192 REPRESENTATIVE SINGLE-CELLED ANIMALS 



rounded by the fluid of the earthworm's seminal vesicles, and the 

 cell of any higher animal (c/. Fig. 60, p. 104), surrounded by intercel- 

 lular lymph, is a close one. Presumably, the same kind of con- 

 structive and destructive metabolic changes occur in either case. 

 Very little can be said regarding the cell behavior in such a form. 

 By expanding and contracting its body, the monocystis effects a 

 slow locomotion, but, living as it does, it has no need for any com- 

 plexity of behavior or for locomotor structures. The life cycle 

 is shown by Fig. 101 and its legend. 



Metabolism, Irritability, and Reproduction in Protozoa 



The types of protozoa that have been described illustrate the 

 manner in which the three great capacities of metabolism, irrita- 

 bility, and reproduction are exhibited by the protoplasm of unicel- 

 lular animals. While comparisons can be made between the proto- 

 zoan cell as an individual animal and the individual composed of 

 many cells as in higher animals, the more exact comparisons are to 

 be made between cell and cell. Thus, in parasitic protozoa, the 

 unicellular organism bathed in the fluids of its host's body, from 

 which it assimilates nutrients and into which it excretes the waste 

 products of its dissimilation, presents a close parallel with the cell 

 of a higher animal as it lies surrounded by its intercellular lymph. 

 If one examines the more representative protozoa with holozoic 

 nutrition like that of the amoeba and paramoecium, it appears that 

 they exhibit all the metabolic processes that it is possible for them 

 to possess in view of their organization. Ingestion, digestion, 

 egestion, assimilation, respiration, dissimilation, and excretion are 

 all present on essentially the same basis as in many-celled forms 

 {cf. p. 103). In the nature of the case, absorption through the 

 mucous membrane of a digestive tract, circulation in the blood, and 

 excretion by excretory organs cannot be present in the protozoan. 

 Metabolism is the same, however, whether in an amoeba cr in a 

 human being. 



Likewise, the response to stimulation, which constitutes irri- 

 tability, appears to be the same kind of process whether in protozoa 

 or metazoa. If we consider the cells of the frog or man individually, 

 they respond to a variety of stimuli: mechanical, chemical, ther- 

 mal, electrical, photic, etc. To all these a protozoan cell may 

 respond equally well, and sometimes to a more marked degree. 



