140 GENERAL ZOOLOGY 



flagellated organisms which are clearly plants. On account 

 of such mixed relationships some biologists make a group, 

 Flagellata, which contains both plants and animals, and 

 which is believed to have given rise to plants by emphasizing 

 certain qualities, and to animals by stressing others. Ac- 

 cording to this view the flagellates were the first living 

 things and hence stand at the base of the evolutionary 

 series. A form like the Amceba may, therefore, not be primi- 

 tive, even though it is simpler in structure than any other 

 animal, but perhaps may have arisen from the more complex 

 flagellates by the loss of some structures. Mastigamceba 

 in such an evolutionary history might serve as a " missing 

 link/' 



Some flagellates almost become metazoans. There are 

 many species in which colonies are formed by cells remaining 

 united after fission. In such colonies the number of cells 

 may be variable or very constant. In certain species there 

 are always four, eight, sixteen, or thirty-two cells. Others 

 have a very large number; a colony of Volvox (Fig. 62, G), 

 for example, may be made of 12,000 cells. In colonies of 

 flagellates there is often a division of labor between the cells 

 some carry on locomotion and feeding, others serve for 

 reproduction only. There is thus a differentiation into 

 somatic (body) and germ (reproductive) cells. In Volvox 

 and some other colonies there is a further division of labor 

 in the germ cells; some developing into large ova (eggs) and 

 others into small sperm cells. This, however, is the limit 

 of cell-specialization in flagellates, or in protozoans, and the 

 colonies cannot properly be classed as many-celled animals 

 because the somatic cells are all alike i.e., there are no 

 tissues. 



There are a number of flagellates which cause serious 

 diseases. Different species of the genus Trypanosoma 

 cause sleeping sickness in man, surra and dourine in horses, 

 nagana in cattle, etc. The Texas fever which is prevalent 

 in cattle throughout the southern United States is caused 



