U>4 REPRESENTATIVK SIXGLE-CELLKD ANIMALS 



known that ivprodiiotioii occurs by cell din'sion or binaiy fission, 

 by which the coll divides into equal halves (Fig. 80 A). Encyst- 

 metit. or the endosun^ of the cell in a (\v^^/ formed as a secretion after 

 the ama^^a ha^; contracted into a sphere and become quiescent, 

 also occurs in some sjxvies of amcvba that have been studied (Fig. 

 SO B 4). The formation of flagellated cells and their union in 

 conjugation, a process comparable with the union of egg and 

 sperm cells in many-celled animals, is known in some of the 

 Sarcodina (,Fig. SO CV However, the details of the life cycle in all 

 its particulars have not yet been established in any one instance 

 for Amcvba prokus and similar fresh-water species. While more 

 is known regarding some of the parasitic species in this sub- 

 division of the Sarcodina. it is not safe to conclude that the same 

 stages occiu" in free-living forms, although thei-e is doubtless some 

 parallelism. 



If we now summarize what has been learned concerning the 

 amcvba in the foregoing paragraphs and in the laboratory study 

 which is presumed to accompany this account: The three gi-eat 

 bodily functions are no less important for the ama^ba than for the 

 vertebrate animal. The imicellular organism cannot exhibit the 

 complexities of the many-celled state, but it does exhibit metabol- 

 ism, irritability, and reproduction, within the limits of a single cell, 

 comparable with these activities in the cells of the metazoan. 

 Waste and repair of protoplasm, growth, and reproduction of 

 the cell thus appear to be fundamentally alike at the two ends of 

 the anim;\l kingdom. Response to stimulation in an amoeba may 

 be as complex as it can be demonstrated to be in a majority, if not 

 all. of the cells of more complex animal bodies. The activities 

 of protoplasm and hence of cells are, therefore, similar in the 

 simplest and in the most complex animals. 



Other Sarcodina. — In the subdivision of the Sarcodina known 

 as Rhizopoda {cf. p. 154), there are many forms that resemble 

 the amcTba. Others, like the genera Arcella and Difflugia (Fig. 

 So), possess shells into which the animal can withdraw and from 

 the mouth of which issue pseudopodia. In another type, called 

 the Foraminifera, there is a shell composed of carbonate of 

 lime, through which pseudopodia pass by one or by numerous 

 openings. These psuedopodia form networks in which the food 

 particles are ingested. With few exceptions, the Foraminifera are 

 marine, occurring in the open ocean in unbelievable niunbers. 



