GENERAL CLASSIFICATION. 95 



time-honoured " distinctions between plants and animals " 

 are apt to be wearisome, I have condensed them in a 

 table. 



General Classification. — Since the Protozoa are for the 

 most part single cells, it is evident that their classification 

 should be harmonious with that of the units in a higher 

 animal. There are three series — (a) those in which the living 

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

 as in the common Amoeba, which we compare with the 

 white blood-corpuscles or leucocytes, many young ova, and 

 other " amoeboid" cells of higher animals ; {b) those in which 

 the units, bounded by a definite rind, bear motile lashes (cilia 

 or flagella), as in the common Paramoicium, which w^e may 

 liken to the cells of «7za^«<f epithelium, or to the active sperm- 

 atozoa of higher animals ; (c) those in which the units, again 

 with a rind, have no motile processes or outflowings, viz., the 

 parasitic Gregarines, which we may compare to degenerate 

 muscle-cells, or to mature ova, or to " encysted" passive cells 

 in higher animals. 



Moreover, this threefold classification represents the three 

 physiological possibilities — (a) the amoeboid units, neither 

 very active nor very passive, forming a median compromise ; 

 {V) the ciliated Infusorians, which are usually smaller, show- 

 ing the result of a relative predominance of expenditure ; 

 (c) the encysted Gregarines representing 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 often 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 the very simplest 

 forms, such as Protomyxa, there is a " cell-cycle " in which 

 all the phases occur in one life-history. 



Therefore our classification must read as follows : — 

 (a) Rhizopods, — which are predominantly amoeboid, 

 and possess changeful but sluggish outflowing pro- 

 cesses, either blunt or thread-like. They represent 

 an equilibrium or mean between the following 

 extremes. 



