170 PROTOZOA 



stage can enduro unfavorable condilions such as lack of food, freezing, 

 or complete evaporation of the w ater. ^\■hen tlius protected they may be 

 blown about by the wind or carried far on the feet of birds. Hence one 

 group bears the name Infusoria, for if dr)- earth or drv plants (e.g., hay) 

 be soaked in water and tlris infusion allowetl to stand for some time, 

 a Protozoan fauna will dc\'elop in it. The encysted animals in tire earth or 

 on the plants are awakened by the moisture to new life and lea\-c the cyst. 

 Spontaneous generation, as was once believed, does not occur here, for 

 if one sterilize the materials and prevent the entrance of germs tlie water 

 will remain uninhabited. 



The protozoa are very important from the pathological standpoint. 

 Each of the four classes includes numerous parasites, the Sporozoa being 

 exclusively parasitic. jMany cause severe infective diseases (malaria, 

 relapsing typhus, sleeping sickness, etc.) especiiilly in the warmer climates, 

 while in the north, at least as far as man is concerned, the bacterial 

 diseases predominate. iNIany protozoan diseases are 'inherited,' that is 

 the egg cells are infected by the parasites. Idiis is the case with the 

 pebrine disease of silkworms, the Texas fever of cattle and others. 



Historical. — On account of their !n:isibility the Protozoa were unknown 

 until 1675; they were discovered in infusions by Leeuwenhoek, the discoverer of 

 the microscope. Wrisberg called them Animalcula infusoria — infusion animals, 

 and Siebold gave them the name Protozoa. Ehrenberg maintained that the 

 Protozoa, like all animals, possessed alimentary canal, ner\ous system, muscles, 

 excretorv and sexual organs. Dujardin denied all this and recognized in them 

 only a single homogeneous substance as sulhcient to jiroduce all vital j.hc- 

 nomena. Siebold discovered that the Protozoa were unicellular. 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 most important link, the first of the chain. 



The dilTerent appearances of Protozoa depend upon the degree of organ- 

 ological and histological differentiation. Since these are most prominent in the 

 nourishing and locomotor structures, these become im]iortant in subdividing 

 the group. In accordance with the motion and taking of food bv pseudo]iodia, 

 fiagella or cilia, there are three classes: Rhizopoda, Flagellata and Ciliata 

 (Infusoria, s. str.). To these are added the Sporozoa, modified in motions and 

 mode of feeding by parasitism. Undoubtedly Rhizopoda, I'lagellata and S]hi- 

 rozoa are much closer to each other than are the Ciliata; hence thev are grouped 

 as Plasmodroma or Cytomorpha in contrast to Ciliomorpha or Cytoida. 



Class I. Rhizopoda. 



First of the Protozoa arc those organisms which lack permanent struc- 

 tures for locomotion ami nourishment, the ])rotoi.)lasm of the boily per- 

 forming these functions. The term Rhizopoda refers to the fact that the 

 protoplasm sends out rootdike j)rocesses or psciidopOiUa for locomotion and 



