PROTOZOA 



Chapter 2 



Protozoa abound in practically every situation where life is possible but, 

 on account of their minute size, they seldom come to our attention. Only the 

 largest of them are visible without the aid of a lens. Consequently they lived 

 almost unnoticed until about 1675, when the early microscopists made for 

 themselves tiny glass drops which served them as lenses, fixed these in ad' 

 justable frames, and sought to examine every minute object available. Since 

 then many arguments have followed as to the structure of these little animals. 

 Some investigators have described vital organs within these Protozoa while 

 others, finding no cell walls within the animals, object to the terms used for 

 the multicellular forms. Some scientists regard the Protozoa as the simplest 

 forms of animal life and so place them as the first and "lowest" group of 

 animals. Other scientists, marvelling at the many functions performed by 

 the tiny creatures and the specialization they show, contend that instead of 

 corresponding to one of the body cells of the other animals, as the term "uni' 

 cellular" implies, we should rather regard a protozoan as corresponding to 

 the whole of a higher animal, but without the subdivision into cells, and 

 therefore apply the term "acellular", meaning without cells. Along the same 

 lines there is a tendency to regard the group of Protozoa not as one of the 

 twelve or more phyla but as a major division of the animal kingdom, parallel 

 to all the other animals, which are then called Metazoa. 



Few of the common Protozoa attain enough size to attract one's atten' 

 tion, except under the microscope. The largest of the celebrated amoebas, 

 when extended, may reach a diameter of one one-hundredth of an inch. 

 Spirostomum, which at certain seasons is extremely abundant in garden lily 

 ponds, is a giant among the common Protozoa, as it reaches a length of one' 

 tenth of an inch. Parameaum, the form most studied in biological classes, 

 is among the larger Protozoa, but it is still so small that eighty-five of them, 

 lined up head to tail like circus elephants, would just reach across the narrow 

 side of a postage stamp, while, if placed side by side, it would take about three 

 hundred and forty to extend from one side of the stamp to the other. 



Protozoa are classified according to their methods of locomotion. The 

 Amoeha group, the Rhizopoda or Sarcodma, have the most changeable shape, 

 usually being spherical when at rest but extending long projections, pseudo' 

 podia or false feet, when crawling or floating. Some of them, like Arcella, 

 secrete vase-like outer covers or shells. Some, like Difflugia, go still further 

 and accumulate sand grains or other foreign particles to supplement these shells. 

 The animals of this group are most common in the layers of ooze or mud on 

 the bottom of quiet ponds or swamps. 



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