24 STRUCTURAL AND SYSTEMATIC ZOOLOGY 



the body there will usually be seen a large, clear, appar- 

 ently empty circle, which from time to time suddenly 

 contracts and disappears from view. This is the con- 

 tractile vacuole, and is supposed to be an organ of excre- 

 tion, since uric acid, one of the forms in which nitrog- 

 enous waste material leaves the body in higher animals, 

 has been found in the vacuoles of certain animals closely 

 related to the amoeba. The vacuole, though apparently 

 disk-shaped, is really spherical. Closer examination will 

 show a small round mass, usually slightly darker than the 

 rest of the body and often distinctly and evenly dotted 

 with fine points, and surrounded by a plainly defined 

 line. This is the nucleus surrounded by its membrane. 

 Its shape is constant, except when the amoeba is in the 

 process of division. Still more careful study will show 

 that the body substance can be rather sharply divided 

 into two regions, an outer, clear, quite homogeneous 

 portion, the ectoplasm, and an inner, granular region, 

 the endoplasm. 



If the animal be watched for a short time, it will proba- 

 bly be seen to ingest food particles, or, possibly, capture 

 another animalcule. In either case, the mode of pro- 

 cedure should be watched and the fate of the captured 

 particle followed. Some of the ingested material will 

 be disgorged, while certain pieces will be seen slowly to 

 disintegrate and to disappear as they dissolve in the 

 droplet of water in which the animal swallowed them. 

 From the behavior of these particles and from the 

 changes seen to take place in substances which have 

 been given the amoeba for experimental purposes, it is 

 believed that the animal produces in its body substances 

 analogous to the digestive juices of higher forms. The 

 disintegration of the food particles, then, indicates that 

 they are being digested. When they have reached the 

 stage of solution, they can, of course, no longer be seen. 



