(ECOLOGY. 209 



bonic-acid compounds become perceptible. But the re- 

 ducing processes become immediately preponderant, if the 

 chlorophyl is absent ; chlorophylless moulds and bacteria 

 have, therefore, the same metabolism as animals. 



Cellulose not a Sure Test. So also it is incorrect 

 to say that only plants have the power to make cellu- 

 lose, for cellulose is found in many lower animals, the 

 rJiizopods, and in the highly-organized group of tunicatcs ; 

 according to recent investigations, it appears to be found 

 even among the arthropods. 



Distinction on a Morphological Basis. Turning to 

 the morphological characteristics, multicellular animals and 

 multicellular plants are readily distinguished, since the 

 former in the formation of the germ-layers have a principle 

 of cell-arrangement which is peculiar to them. With the 

 appearance of the gastrula stage, each organism is char- 

 acterized as undoubtedly animal. But in unicellular ani- 

 mals 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 mem- 

 brane, 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- 

 c ;11 is early surrounded with a firm armor, 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 

 inconceivable multiformity which animal histology and 

 organology disclose. The numerous higher stages of organ- 

 ization 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 



