CHAPTER V 

 SURVEY OF UNICELLULAR ANIMALS 



The most important discoveries of the laws, methods, and progress 

 of Nature have nearly always sprung from the examination of the 

 smallest objects which she contains, and from apparently the most 

 insignificant enquiries. — Lamarck. 



The invention of aids to the human senses, in particular the 

 microscope, has made us aware of a "world of the infinitely little," 

 whose representatives — Protococcus, Bacteria, and Amoeba — 

 have been used in our study of the nutritional interdependence of 

 organisms: animals cannot live without plants. We turn now to 

 a brief survey of the close allies of Amoeba in the great group of 

 unicellular animals, the Protozoa. 



The Protozoa are the simplest forms of animal life, each in- 

 dividual comprising typically but a single unit of living matter, 

 a cell. But it does not follow that they are devoid of complex 

 organization. Indeed some Protozoa exhibit a complexity of struc- 

 ture within the confines of a single cell that is not exceeded, per- 

 haps not equalled, in the cells of higher animals. The Protozoa 

 are the simplest, but by no means simple animals. 



Because these microscopic forms afford the nearest available 

 approach to primitive animal life, innumerable studies on their 

 physiology have been made in the hope that they would more 

 readily supply the key to life processes — e.g., nutrition, growth, 

 reproduction, sex — that find such complex and bewildering ex- 

 pression in higher animals. Furthermore, not a few of the Protozoa 

 live as parasites in Man and beast and cause some of the most 

 serious diseases and greatest economic loss. So on both the theo- 

 retical and practical side, protozoology claims attention. 



In any survey of animals or plants, first of all it is necessary to 

 arrange the various kinds, or species, in some order — to classify 

 them. To place nearer together those that appear related in struc- 

 ture and function and separate farther those that seem to be less 

 related. And if relationship is the basis of classification, we should 



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