DESCRIPTION AND HISTORY 13 



individuals which conjugate may be either (1) similar full-sized animals, 

 (2) full-grown animals of dissimilar size, (3) reduced individuals of similar 

 size (similar swarm-spores), or (4) reduced individuals of dissimilar size 

 (specialized gametes). In the last mentioned case the gametes resemble 

 the male and female reproductive cells of the Metazoa. 



Conjugation was formerly thought to be a process of rejuvenation by 

 which the vital energies of the animals are renewed after the appearance 

 of senile changes and a decrease in size and strength. It is now believed 

 to be rather a method for the introduction of variation into a race or 

 species, and to have thus a meaning similar to that of sexual reproduction 

 among the Metazoa, a variable race being better fitted to adapt itself 

 to a changing environment and to overcome unfavorable life-conditions. 



The Protozoa are all, with a few exceptions, aquatic animals. When 

 the water in which they are living dries up and at certain other times 

 they encyst themselves, and in this condition can withstand complete 

 desiccation a long time. Protozoa are easily transported by the wind, 

 especially when encysted, and many species have a world-wide distri- 

 bution. 



Protozoa feed upon organic matter in every form. Certain species 

 are carnivorous ; others feed exclusively on plants ; many feed on decaying 

 substances; and many are parasitic. Of this latter kind many, especially 

 among the Sporozoa, are the cause of disease both in man and animals. 

 Many contain chlorophyll and live like plants and are consequently near 

 the border line between plants and animals. 



History. Microscopic animals were first studied in 1675 by the Dutch 

 naturalist Leeuwenhoek, who first used the microscope in the study of living 

 organisms. About a hundred years later Otto Friedrich Miiller described 

 a large number of them, adopting the binomial nomenclature, and thus 

 laid the foundation of the present classification. The name Protozoa 

 originated with Goldfuss in 1820, who, however, included in the group 

 jellyfish, liydroids, and all of the lowest animals. In 1838 Ehrenberg 

 published his epoch-making work on Infusoria, including in this term all 

 the microscopic animals, the significance of his work consisting in the fact 

 that he brought together accurate descriptions of great numbers of these 

 organisms. Ehrenberg was followed by Dujardin and others and in 

 1845 von Siebold, interpreting these simple creatures in the light of the 

 newly established cell-theory, separated them from the Radiata, with 

 which they were classed, and applied to them the name Protozoa. 

 Biitschli (1880-1889) gave the classification of the group its present 

 form. 



The Protozoa contain 4 classes and about 8,000 species, of which the 

 majority are radiolarians. 



