SINGLE-CELLED ANIMALS AS ORGANISMS 169 



fission. M, mouth ; 

 MAC., macronucleus ; 

 MIC., micronucleus. 

 (After Sedgwick and 

 Wilson.) 



structure, consisting of one large and one 

 small portion, called, respectively, the ma- 

 cronucleus and the micronucleus. 



Reproduction of Paramoecium. Some- 

 times a paramcecium may be found in the 

 act of dividing by the process known as 

 fission, to form two new cells, each of which 

 contains half of the original cell. This is a 

 method of asexual reproduction. The origi- 

 nal cell may thus form in succession many 

 hundreds of cells in every respect like the Paramoecium dividing by 

 original parent cell. 



Amoeba. 1 In order to understand more 

 fully the life of a simple bit of protoplasm, 

 let us take up the study of the amoeba, a 

 type of the simplest form of animal life. Unlike the plant and 

 animal cells we have examined, the amoeba has no fixed form. 



Viewed under the compound micro- 

 scope, it has the appearance of an 

 irregular mass of granular proto- 

 plasm. Its form is constantly 

 changing as it moves about. This 

 is due to the pushing out of tiny 

 projections of the protoplasm of 

 the cell, called pseudopodia (false 

 feet). The locomotion is accom- 

 plished by a streaming or flowing 

 of the semifluid protoplasm. The 

 pseudopodia are pushed forward in 

 the direction which the animal is 



Amoeba, with pseudopodia (P.) ex- 

 tended ; EC, ectoplasm ; END, en- to gO, the rest of the body follow- 

 doplasm; the dark area (tf.) is mg j n the centra l part O f the 

 the nucleus. (From a photograph 

 loaned by Prof essorG.N. Calkins.) Cell IS the nucleus. This im- 



1 Amoebae may be obtained from the hay infusion, from the dead leaves in the bot- 

 tom of small pools, from the same source in fresh-water aquaria, from the roots of 

 duckweed or other small water plants, or from green algae growing in quiet localities. 

 No sure method of obtaining them can be given. 



