CECOLOGY 159 



which often Hve in animals. The radiolarians so constantly contain green- 

 er yellow-colored cells that for a long time these were regarded as constituent 

 parts of the animal. Similar yellow and green cells inhabit the stomach epithe- 

 lium of many actinians, corals, and even of many worms. The Zooxanthellse 

 are nourished by the carbon dioxide formed by the animal tissues, and breathe 

 out oxygen, which in turn is used by the animal; further, they form starch and 

 other carbohydrates, and any surplus thus formed may be food for the animal. 

 Thus there is on a small scale that cycle whicli exists on a grand scale in nature 

 between the animal and vegetable kingdoms. By aid of chlorophyl and sunlight 

 plants decompose water and carbon dioxide and form from them oxygen, which 

 they respire, and compounds rich in carbon, which they store in their tissues: 

 they are reducing organisms. On the contrary, animals give off carbon dioxide 

 and water, but take their oxygen from the air, and carbon compounds in their 

 food; they use oxygen to break down the chemical combinations, to oxidize: they 

 are oxidizing organisms. This explains why the favorable influence of plants upon 

 animals ceases immediately when they change the character of their metabolism. 

 With the disappearance of their chlorophyl fungi and bacteria have lost the 

 power of reducing carbon dioxide; they derive their food from other organisms 

 and decompose this into carbon dioxide, water, etc.; like animals, they are oxi- 

 dizing organisms, and consequently dangerous competitors and are the cause 

 of many serious ailments. 



IV. ANIMAL AND PLANT. 



Distinction between Animal and Plant. — The consideration of symbiosis 

 leads to the fact that a distinction exists between plants and animals in the mode 

 of metabolism, which may be expressed thus: plants usually take in carbon 

 dioxide and give off oxygen, while animals absorb oxygen and give out carbon 

 dioxide. Hence it might be concluded that it is easy to discover universal 

 distinctions between plants and animals. But the more one studies this ques- 

 tion, the more difficult becomes its solution. The old 

 zoologists believed that there are organisms which stand 

 on the limits between the animal kingdom and the 

 vegetable, and named these zoophytes or plant-ani- 

 mals. Now v/e know that these are true animals with 

 but a superficial similarity to plants; but, by means of 

 the microscope, we have become accjuainted with 

 numerous lower organisms, and it is still doubtful in 

 which of the two realms some of these, like the Myxo- 

 mycetes and many Flagellata, belong. 



Physiological Distinctions. — In the search for 



distinctions both physiological and morphological 



characters may be considered. Starting from the 



physiological side Linnaeus ascribed to plants only the 



capacity of reproduction and nutrition, but to animals ^'^'^"\^l^'~''^^^ '^"f'C 



the power of motion and sensation in addition. Now ■'"'^ ? ^\ X ™ '"'^ 



i , , , 1-, -1 , T ■ c, carina; t, tergum; s, 



we know that vegetable, like animal, protoplasm is scutum 



irritable and is capable of motion, as is shown by the 



active movements of the lower Algse, the great sensitiveness of the Mimosa, and 



other plants; but further, we know that even many highly organized animals, 



e.g., Crustacea (fig. 115), lose the power of locomotion and become fixed, and 



many fixed forms, e.g., the sponges (fig. 88), appear immovable and unaffected 



by stimulation; thus the so-called animal functions cannot be regarded as 



affording accurate distinctions. 



