CHAPTER V 



THE PROTOZOA 

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Free living Protozoa, Amoeba and Paramaecium Parasitic Protozoa, 

 Monocystis and Plasmodium. 



WE have already seen that the Protozoa comprise the 

 simplest animal forms. In their adult stage they are composed of a 

 single cell with one or more nuclei ; or, if of more than one cell, 

 then they form an aggregation of equivalent cells. They are for the 

 most part very small, invisible to the naked eye and requiring a 

 microscope to make them out ; but in spite of their small size and 

 simplicity each one is a complete living unit. Each possesses all the 

 characteristics of living matter, and each is capable of exhibiting all 

 the vital phenomena. A certain amount of specialisation within 

 the limits of the cell is nearly always met with, and sometimes quite 

 a number of cell organs are produced each serving some special 

 function. They are widely spread over the whole surface of the 

 globe from the arctic regions to the tropics, in fresh and salt water, 

 as well as in or on the soil, but, as a rule, a fair amount of moisture 

 is necessary for them to keep alive. A very large number of them 

 live on and in other animals, and they bring about a diseased con- 

 dition in the animal in which they live. Those producing diseases 

 in man are naturally of much importance from the medical point of 

 view. For convenience of study we may divide the Protozoa with 

 which we are immediately concerned into free-living and parasitic 

 forms, although this is by no means a natural classification. To start 

 off with we will take Amceba, which is a lowly and little specialised 

 form. 



Free-Living Protozoa i. Amoeba. 



The genus Amceba contains a number of species, some even 

 parasitic, and one of the largest and most common is A . proteus, the 

 " proteus animalcule " as it was termed by the older naturalists on 

 account of one of its most striking characteristics, that of constantly 

 altering its shape. It is found on the surface of the mud and 

 decaying vegetable matter in ponds and ditches. The animal is 



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