AMOEBA 27 



a phylum. Every known animal is placed in one of these phyla. 

 Thus the most primitive forms of animal life, including all the 

 unicellular forms such as Amoeba, are placed in the first phylum, 

 which is known as the Protozoa. Because of the great numbers 

 of animals and their many points of divergence it is necessary to 

 subdivide the phyla into many smaller divisions, the exact number 

 of these divisions in any phylum depending upon the number of 

 diverging organisms to be considered. (W. pp. 457-459.) 



Considering the phylum Protozoa, there are four primary divi- 

 sions, termed classes. Amoeba and related forms which possess 

 pseudopodia are put in the first class, the Sarcodina, which is again 

 split into two subclasses. Amoeba belongs to the subclass Rhizo- 

 poda, which are creeping forms as distinguished from the subclass 

 Actinopoda, which are floating forms. Further subdivision can 

 be made through order, family, genus, and, finally, species. 

 Animals belonging to a particular species conform very closely in 

 their characteristics, but still show slight individual variations. 



In naming an animal or plant the binomial system of nomen- 

 clature is used. The term binomial means that two names are 

 employed which are those of the genus and the species. Thus 

 in the case of Amoeba, the scientific name for the common species 

 studied is Amoeba proteus, in which Amoeba is, of course, the name 

 of the genus and proteus the name of the species. In many cases 

 where exact scientific data are not involved it is customary, just 

 as in this chapter, to omit the name of the species in referring to an 

 organism. 



Other Noteworthy Types of Sarcodina. There are a num- 

 ber of common species of Sarcodina which, unlike Amoeba, possess 

 a shell. Thus Difflugia has a beautiful, cone-shaped shell which 

 has been made by cementing bits of sand together. Another 

 type of shell is found in forms like Arcella or in the Foraminifera 

 which are formed as a secretion by the peripheral ectoplasm. The 

 chemical composition of the shells varies greatly in the different 

 groups. Great chalk deposits, which are of economic importance, 

 have been formed in the past ages by the gradual accumulation of 

 shells from the marine forms, particularly the Foraminifera. The 

 Sun-animalcule, Actinophrys, shown on page 21, is a beautiful, 

 fresh-water form with many fine pseudopodia radiating from the 

 spherical cell body. ( W. fs. 19-21, 244.) 



There are several species of Amoebae which are parasitic in the 



