PREFACE TO SIXTEENTH THOUSAND. 



Zoological science lias, during tlie last few years, made 

 very considerable advances, especially as regards the lower 

 nr invertebrate animals. Some of the conclusions arrived at, 

 though of high interest to the philosophic zoologist, cannot 

 with propriety be introduced into a work so elementary as 

 this. Others are briefly indicated, so far as the space 

 allowable for foot-notes, without disturbance of the pages, 

 would permit. In the text itself scarcely any change has 

 been made. 



Eecent discoveries regarding the development and struc- 

 ture of these animals have brought to light unexpected 

 affinities between different groups, which naturally suggest 

 corresponding changes in arrangement and nomenclature. 

 In fact, the best classification that can at any period be 

 proposed must be regarded only as provisional; with the 

 advance of knowledge it must be modified or changed. In 

 some departments, such changes are even no tv approaching ; 

 and "coming events cast their shadows before.** 



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