86 TEXTBOOK OF ZOOLOGY 



Structure 



Amoeba proteus is one of the largest of the fresh-water forms. 

 Its average diameter is about M.00 iJ^ch (0.25 mm.), while its ex- 

 treme diameter is Y^o inch or barely visible as little specks to the 

 unaided human eye. The animal owes its irregular shape to the 

 fact that protrusions of its own substance are formed at its surface. 

 These are known as pseudopodia, and they are constantly changing 

 in shape in the active animal by the flowing of the protoplasm. 



Under favorable conditions the protoplasm can be differentiated 

 into two portions. The firmer, somewhat tougher outer portion, the 

 ectosarc (ectoplasm), is nearly homogeneous and includes the plasma 

 membrane (or plasmalemma) : the more fluid inner portion, endosarc 

 (endoplasm), is much more granular and contains the cytosome, cell 



FOOD 

 'fj VACU01_E 



I ^^ " '^T^'- '^ VACUOLE 



I 1 Z-^Z-Z^^^> ^ NUCl_EUS 



> f '.'©**« V-''^^S? -^ PSEUDOPODIUM 



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U", 



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^i^ ECTOPL.ASM 



Fig. 33. — Drawing to show the appearance and structure of a living Amoeba 



proteus. 



inclusions as well as the nucleus. The larger bodies in the cytosome 

 are food vacuoles, single, shiny, contractile vacuoles containing 

 watery fluid and varying in size; water vacuoles; various granules; 

 mitochondria; fat globules; and crystals. Some authors distinguish 

 two types of protoplasm in the endosarc; the inner more fluid, plas- 

 masol in which the streaming movements take place and, surrounding 

 this a more viscous, passive portion, the plasmagel. The nucleus 

 usually appears somewhat dense and granular, and is located in the 

 portion away from the end which is advancing in a moving specimen. 



Metabolism 



This refers to the constant building up (anabolism) of living 

 protoplasm and its concurrent oxidation (eatabolism). It includes 

 all activities necessary for maintenance of itself and its race. These 



