86 THE MAIN CURRENTS OF ZOOLOGY 



vertebrates, arthropods and molluscs and thus ob- 

 tain, especially in the eyes of the beginner, an 

 importance which does not belong to them." 



For our present purpose, it will be sufficient to 

 enumerate and briefly characterize the ten phyla 

 which embrace nearly all animals except those of 

 somewhat uncertain position. While merely the 

 phyla are mentioned here, it is to be understood that 

 the division of these large groups into classes, orders 

 and families brings us by groups of different rank to 

 genera and species. The latter divisions supplying 

 the generic and specific name by which individual 

 animals are known. 



I. Protozoa. The simplest microscopic, usually 

 aquatic, single-celled animals. In these animals, the 

 manifestations of life are reduced to their simplest 

 expression and a study of them is the best introduc- 

 tion to the more complex animals and also the best 

 introduction to general physiology. Certain forms 

 are disease-producing or pathogenic, as the germ of 

 malaria, of sleeping sickness, etc. Although very 

 minute, certain kinds of protozoa commonly occur as 

 fossils. Those producing shells of carbonate of lime 

 making beds of chalk, and those forming shells of 

 silica occurring in fossiliferous earths. The known 



