38 AN INTRODUCTION TO ZOOLOGY 



Habitat and Collecting. Ameba proteus lives in fresh- water 

 ponds and streams.. It may be obtained for laboratory use from 

 a variety of places, such as the organic ooze from decaying vege- 

 tation or the lower surface of lily pads. Perhaps the most cer- 

 tain method is that suggested by Professor Jennings (37). About 

 two weeks before the specimens are needed, a mass of pond weed 

 (Ceratophyllum is the best) should be gathered, placed in flat 

 dishes, and immersed in water. The vegetation soon decays, and 

 a brown scum appears on the surface. In this scum Amebas 

 may be found. 



General Anatomy. Two regions are distinguishable in the 

 body of Ameba, an outer colorless layer of clear cytoplasm, the 

 ectosarc, and a comparatively large central mass of granular cyto- 

 plasm, the endosarc. A single clear spherical body, usually lying 

 near the end of the animal away from the direction of motion, and 

 disappearing at more or less regular intervals, is the contractile or 

 pulsating vacuole (Fig. 9, 2). Suspended in the endosarc is a 

 nucleus (Fig. 9, /), usually one or more food v'acuoles (Fig. 9, 4), 

 material ready for excretion, foreign substances such as grains of 

 sand (Fig. 9, 5), and undigested particles, the amount of the 

 latter depending upon the feeding activity of the specimen at the 

 time when examined. 



From this description it will be noted that Ameba proteus 

 contains all of the essential constituents of a cell. It is, moreover, 

 simple in structure, shows a number of physiological activities 

 in their simplest form, is one of the most primitive of all animals, 

 and is easily obtained. For these reasons it has been, and still is, 

 a favorite subject for study. 



Detailed Anatomy. The ectoplasm (Fig. 9, 3) is easily dis- 

 tinguished from the endoplasm because of the absence within it 

 of granules. The ectoplasm is firmer than the endoplasm, prob- 

 ably because the outer protoplasm tends to stiffen under the 

 influence of surface tension. 



The endoplasm occupies the central portion of the body. Be- 

 ing less dense than the ectoplasm, it contains within it all of the 



