726 THE POPULAR SCIENCE MONTHLY. 



ed, we seek in vain. Yet watch it for a moment as it lies in a drop of 

 water beneath our microscope. Some living denizen of the same drop 

 is in its neighborhood, and its presence exerts on the protoplasm of 

 the Amoeba a special stimulus which gives rise to the movements ne- 

 cessary for the prehension of nutriment. A stream of protoplasm 

 instantly runs away from the body of the Amoeba toward the destined 

 prey, envelops it in its current, and then flows back with it to the cen- 

 tral protoplasm, where it sinks deeper and deeper into the soft, yield- 

 ing mass, and becomes dissolved, digested, and assimilated in. order 

 that it may increase the size and restore the energy of its captor. 



But again, like all living things, Amoeba must multiply itself, and 

 so after attaining a certain size its nucleus divides into two halves, and 

 then the surrounding protoplasm becomes similarly cleft, each half 

 retaining one half of the original nucleus. The two new nucleated 

 masses which thus arise now lead an independent life, assimilate nutri- 

 ment, and attain the size and characters of the parent. 



We have just seen that in the body of an Amoeba we have the 

 type of a cell. Now, both the fresh waters and the sea contain many 

 living beings besides Amoeba which never pass beyond the condition 

 of a simple cell. Many of these, instead of emitting the broad, lobe- 

 like pseudopodia of Amoeba, have the faculty of sending out long, 

 thin threads of protoplasm, which they can again retract, and by the 

 aid of which they capture their prey or move from place to place. 

 Simple structureless protoplasm as they are, many of them fashion for 

 themselves an outer membranous or calcareous case, often of symmet- 

 rical form and elaborate ornamentation, or construct a silicious skele- 

 ton of radiating spicula, or crystal clear concentric spheres of exqui- 

 site symmetry and beauty. 



Some move about by the aid of a flagellum, or long whip-like pro- 

 jection of their bodies, by which they lash the surrounding waters, 

 and which, unlike the pseudopodia of Amoeba, can not, during active 

 life, be withdrawn into the general protoplasm of the body ; while 

 among many others locomotion is effected by means of cilia micro- 

 scopic vibratile hairs, which are distributed in various ways over the 

 surface, and wbich, like the pseudopodia and flagella, are simple pro- 

 longations of their protoplasm. 



In every one of these cases the entire body has the morphological 

 value' of a cell, and in this simple cell reside the whole of the prop- 

 erties which manifest themselves in the vital phenomena of the organ- 

 ism. 



The part fulfilled by these simple unicellular beings in the economy 

 of nature has at all times been very great, and many geological for- 

 mations, largely built up of their calcareous or silicious skeletons, bear 

 testimony to the multitudes in which they must have swarmed in the 

 waters of the ancient earth. 



Those which have thus come down to us from ancient times owe 



