INTRODUCTION. 11 



alimentary apparatus, and require, when they take in 

 nutriment, to extemporize a cavity foi its reception. 



7. The Nature of the Pood is the most widely applicable 

 of the distinctive features of the two kingdoms. This 

 distinction may be briefly stated thus : Plants are able 

 to convert inorganic matter into organic; but no animal, 

 so far as our present knowledge extends, is able to subsist 

 upon inorganic substances. The food of plants consists 

 principally of carbonic acid, water, and ammonia, together 

 with small quantities of various salts. These substances 

 are derived partly from the air, and partly from the earth. 

 The carbon of the carbonic acid is retained by them, and 

 the oxygen set free. Animals, on the other hand, deprive 

 the air of its oxygen, and give off carbonic acid. Animals 

 are able to subsist only on the complex organisms pre- 

 pared for them by plants, the food of these being of a 

 more simple character. " Plants are the great manu- 

 facturers in nature, animals the great consumers." (Dr. 

 Nicholson.) 



These distinctions, however, are not universally true. 

 On the one hand, several of the fungi are as dependent 

 on organic food as animals are; while, it is probable, that 

 some microscopic animals may be able to subsist on 

 inorganic compounds. 



8. Classification. Various systems of classification have 

 been adopted by different naturalists. Lamarck divided 

 the animal kingdom into two great divisions, the Verte- 

 brata and Invertebrata. The first of these is still re- 

 tained as a primary division, but the second contained 

 such a heterogeneous assemblage that further division 

 became necessary. Accordingly, Cuvier, another French 

 naturalist, subdivided the Invertebrata into Mollusca, 

 Articulata, and Radiata. As the new world of animal 

 life revealed by the microscope could not be assigned to 

 any of these groups, it became necessary to form a 

 separate division, to which the name of Protozoa has 

 been given. 



Careful examination showed that Cuvier's Badiata 



