96 PROTOZOA THE SIMPLEST ANIMALS. 



condensation of reproductive possibilities, may produce ova which 

 develop parthenogenetically. 



Here, then, we have an organism, on the border line between plant 

 and animal life, just across the line which separates the unicellular from 

 the multicellular, illustrating the beginning of that important distinc- 

 tion between somatic or body cells and reproductive cells, and occurring 

 in asexual, hermaphrodite, and unisexual phases. Klein records no less 

 than twenty-four different forms of V. aureus from the purely vegetative 

 and asexual to the parthenogenetic, for there may be almost entirely male 

 colonies, almost entirely female colonies, and other interesting transi- 

 tional stages. Klein has also succeeded to some extent in showing 

 that the occurrence of the various reproductive types depends on outside 

 influences. 



General Classification of Protozoa. 



Since the Protozoa are unicellular organisms (except the 

 few which form loose colonies), their classification should 

 be harmonious with that of the cells in a higher animal. 

 This is so. Thus (a) the Rhizopods, in which the living 

 matter flows out in changeful threads or " pseudopodia," as 

 in the common Amoeba, are comparable with the white 

 blood corpuscles or leucocytes, many young ova, and other 

 " amoeboid " cells of higher animals ; (b) the Infusorians, 

 which have a definite rind and bear motile lashes (cilia 

 or flagella), e.g. the common Paramecium, may be likened 

 to the cells of ciliated epithelium, or to the active sperma- 

 tozoa of higher animals ; (c) the parasitic Sporozoa, which 

 have a rind and no motile processes or outflowings, may 

 be compared to degenerate muscle cells, or to mature ova, 

 or to "encysted" passive cells in higher animals. 



This comparison has been worked out by Professor Geddes, who also 

 points out that the classification represents the three physiological 

 possibilities — (a) the Amoeboid units, neither very active nor very passive, 

 form a median compromise ; (b) the ciliated Infusorians, which are 

 usually smaller, show the result of a relative predominance of expendi- 

 ture ; (c) the encysted Gregarines represent an extreme of sluggish 

 passivity. 



But, as Geddes and others have shown, the cells of a higher animal 

 often pass from one phase to another, — the young Amoeboid ovum 

 accumulating yolk becomes encysted, the ciliated cells of the windpipe 

 may, to our discomfort, sink into amoeboid forms. The same is true of 

 the Protozoa ; thus in various, conditions the ciliated or flagellate unit 

 may become encysted or amoeboid, while in some of the simplest fonns, 

 such as Protomyxa, there is a " cell-cycle" in which all the phases occur 

 in one life history. 



It is also important to notice Professor Ray Lankester's division of 



