COMPARISON OF ANIMAL AND PLANT BIOLOGY 129 



water snails and aquatic plants live together for a long time 

 without change of water or addition of food, is to be found 

 in the existence of the double cycle of organic matter. The 

 snails feed on the plants. (Fishes which would eat the plants 

 might be kept in the same way.) The carbon dioxide excreted 

 by the snails is used by the plants in making carbohydrates. 

 Plants also absorb from the water the nitrogenous excretions 

 of the snails. Thus the excretions of the snails furnish the 

 important elements, carbon and nitrogen, for the growth of 

 the plants. When the plants use the carbon dioxide ex- 

 cretion in starch-making there is an excess of oxygen, and 

 this, absorbed by the water, is breathed by the snails. A 

 small quantity of the oxygen in the water is absorbed by the 

 plants, especially when in darkness. The plants and the 

 snails when properly balanced in an aquarium so supplement 

 each other that they may live together indefinitely. 



Such a " balanced " aquarium wherein animals and plants 

 supplement each other is a sort of miniature of the world in 

 general, for all plants and animals have the relations of food, 

 oxygen, and excretions illustrated by the plants and animals 

 in the aquarium. 



119. Irritability or Nervous Reaction. We have pre- 

 viously noted that higher animals have a system of nervous 

 organs; but many of the lowest animals and all plants have 

 no special organs corresponding to the nervous organs of 

 higher animals in which are concentrated the powers of 

 responding to stimuli. Nevertheless, there is in all living 

 things the power of responding to various kinds of stimuli. 

 This power is known as irritability, and it seems to be one 

 of the fundamental characteristics of all living matter. 



120. Digestion occurs in both animals and plants, and in 

 each case it is a process of rendering foods soluble in water 

 in preparation for osmosis through cell-walls. In both plants 

 and animals digestion is accomplished by peculiar substances 

 (enzymes), and there is a great similarity between these sub- 



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