138 GENERAL ZOOLOGY 



in the succeeding generations to form other amoebas and 

 after a time maturity, old age, and death are attained. 

 Any one individual is young, or mature, or old, through all 

 its " life." 



If binary fission were the only process concerned in the 

 reproduction of amoebas, they would probably all have died 

 of old age long ago. But there is another way in which in- 

 dividuals may be produced. This is by conjugation. At 

 intervals two amoebas come together and, after a prepara- 

 tory or ripening process, exchange some nuclear material. 

 When they separate again they are not the same, for each 

 contains part of the other. There is also another and more 

 important change resulting from conjugation. After it is 

 completed each of the conjugants is young, no matter what 

 its condition was before. An amoeba may be mature or in 

 the last stages of old age, and after conjugation be rejuve- 

 nated, starting anew with all the vigor of youth. 



When compared with other rhizopods (Fig. 62), the 

 amoeba shows great simplicity of structure. It is a 

 bit of naked protoplasm with a nucleus and a contrac- 

 tile vacuole. Some of its relatives have little shells about 

 the body. Arcella (B) has a hemispherical shell composed 

 of chitin; Difflugia (C) encloses itself by sticking sand 

 granules together to make a beautiful little case. In one 

 order of rhizopods, the Foraminifera, a calcareous shell 

 is secreted within the cytoplasm of the cell. These shells 

 are of various forms in different species like rods, wheels, 

 strings of beads, etc. The largest protozoan known is a 

 fossil foraminifer, Nummulites, which is disc-shaped and 

 sometimes' has a diameter of about two inches. Chalk 

 deposits like those at Austin, Texas, are made up mostly 

 of the shells of Foraminifera. The rhizopods in the order 

 Heliozoa (D) do not have a shell but possess very slender 

 pseudopodia which radiate from the body and have per- 

 manent rod-like supports in them. The Radiolaria are 

 marine rhizopods which have slender pseudopodia like 



