(ECOLOGY. 



173 



Morphological Distinctions. Turning to the morphological 

 characteristics, multicellular animals and multicellular plants are 

 readily distinguished, since the former in the germ-layers have a 

 principle of cell arrangement peculiar to them. With the appear- 

 ance of the gastrula each organism is undoubtedly an animal. 

 But in unicellular animals the arrangement of the cells is lacking, 

 and only the constitution of the single cell guides us. Now are 

 there unmistakable morphological differences between the animal 

 and the vegetable cell ? 



Plant-cells have a Cellulose Membrane. In the structure of 

 plant and animal cells an important distinction is found in the 

 fact that the- former has a cellulose membrane, but the latter is 

 usually membraneless. To this distinction must be referred in the 

 last analysis the widely different appearance of the two realms. 

 Since the plant-cell is early surrounded with a firm coat, it loses a 

 large part of its power of further changing its form; hence vege- 

 table tissues and organs are uniform in comparison with the incon- 

 ceivable multiformity which animal histology and organology 

 disclose. The numerous higher stages of organization which the 

 animal kingdom reaches, even in its lower classes, is in great part, 

 indeed, the result of the fact that the cells of animals do not 

 become encapsuled, but have preserved the capacity for more 

 varied and higher development. 



Transitions. But even here transitions are found between the 

 lower plants and animals. In the lower 

 Algae the cells have power to emerge from 

 their cellulose membrane, and to swim 

 about freely (fig. 115), before they encapsule 

 themselves anew. On the other hand, most 

 unicellular animals encyst; they pause in 

 their ordinary functions of life, become 

 spherical, and surround themselves with a 

 firm membrane, in some cases even of cel- 

 lulose. Since in both cases an alternation 

 between the encapsuled and the free-living 

 condition occurs, only the longer duration 

 of the one or of the other can lead to a 

 distinction. But here occurs the possibility 

 that undifferentiated intermediate forms ex- 

 ist ; their actual existence prevents, even yet, 

 a sharp distinction between the animal and 

 vegetable kingdoms. 



7? 



FIG. 115. (Edngonium in 

 spore-formation. (After 

 Sachs.) A.) a piece of 

 the filament of the alga 

 with escaping cell-con- 

 tents; _B, zoospore 

 formed from the con- 

 tents; C, zoospore fixed 

 and germinating. 



