170 PROTOZOA 



stage can endure 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 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 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 generation, as was once believed, does not occur here, for 

 if one sterilize the materials and prevent the entrance of germs the 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. Many cause severe infective diseases (malaria, 

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

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

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

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

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



Historical. On account of their invisibility 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, nervous system, muscles, 

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

 only a single homogeneous substance as sufficient to produce all vital phe- 

 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 different 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 important in subdividing 

 the group. In accordance with the motion and taking of food by pseudopodia, 

 flagella 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, Flagellata and Spo- 

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

 as Plasmodroma or Cytomorpha in contrast to Ciliomorpha or Cytoida. 



Class I. Rhizopoda. 



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

 tures for locomotion and nourishment, the protoplasm of the body per- 

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

 protoplasm sends out root-like processes or pseudopodia for locomotion and 



