156 REPRESENTATIVE SINGLE-CELLED ANIMALS 



reasons for believing that some of the Mastigophora are more 

 primitive from an evolutionary standpoint. Notable for their 

 simplicity are the members of the genus Amoeba (Fig. 86). The 

 following descriptions of the structure and activities of Amoeba 

 proteus and related species are intended to supplement observa- 

 tions which are presumed to have been made in the laboratory. 

 The amoeba is composed of a gelatinous, semi-fluid substance, 

 with an outer portion, the ectoplasm or ectosarc, which is almost 

 homogeneous even under the highest powers of the microscope, 

 and an inner portion, the endoplasm or endosarc, which contains 

 vacuoles and particles of various sorts in addition to the nucleus. 

 The ectosarc appears as a firmer substance, in spite of its manner 

 of flowing as the amoeba moves, and the endosarc as a more fluid 

 region in which the endosarcal particles are suspended. Some 

 investigators have claimed that there is an outermost layer, 

 so thin that it cannot be seen, but demonstrable because objects 

 which stick to the surface move at a different rate from that of the 

 underlying ectoplasm. Others question the existence of this outer- 

 most layer. 



Like other cells, the amoeba, therefore, possesses a nucleus and 

 cytosome or cell body, but the cytosome is differentiated into 

 ectoplasm and endoplasm. The nucleus contains chromatin, 

 arranged in a characteristic manner in different species. The 

 cytosome is composed of cytoplasm, in which may be recognized 

 inclusions of various sorts, as is the case with most cells in other 

 animals. The larger bodies suspended in the endoplasm are of 

 several kinds: the /ood vacuoles, in which the digestion of ingested 

 food occurs; the contractile vacuole, connected with the ectoplasm; 

 ordinary water vacuoles', various granules; and, particularly, 

 crystals of definite shapes, which are perhaps distinctive for par- 

 ticular species of amoeba. The significance of these several parts 

 will be discussed in connection with their activities. 



Movements, Locomotion, and Behavior. — The apparently 

 simple manner in which an amoeba effects locomotion by the 

 " flowing " of its irregularly shaped body has attracted attention 

 since the animal was first observed by the early microscopists, who 

 called it the " proteus animalcule," meaning " changing little 

 animal." We seem to have before our eyes, in the amoeba, the 

 simplest of all forms of protoplasmic movement, and one, perhaps, 

 reducible to physico-chemical phenomena. However, the simpler 



