372 GENERAL ZOOLOGY 



crystals and grains of sand. The ectoplasm is flexible and 

 transparent but somewhat tougher and denser than the 

 endoplasm. The larger granules in the endoplasm represent 

 food substances that have been engulfed and that are in the 

 process of digestion. The spaces occupied by the food, the 

 food vacuoles, are temporary digestive organs which disappear 

 when the food is digested. Near the surface of the body is a 

 clear circular area, which disappears and reappears at regular 

 intervals. This is the contractile vacuole. Careful observa- 

 tion shows that when the vacuole contracts, the contents are 

 expelled through the ectoplasm. The vacuole then fills from 

 the confluence of streams of fluid drawn from the cytoplasm, 

 and accumulating at the site of the contractile vacuole. The 

 fluid is water containing small amounts of metabolic products 

 dissolved in it. Water in regulated amounts probably enters 

 the amoeba through the surface, though this cannot be demon- 

 strated by direct observation. The vacuole seems to be the 

 focal point where metabolic products dissolved in water 

 accumulate before being expelled. 



There are no special respiratory organs in Protozoa for 

 absorbing molecular oxygen and giving off carbon dioxide; such 

 respiratory changes are functions of the surface of the body. 

 In this respect the amoeba behaves in the same way as a cell 

 in the body of a metazoan, with the difference that in the latter 

 oxygen is absorbed from the blood and carbon dioxide is given 

 off into the blood, while in the amoeba, the water in which the 

 animal lives supplies it directly with oxygen and absorbs carbon 

 dioxide from it. A nervous system and sense organs are 

 lacking in amoeba, yet the animal reacts to stimulation of 

 touch, light, heat, etc., by movements toward or away from 

 the source of stimulation, thus indicating that irritability 

 and contractility are present as properties of the animal as a 

 whole. There is no indication of differentiation in the cyto- 

 plasm to account for the conduction of nervous impulses or 

 the contractions that characterize the active animal. The 

 semifluid character of the animal precludes the development of 

 rigid structural features. Even in the case of the contractile 

 vacuole, which has the appearance of stability, microscopic 

 examination fails to reveal any structure that might account 

 for its functional activity. 



