714 



PROTOZOA. 



The lowest of the nucleated Protozoa are the Rhizopoda or root-footed 



animals, a name given to them on account of their manner of moving by 



means of root-like prolongations of the body. The 



The Rhizopoda. simplest of these Rhizopoda is the Amoeba, which has 



been called the Proteus animalcule because of its constant 



changes of shape. These animalcules are to be found in most rain-water 



puddles and in ponds, and when seen under the microscope might escape 



observation as mere specks of clear jelly-like matter, yet the careful observer 



will find that each such speck is capable of moving about and feeding, and, 



indeed, can be proved to be living, carrying on all those functions which, 



taken together, we designate as life. 



If a Proteus animalcule is magnified about three or four hundred times the 

 following important points can be noted : (1) It is naked, i.e., it has no 

 cuticle or shell. (2) The surface layer of protoplasm, 

 The Amoeba. though not forming a skin, is clear and glassy, and com- 

 pletely envelops the more fluid and granular inner portion 

 of the Amoeba. (3) Among these granules can be seen the nucleus and 

 foreign particles, taken in as food, the remains of such particles, and 



other bodies not understood. (4) Lastly, if the 

 Amoeba is carefully watched, a round space, like 

 a bubble, appears, increases in size, and then 

 instantaneously disappears, forming again at 

 another spot. This is the contractile vesicle 

 which is thought to collect the waste products 

 of the body and to eject them when it con- 

 tracts. 



The Amoeba is seen to change its shape 

 slowly by protruding at certain points finger- 

 like or knob-like processes which are called 

 pseudopodia or make-believe feet. Its exceed- 

 ingly slow movements over the surface of plants 

 or other submerged objects, or through the 

 water, are all made by the extension of pseudo- 

 podia in one direction and corresponding with- 

 drawal on another side, the whole body seeming 

 to flow on along its pseudopodia. If, in its course, 

 it meets with minute plant cells suitable for 

 food, it admits them into its body at any point, retains them within it for a 

 time, while it evidently assimilates some nourishment from them, and then 

 flows on, the indigestible remains of its meal being left behind. The 

 manner in which the Amoeba breathes cannot be seen, but the fact that it 

 breathes is indisputable, for if the water in which Amoebae are kept is 

 deprived of its free oxygen, the assimilation of which is what is meant by 

 breathing, the animals become rigid and lifeless. 



When, in consequence of the nourishment derived from the assimilated 

 particles, an Amoeba has grown to a certain limit, its shape becomes that of 

 a dumb-bell (Fig. 1, B), the nucleus divides into two, each end of the dumb- 

 bell containing one of the new nuclei, the connecting part narrows more and 

 more, and finally the two ends separate completely (Fig. 1, C), and each 

 half (or '* daughter ") repeats the simple life activities of the original whole 

 (or "mother* "). These terms mother and daughter at first sight appear 

 hardly justifiable, yet we now know that all reproductive processes, even in 



Fig. 1. A, B, C. 



A PROTEUS ANIMALCULB (Amoeba} 



in three stages. 



