186 PROTOZOA. 



are aquatic. A few forms, like Amoeba terricola, are terrestrial, and 

 these only occur in moist places. Salt and fresh water, of the latter 

 .stagnant pools rich in vegetation, are the favorite places for Pro- 

 tozoa. The fresh-water forms are cosmopolitan, so that the forms 

 in the most diverse lands are very similar. This depends upon cer- 

 tain peculiarities. The fresh-water Protozoa can become encysted 

 independent of reproduction, and in the encysted stage can endure 

 times of unfavorable conditions such as lack of food, freezing, or 

 complete evaporation of the water. When thus protected they 

 may be blown about by the wind or carried far on the feet of 

 birds. Hence it is that one group bears the name Infusoria, for if 

 dry earth or dry plants (e.g., hay) be soaked in water and this 

 infusion allowed to stand for some time, a more or less rich 

 Protozoan fauna will develop in it. The encysted animals in the 

 earth or on the plants are awakened by the moisture to new life 

 .and leave the cyst. Spontaneous germination, as was once believed, 

 does not occur here, for if one sterilize the materials and prevent 

 xthe entrance of germs the water will remain uninhabited. 



Historical. On account of their practical invisibility the Protozoa were 

 unknown until 1675 ; they were discovered in infusions by the Dutch 

 Leeuwenhoek, the discoverer of the microscope. Wrisberg in the eight- 

 eenth century called them Animalcula infusoria infusion animals, and 

 Siebold in the century just closed gave them the name Protozoa. The 

 proposition of Haeckel to place a portion of the Protozoa in a kingdom 

 Protista between animals and plants has found but little acceptance. In 

 the accounts of the structure the views of Dujardin and Ehrenberg were 

 long at variance. Ehrenberg maintained with all confidence that the 

 Protozoa like all animals possessed the most important organs, alimen- 

 tary canal, nervous system, muscles, excretory and sexual organs. Du- 

 jardin denied all this and ascribed to the Protozoa only a single homo- 

 geneous substance, ' sarcode ' (p. 60) which was sufficient to produce all 

 vital phenomena. Dujardin's view later found important support in 

 ^iebold's discovery that the Protozoa were unicellular. Still for a long 

 time Ehrenberg's ideas persisted in various modified forms and were not 

 totally overthrown until after the middle of the nineteenth century. The 

 fact that there are unicellular animals without organs and yet capable of 

 existence was an extremely valuable addition to knowledge, for it not only 

 broadens our conception of animal life, but it furnishes ; for the theory of 

 evolution from simple organisms the strongest link in the chain, the first 

 of the series. 



''TJie -different appearances of Protozoa depend upon the grade of 

 organological and histological differentiation. Since these are most promi- 

 nent in the nourishing and locomotor structures, these become important 

 in subdividing the group. The organs for these purposes pseudopodia, 



