ANIMALS 



The lines into which life branches do not possess the same 

 power to evolve ; along the animal pathway the three main 

 branches are those of the mollusks, arthropods and vertebrates. 

 In very ancient times each of these phyla had partly at least 

 followed the example of the plants in encasing themselves in a 

 hard external skeleton through which stimuli could with greater 

 difficulty penetrate, and because of which freedom of move- 

 ment was greatly curtailed. Before this condition had become 

 universal, however, branches developed from each, which, an- 

 ticipating historic warfare in this respect, gradually eliminated 

 this protecting but cumbersome armor and developed instead 

 a higher type of nervous system and more responsive muscles. 

 These more progressive branches terminate at present in the 

 squids and devil-fish (Dibranchiata) of the mollusks, the insects 

 of the arthropods and the mammals of the vertebrates. These 

 three .phyla pursued the same pathway through the Protozoa, 

 Coelenterata and possibly also through the worms. 



From the protozoons to the chordates, as in the human social 

 life, the advance of life is measured by sub-division of labor. In 

 the Protozoa the single cell composing the entire animal performs 

 all the functions which in the Chordata are accomplished by 

 the millions of cells working through hundreds of organs. Natu- 

 rally the work is better done by the latter than by the former. 

 Upon this division of labor as well as upon the varying form of 

 the organs through which the w^ork is performed, the Animal 

 Kingdom is divided into twelve great groups or phyla. 



PHYLUM I, PROTOZOA 



The Protozoa are simple, one-celled, aquatic animals consist- 

 ing of protoplasm, usually microscopic in size, with or without 



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