54 STRUCTURAL AND SYSTEMATIC ZOOLOGY 



The diagram on the opposite page roughly represents 

 (for the relations of animals can not be expressed on a 

 plane surface) the relative positions of the branches and 

 classes according to affinity and rank.* 



SERIES I. PROTOZOA 



Animals whose bodies consist of a single cell, the 

 process of reproduction being by division or by budding, 

 but never by means of true eggs. 



Branch I. PROTOZOA 



In structure the Protozoa are the simplest of animals, 

 consisting of only a single cell. They are microscopic 

 in size and aquatic in habit, though in certain stages 

 of their lives (encystation) many of them may endure 

 dryness for weeks or months. Their bodies consist 

 mainly or wholly of protoplasm, which may or may not 

 be covered by a cuticle or by a shell-like excretion of 

 lime, chitin, or flint, or inclose spicules of the latter 

 substance. The various individuals may live separately 

 as single, independent organisms, or they may be or- 

 ganically joined together in clusters called colonies. 

 They exhibit all the essential functions of life nutri- 

 tion, growth, nervous properties, and reproduction. 

 They feed upon minute algae, bacteria, vegetable de- 

 bris, and upon other microscopic animals. Some forms 

 are parasitic. It has been shown by experiment that 

 many species are sensitive to changes in the amount of 

 illumination to which their bodies are exposed and to 

 various colors of light; that they are attracted or re- 



* The student should master the distinctions between the great groups, 

 or classes, before proceeding to a minuter classification. "The essential 

 matter, in the first place," says Huxley, " is to be quite clear about the 

 different classes, and to have a distinct knowledge of all the sharply de- 

 finable modifications of animal structure which are discernible in the Ani- 

 mal Kingdom." 



