DIFFERENCES BETWEEN ANIMALS AND PLANTS. 17 



SECTION III. 



DIFFERENCES BETWEEN ANIMALS AND PLANTS. 



49. At first thought, nothing would seem more widely 

 different than animals and plants. What is there in com- 

 mon, for instance, between an oak or an elm, and the bird 

 which seeks shelter under their foliage ? 



50. The differences are usually so obvious, that this 

 question would be superfluous had we to apply it to only the 

 higher forms of the two kingdoms. But this contrast di- 

 minishes, in proportion as their construction is simplified ; 

 and *as we descend to the lower forms, the distinctions 

 are so few and so feebly characterized, that it becomes 

 at length difficult to pronounce whether the object we have 

 before us is an animal or a plant. Thus the sponges have 

 so great a resemblance to some of the polypi, that they have 

 generally been classed among animals, although in reality 

 they belong to the vegetable kingdom. 



51. Animals and plants differ in the relative predomi- 

 nance of the elements, oxygen, carbon, hydrogen and nitro- 

 gen, of which they are composed. In vegetables, only a 

 trace of nitrogen is found, and that merely in the seeds, and 

 some other products of the plant ; while it enters largely 

 into the composition of the animal tissues. 



52. Another peculiarity of the Animal Kingdom is, the 

 presence of large, distinctly limited cavities, destined for 

 the lodgment of certain organs ; such is the skull and 

 the chest in the higher animals, the cavity of the gills in 

 fishes, and of the abdomen or general cavity of the body, 

 for the reception of the digestive organs, which exists in all 

 animals, without exception. 



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