PRINCIPLES OF 



ECONOMIC ZOOLOGY 



BRANCH PROTOZOA 



The animals of this branch are one celled and microscopic, 

 or very small. These cells may unite, but as the union is not 

 organic, it is said to form a colony, and not an individual animal 

 as is the case in the higher forms. A colony may consist of a 

 few cells, as in Gonium, or of many cells, as in Volvox. 



Since protozoans are so minute and their soft protoplasmic 

 substance is so easily dried up, they are usually aquatic, but 

 some forms are parasitic, while others, as Amoe'ba terric'ola, are 

 terrestrial, but these live or remain active in moist places only. 

 Protozoans are most abundant in salt water, or in stagnant pools 

 of fresh water, and are found in almost all parts of the globe. 



Since, by reason of their simplicity, protozoans are adapted 

 for living where other animals could not exist, they are supposed 

 to be the oldest or first animal life, and it is believed that they 

 existed in the Archaean time. (See Fig. 302.) 



Numbers. — There are many thousands of species of these 

 protozoans, each species differing from all others in some 

 detail, yet all agreeing in their unicellular simplicity. Only a 

 few of the typical forms can be mentioned. 



CLASS I. RHIZOPODA 



The lowest class, or Rhizop'oda, is represented by the Amoeba 

 (Fig. 1). It is an irregular mass of colorless, semifluid, or jelly- 

 like living protoplasm destitute of a cell wall. There is no dis- 



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