THE CLASSIFICATION OF ANIMALS. 127 



system of classification can be compreliended in the least. 

 While it was customary in old-fashioned teaching of Zoology 

 to begin by teaching the system of classification, this is now, 

 on the contrary, for the reasons above indicated, the last 

 thing to be learnt ; and the student's first lessons in the 

 laboratory are, on the contrary, the careful examination and 

 description of the details of structure in a few kinds of animal. 

 (For mention of text-books arranged according to this plan, 

 see Part III., Chap. I, p. 312.) 



Although a real understanding of their meaning is not pos- 

 sible until a considerable amount of detailed knowledge of 

 several different types has been acquired, yet some pre- 

 liminary acquaintance with the names of the chief divisions 

 of the animal kingdom is useful even to the begiimer, and 

 therefore a short summary of the facts of classification is 

 given in the subsequent chapters. 



Classification by Type. 



The following brief descriptions of Amisba, Vorticella, 

 Hydra, and Lumbricus, are given as examples of classification 

 by type. 



I. Types of the Protozoa: first grade of animal life, 

 unicellular organisms. 



Protozoa : animals more or less resembling the Proteus 

 Animalcule Amceba (various species exist ; proteun, quarta, 

 verrucosa, etc.) : see figs. 26 and 27, p. 13-5. 



Anusba is a nucleated unicellular organism which reproduces 

 itself simply by division into two equal parts (fission) ; a pro- 

 cess of conjugation between two individuals has, however, 

 _been observed, and interpreted as a sexual process. The 

 protoplasm of the cell consists of an inner granular layer, 

 the endosarc, surrounded by a clear outer layer, the ectosarc; 

 and contains an excretory structure, the contractile vacuole 

 (or contractile vesicle), a contractile space filled with fluid 



