PROTOZOA 187 



Protozoa. We are obliged to confess that there is no 

 perfectly valid distinction between the lowest animals 

 and the lowest plants ; they grade one into the other. 



2. Protozoa abound in the sea and in fresh waters; Variety and 

 they occur also in damp soil, while vast numbers of of p ro tozoa 

 species are parasitic. The Mycetozoa may be regarded 



as Protozoa adapted to life in air. The species of Pro- 

 tozoa are excessively numerous, and in some cases they 

 appear almost indefinitely so. The marine Radiolaria, 

 described by Professor Haeckel of Jena, construct elabo- 

 rate and beautiful shells of almost every conceivable 

 pattern, reminding us of the infinite variety of snow 

 crystals. The thousands of "species" named all have 

 characteristic forms, but more recent researches indi- 

 cate that many can be grouped as phases of variable 

 species. Even so, however, the number of distinct 

 kinds is very great, and the same may be said of another 

 marine group, the Foraminifera. It is a remarkable 

 fact that in spite of the low type of organization and the 

 multitude of species, the different types of Protozoa are 

 on the whole extraordinarily constant and of great an- 

 tiquity. The very same species may be found in fresh 

 waters on continents and islands, in the tropics and in 

 cool countries, at sea level and in the mountains. Con- 

 sequently the student who believes he has a new Proto- 

 zoan is obliged to consider in comparison the species of 

 the whole world, for the animal he has discovered in 

 New York may have been described from Tasmania. 



3. The principal types of Protozoa may be classified Flagellates 

 in groups or subphyla by the use of a few simple char- 

 acters. The Mastigophora or flagellates move by means 



of a slender, undulating or vibratile thread of proto- 

 plasm called the fiagellum (little whip). In certain 

 types there are two or even more of these flagella, and 



