UNICELLULAR ORGANISMS 



59 



Amoeba. This is very unusual, however, and has been seen by 

 only one observer (Sheel). In this method the animal draws in 

 its pseudopodia, assumes a spherical form and secretes around 

 itself a thin shell called a cyst. Inside this cyst the nucleus 

 divides into many parts, some five or six hundred nuclei thus 

 finally arising by division. After this the rest of the substance 

 divides so that each nucleus finally becomes surrounded by a 

 little protoplasm, the contents of the cyst coming thus to con- 

 sist of some hundreds of little bodies, each with its nucleus. 

 Eventually the cyst bursts and the little cells escape, each being 

 now a minute Amoeba, which has only to grow, to be like the 

 original. This method of reproduction is also evidently a divi- 

 sion. It is a type of division called spore formation. The whole 

 process takes two and a half to three months, and the condi- 

 tions which bring it about are unknown. 



FIG. 20. SINGLE-CELLED ANIMALS RELATED TO AKKEBA 



A, Difflugia, an Ama>ba-\ike animal with a shell made of pebbles; B and C, Podophrya 

 and Acineta, animals with stiff protruding tentacles of protoplasm; /, food; D, Arcella, an 

 Amoeba-like animal with a secreted shell. 



PARAMBCIUM 



Paramedum can usually be found in the same localities as 

 Amoeba and can easily be obtained by allowing lily pads to 

 decay in a dish of water. A quantity of living organisms soon 



