THE AMCEBA. 



333 



III. The above-mentioned characters may be readily observed in one of the most common forms 

 of Rhizopods, namely, the Amoeba,* or proteus-animalcule, so called on account of the ever-changing 

 shapes which a well-conditioned active individual puts on while moving under the microscrope, and 

 pushing out and drawing in the various projections on its surface, sometimes like fingers or threads, 

 called pseudopods. 



There are, however, certain living atoms of pi-otoplasm so simple in condition, being quite 

 structureless, except in having constituent granules, that some naturalists have separated them from 

 the Amcebce and the other Rhizopoda in classification, and called them Monera ;t not so much in view 

 of their singleness, as on account of their unity of composition. They may, however, be intermediate 

 forms, or passages from one stage to another in the growth and development of certain animalcules. 

 Some of them may even be the germ-products of low plant-structures. 



Some appear to be so destitute of any structural features that their slime-body shows no 

 distinction between the outer and inner parts, and has no nucleus ; and their free, homogeneous, 

 jelly-like substance, in moving, stretches itself out in one direction 

 or another in lobular, finger-like, or filamentous prolongations, and 

 contracts again, either over such organic atoms as seem to be its 

 food, or towards a point where such a protruded part has adhered 

 and fixed itself. 



Some such amoeboid creatures are shown in Fig. 1 ; and 

 their elementary simplicity has originated for one kind the name 

 of Protamceba^ and for another, Protogenes. The latter is a 

 relatively large and outspread mass (three or four millimetres in 

 diameter) of such protoplasm as is known as a " plasmodium."|| 

 This is similar to the protoplasm of the much smaller Protamoeba 

 (scarcely ^th of a millimetre in width), but is made up of a com- 

 bination of many such individuals. 



Reproduction of the species is carried on either by the separa- 

 tion of individuals from the parent mass, by their splitting off, or by A, in the contracted 

 the parent dividing into two. 



Other Monera begin life like the Protamceba, but after a while 



they cease to be active, becoming quite still, and enter on what is known as the " resting- 

 stage " in Infusoria, the Amcebce, and other animalcules. In this quiescent state they are 

 round, and become enclosed in a tough coat, and are said to be "encysted," 11 until before long 

 the enclosed morsel of protoplasm resolves itself into numerous definite minute bodies, each 

 capable of living by itself when set free, and hence termed " Zoospores."** Sometimes these tiny 

 corpuscles, combining together, form a new gelatinous mass (plasmodium), like that of the parent, 

 as in Protomyxa\\ and some other relatively large Moneres, not nucleated in every stage of 

 their growth, creeping by means of their soft mobile body at first, and afterwards by the 

 contractile filaments of their sarcode, which branch out and form delicate reticulations, with 

 irregular meshes, as in some Rhizopoda. As these animalcules, closely as they may be related 

 to the Rhizopods, differ from them somewhat in their mode of growth, and in their changes 

 from one stage to another, they have been grou]>ed in some classifications under other distinc- 

 tive names, such as Myxomyceta,^ Myxoyastrea,^ and Mijcetozoa.\\\\ It is difficult for botanists 

 to regard them as belonging to the animal kingdom. 



IV. The above-mentioned lowly creatures of the Amoeba and Protamceba types show a close 

 analogy to the elementary " cell," which, in some condition or other, is known to be at the foundation 

 or commencement of all kinds of animal and vegetable tissues. A " cell " consists of a minute sac, or 

 bladder-like envelope (the "cell-wall"), and an enclosed morsel of fluid or semi-fluid gelatinous 



* Greek, amoiti: (ameilo) exchange. t Greek, monos, alone. 



+ Greek, prOtos, first ; amoeba. Greek, protos, first ; ytinomai, I am born. 



|| Greek, plasma, a formation ; eidos, appearance. T Greek, en, in ; kustis, a bladder. 



** Greek, zoe, life; s}x>ros, a seed. tf Greek, prutos, first ; myxa, mucus. 



+J Greek, mijxa, mucus ; myccs (gen. mycetos), a fungus. 



Greek, mitxa ; f/astrc, the bottom of a vase, or the hold of a ship. |!|| Greek, myccs, a fungus ; zoo, an animal. 



Fig. 1. AMCEBA PKOTEUS. 



C',l>, in motion; ;>,;>, ;>, iisciulojiod.s pushed 

 tit in action. Hiuhly magnified. 



