206 GENERAL PRINCIPLES OF ZOOLOGY. 



with its nettle-batteries the entrance to the shell, and con- 

 sequently wards off intruders which might slide into the 

 interior of the shell and become injurious to the soft hinder 

 part of the crab's body. 



Occurrence of Symbiosis. That animals in general 

 rarely live with one another in symbiosis rests mainly upon 

 the fact that the conditions of life of all animals up to a 

 certain degree are similar or identical. They all take in 

 compounds rich in carbon and nitrogen, decompose them, 

 and, in the presence of the oxygen of the air, separate them 

 into carbonic acid, water, and oxidation products contain- 

 ing nitrogen. All animals consequently are competitors in 

 the common struggle for food. For the same reason, con- 

 versely, symbiosis between plants and animals is not at all 

 uncommon. In particular there are certain lower algse, 

 the ZooxantJiclke, which often live in animals. Certain 

 rJiixopods, particularly the radiolarians, contain with such 

 constancy in their soft bodies green- or yellow-colored cells 

 that for a long time these were regarded as constituent 

 parts of the rliisopod. Quite similar yellow and green cells 

 inhabit the stomach epithelium of many actinians, corals, 

 and even of many worms. The Zooxanthella are nourished 

 by the carbonic acid which is formed by the animal tissues, 

 and breathe out oxygen, which in turn serves as food for the 

 animal ; further, they form starch and other carbohydrates, 

 and there is nothing to prevent any surplus thus formed 

 from becoming food-material for the animal. Thus, on a 

 small scale, that cycle of matter obtains which exists on a 

 grand scale in Nature between the animal and vegetable 

 kingdoms. By aid of chlorophyl and of the chemical influ- 

 ence of sunlight, the plants decompose water and carbonic 

 acid and form from them oxygen, which they breathe out, 

 and compounds rich in carbon, which they store up in 

 their tissues : they are reducing organisms. On the con- 

 trary, animals breathe out carbonic acid and water, but 

 take their oxygen from the air, and carbon compounds 

 in their food ; they use oxygen to break down the chemical 



