CHAPTER XVIII 



PROTOZOA 



Vocabulary 



Protozoa, " first animals," that is, simplest in structure: one-celled. 

 Microscopic, minute, so small as to be seen only with microscope. 

 Fission, reproduction by division into two parts. 

 Conjugation, reproduction by union of parts of the nucleus. 

 Stagnant, not flowing, as applied to water. 

 Vacuoles, bubble-like cavities in protoplasm, used in excretion. 



In the study of plants we have seen how various forms start in 

 a one-celled stage, the egg, and develop into very complicated 

 forms with separate tissues of various kinds of cells. We have 

 seen also that there are plants so simple that they never have 

 more than one cell, in which is performed all the functions neces- 

 sary to the plant. With animals the same conditions are found; 

 there are the very complex types such as birds, insects, and man 

 where each function has many sorts of cells (tissues) concerned in 

 its performance while at the other extreme, there are simple 

 one-celled animals, all of whose life functions are performed in their 

 single, microscopic ceils. 



These simplest forms are called the protozoa (first animals) and 

 though vastly numerous and widely distributed, they are not 

 familiar because of their small size. Small as they are they are very 

 important in nature, forming food for higher animals, acting as 

 scavengers, causing disease in a few cases, and even forming layers 

 of the earth by the deposit of their countless shells, as in the case 

 of the chalk-making forms. 



Amoeba. One of the simplest of these simple animals is the 

 amoeba which lives in the slime at the bottom of most streams 

 and ponds. Though barely visible to the naked eye, under the 



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