160 



GENERAL PRINCIPLES OF ZOOLOGY 



Even the difference in metabolism is by no means sufficient. Every plant 

 has a double exchange of material. In its movements and other vital functions 

 the vegetable protoplasm produces carbon dioxide and consumes oxygen; at the 

 same time there goes on, under the influence of sunlight and of chlorophyl, the 

 reduction of carbon dioxide and the giving off of oxygen. In chlorophyl- 

 containing plants the reducing process preponderates so considerably during the 

 day that they give off a quantity of oxygen, and only at night, when the reducing 

 process becomes interrupted on account of the lack of sunlight, does the pro- 

 duction of carbonic-acid compounds become perceptible. But if the chlorophyl 

 be absent the reducing processes disappear; chlorophylless fungi and bacteria, 

 have, therefore, the same metabolism, so far as carbon dioxide is concerned, as 

 animals. So also it is incorrect to say that only plants have the power to make 

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

 highly organized group of tunicates and even among the arthropods. 



Morphological Distinctions. Turning to the morphological characters, 

 multicellular animals and multicellular plants are readily distinguished, since 

 the former in the germ-layers have a principle 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 can guide 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 membrane- 

 less. 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 vegetable tissues and organs, 

 in spite of the manifold' intracellular differentiations, 

 like the chlorophyl granules, are uniform in comparison 

 w^ith the inconceivable multiformity which animal 

 structures disclose. The higher stages of organization 

 which the animal kingdom reaches, even in its lower 

 classes, is in great part the result of the fact that the 

 cells of animals do not become encapsuled, but have 

 preserved the capacity for more varied and higher de- 

 velopment. But even here transitions are found be- 

 tween the lower plants and animals. In the lower 

 Algae the cells can leave their cellulose membrane, and 

 swim about freely (fig. 116), before they enclose them- 

 selves anew. On the other hand, most unicellular 

 animals encyst; they pause in their ordinary functions, 

 become spherical, and surround themselves with a firm 

 membrane, sometimes even of cellulose. Since in both 

 cases an alternation between the encapsuled and the 

 free condition occurs, only the longer duration of the 

 one or of the other can lead to a distinction. But here occurs the possibility 

 that indifferent intermediate forms exist; their actual existence prevents, even 

 yet, a sharp distinction between the animal and vegetable kingdoms. 



V. GEOGRAPHICAL DISTRIBUTION OF ANIMALS. 



The Different Faunal Regions. Even a superficial knowledge cf the 

 distribution of animals shows that the animal population, the fauna, in different 



FIG. 1 1 6 . CEdogo- 

 nium in spore-formation 

 (after Sachs) . A , a piece 

 of the alga with escap- 

 ing cell-contents: B, 

 zoospore formed from 

 the contents; C, zoospore 

 fixed and germinating. 



