10 INTRODUCTION. 



Biology, comparative anatomy, embryology, paleontology, che- 

 mistry, and microscopy now help to throw light on the great work 

 of Zoology. To one or other of these branches scientific men of 

 the world are devoting themselves, and especially to the study of 

 Cuvier's " Radiate mob," in which debate is still active and 

 decision uncertain. 



In comparing the following tables, it will be seen that for the 

 very lowest forms the name Protozoa (first animal) is generally 

 accepted. So, also, is Mollusca for shell-fish, the word having 

 been retained ever since Aristotle, who thus distinguished the 

 group. Darwin describes the Protozoa as " animals composed of 

 gelatinous material, showing scarcely any traces of distinct organs." 

 " Uni-cellular animals," Dallas calls them, " in which the functions 

 of life are performed by its simplest element, the cell." They 

 are mostly microscopic. The Protozoa are sometimes placed 

 first, sometimes last in the arrangement of " Types," " Divisions," 

 or "Groups," which words seem to be displacing the old-fashioned 

 and somewhat unmeaning term " Kingdoms." The later and 

 more intelligent arrangement is to begin with the lowest forms, 

 the mere specks of protoplasm, and work upwards through the 

 more complex organisms to the Vertebrata. Dallas, in The 

 Natural History of the Animal Kingdom (1856), was, I believe, 

 the first English naturalist who introduced this rational method. 

 His divisions were then five, viz. — 



1. Protozoa. — Composed of a simple cell, or an aggregation of cells. 



2. Radiata. — With parts arranged round a common centre. 



3. ARTICULATA. — Including an immense diversity of forms, vermes, 



insecta, Crustacea, etc., etc. 



4. Mollusca. — Shell-fish and cephalopods. 



5. Vertebrata. 



Huxley, in his Classification of Animals (1869), wrote: — " It 

 seems to me that the whole animal kingdom cannot be divided 

 into fewer than eight primary groups, no two of which are sus- 

 ceptible, in the present state of knowledge, of being defined by 

 characters which shall be at once common and diagnostic." He 

 arranges them thus, — 



Vertebrata. 

 Mollusca. Annulos^e. 



molluscoid^e. annuloid.e. 



Ccelenterata. Infusoria. 



Protozoa. 



