AN INTRODUCTION TO ZOOLOGY. 



Another feature often present in cells is the vacuole. 

 This is a clear space which may contain fluid, or 

 sometimes air. In plant cells there are often many 

 vacuoles, so that the remaining part of the protoplasm 

 is reduced to a mere set of threads passing between 

 the spaces. The same is the case with some animal 

 cells (see iig. 20, p. 76). In other cells a different 

 kind of vacuole is met with, which is contractile. 

 The vacuoles just described are fixed and do not 

 move, but a contractile vacuole is formed when the 

 surrounding protoplasm every now and then tightens 

 round it, and squeezes it to nothing. The object of 

 this is usually to get rid of some fluid the vacuole 

 contains, thereby squeezing it outside the cell. 



Amceba, one of the Protozoa, presents simply the 



ccs<iaoo<3 



Fig. 7. 



Pig. 8. 



P^S-'— Magnified specimeBS of Am.a\>a (small ones): m, contractile vacuole ; 

 », nucleus ; ps, pseudopodia ; /, small organisms taken in as food. The three 

 smaller specimens are shown in a more or less retracted state, the two larger 

 ones more active. All show an inner granular layer, the endoplasm, and an 

 outer clear layer the ectoplasm. (The boundary line between these appears in 

 the cut somewhat too distinctly marked.) 



„ ^-^^ 8.—0hanges of outline presented by a eingls Anaho.-. 1 to 16, during a 

 period of four minutes ; 17 to 26, during a further period of three minutes. 



cell- characters above described. Its protoplasm con- 

 tains a nucleus and a contractile vacuole ; it has the 

 power of movement, and can temporarily push out 

 parts of its substance (sarcode, as the protoplasm 



