A TREATISE ON ZOOLOGY. 



INTRODUCTION. 1 



THERE are certain matters which require brief treatment by way 

 of introduction to the present treatise on Zoology. 



The first concerns the limitation of the subject-matter indicated 

 by the term " Zoology," requiring a statement of what living things 

 are here considered as animals and what are excluded from that 

 title. The second concerns the grouping of animals in large series 

 corresponding to the indications afforded by their structure as to 

 their genetic affinities. The method adopted in the present work 

 has been to take large divisions of the Animal series such as are 

 often called " sub-kingdoms " or " phyla " (or in some instances less 

 comprehensive divisions) one by one for systematic description and 

 for more detailed enumeration and justification of the classes, orders, 

 and families recognised than is usual in handbooks of Zoology. 

 These large divisions have been assigned for treatment to separate 

 authors, and in each case the author has given a description of the 

 characters which justify the recognition of the group which he 

 treats as an independent series ; to this he has added a more 

 extended discussion of the range of variety in the structure of the 

 forms held to be reasonably considered as members of the series. 



A special chapter written by me forms the introduction to 

 volume ii. of this work. It may be regarded as a continuation of 

 the present chapter, and treats of the division of the higher grade 

 of animals, which is called the Metazoa (the lower being the 

 Protozoa), into two branches, the " Parazoa " and the " Enterozoa." 

 It is, however, chiefly occupied with a discussion of the division 

 of the Enterozoa into two grades of higher and lower structural 

 complexity, which are designated respectively the " Enterocoela " 

 and " Coelomocoela." The chief phyla or large branches of the 

 animal pedigree are there enumerated, whilst each is subsequently 

 treated by independent authors. 



In the present introductory chapter I have therefore to consider, 

 besides the question as to what distinctions separate animals from 



1 By Sir Ray Laukester, K.C.B. 



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