely understood but it 

 seems certain that the nucleus is a controlling center for the cell's activities. 

 Its functions, therefore, are, roughly, two-fold; they are either concerned 

 with the maintenance of the cell, or they are concerned with its reproduc- 

 tion; that is, they are either somatic or sexual. Usually, both functions 

 are subserved by a single nucleus; sometimes, however, as in the flagel- 

 lates, they are divided between two nuclei. 



The activities of a protozoon may be divided into three classes: 



LOCOMOTION, METABOLISM* and REPRODUCTION. 



LOCOMOTION. The protozoa have several different methods of 

 moving themselves about. Some of them move by the formation of 

 pseudopodia; in this method of progression, the protoplasm flows out, 

 in finger-like processes, from the body of the parasite and, as the proto- 

 plasm flows into these processes, the whole organism progresses, literally, 

 by flowing along. Some of the gregarines move about by means of a 

 flowing of the protoplasm which always takes place in one direction; it 

 is probable that the control of the direction of the flow in these parasites 

 is effected by the contraction of myonemes. Myonemes are contractile 

 fibers, which usually lie near the surface of the organism possessing them. 

 Through their contraction, the form of the body of the parasite may be 

 altered and, in this way, motion may be produced. Cilia are small hair- 

 like multiple, often numerous processes, which may be placed either 

 in definite areas or over the whole surface of a protozoon. They produce 

 motion by waving; they usually act together and their motion has a strong 

 simultaneous stroke in one common direction. Flagella are larger than 

 cilia; they are whip-like processes which have a lashing movement. They 

 are usually few in number and are often placed at the ends of the organism. 



REPRODUCTION. The protozoa reproduce in many different ways and 



*For a consideration of metabolism, seep. 82, Part II, Physiology. 



