84 MANUAL OF PHYSIOLOGY. 



erable rapidity from the body, commonly encircle particles of 

 nutrient material, and then closing in around them, press them 

 into the midst of the granular central mass. Here they sojourn 

 some little time, and during this period, no doubt, any nutritive 

 properties they possess are extracted from them, and they are 

 then ejected from the plastic substance. This form of assimila- 

 tion demands no previous preparation of the food such as we shall 

 see takes place in the alimentary tract of man, and in the special 

 organs of the higher animals ; yet it is a form of digestion ade- 

 quate at least to the requirements of this simple organism. The 

 repeated alteration of the different parts of the protoplasm in 

 relation to one another and the surrounding medium during the 



FIG. 40. 



Two different forms of Amoebae in different phases of movement. Those on 

 the left after Cadiat. A and B show an outer clear zone (Gegenbaur). 



flowing hither and thither of the currents, produces not only a 

 change in the shape and position of the animal, but also acts as 

 a means of distributing the nutriment to the different parts of 

 the body, and of collecting and carrying to the surface the vari- 

 ous products of tissue-decomposition ; thus the streaming proto- 

 plasm does the work of a circulating fluid such as we see in the 

 more elaborate organisms for the distribution of nutriment and 

 elimination of waste materials. The surface of the amoeba is 

 sufficient to allow of the gas-interchange necessary for life, and 

 by means of the ever-changing material exposed, sufficient oxy- 

 gen is taken for its tissue combustions, and so a function of respi- 

 ration is established. The growth that results from the perfect 



