PROTOZOA 313 



size, and inclose granules of food material. At intervals clear globules 

 may be seen to gradually form within the cytoplasm and then suddenly 

 contract and disappear. These are the contractile vacuoles which on 

 contracting empty their fluid contents to the exterior. They are rudi- 

 mentary cell organs for the elimination of injurious substances and differ 

 from the food vacuoles in having a definite place in the cell as well as in 

 their approximately constant number. Young amebae usually have 

 within the endoplasm a single nucleus but they may early become 

 multinucleate. All of the vital functions appear to be under the con- 

 trol of the nucleus; experimental removal of the nuclei has shown that 

 Protozoa thus treated cannot properly perform their functions and soon 

 perish. 



In feeding the ameba merely flows around the object which it is to 

 use as food; becoming thus inclosed in the cytoplasm the nutritive 

 elements are digested and assimilated. Circulation is limited to the 

 streaming movements of the cytoplasm, and respiration is carried on by 

 absorption of oxygen from the surrounding water. 



Reproduction in ameba is by fission or budding. Before division of 

 the cell changes occur in the nucleus involving a separation of the 

 nuclear parts with the formation of two distinct nuclei. These separate 

 and during the process the cell constricts, finally dividing completely 

 with each part inclosing one of the new nuclei. In some cases the cell 

 becomes spherical and secretes a protecting membrane around itself 

 before division; the outer membrane becomes hard and adapted to re- 

 sist drying and extremes of temperature, the organism assuming in this 

 condition a resting or encysted stage. Encysted individuals usually 

 divide into more than two; there may be four, eight, or even hundreds 

 of small amebae resulting from the reproductive process. In multi- 

 nucleate forms it frequently happens that the division is into as many 

 parts as there are nuclei. 



Parasitism of the Protozoa 



In 1881 Laveran, a physician in the French army, distinctively 

 directed attention to the Protozoa as a cause of disease in animals by 

 his discovery that the cause of malaria in man is a protozoan which, 

 entering the red blood cells, destroys them and in this way causes the 

 anaemia characteristic of the disease. Later it was demonstrated that 

 this malarial organism is transmitted by a mosquito and that this is the 

 only way that the disease can be acquired. This discovery served to 

 indicate lines of research looking to insects and other arthropods as 

 essential carriers of other forms of pathogenic Protozoa, in which field 

 much has already been accomplished. 



Theobald Smith, in 1892, found that Texas fever of cattle is caused 



