CHAPTEK IIL 



THE GENERAL STRUCTURE OF ANIMALS. 



BEFORE proceeding to a comparative account of the 

 structure and functions of the organs of different 

 animals, it will be necessary to introduce the student 

 to the broader characteristics of the groups into which 

 the animal kingdom has been divided. What fol- 

 lows in this chapter is to be regarded as having that 

 aim alone ; it is in no way to be looked upon either 

 as a classification of animals, or even as an intro- 

 duction to it, and it is to be used rather as a kind of 

 guide to the relative position of any animal that may 

 be mentioned in the succeeding chapters. So far as is 

 possible in the necessities of the case, it has been so 

 prepared as to hinder rather than to aid the student 

 in any attempt to commit to memory a system of 

 classification ; for it is certain that there is nothing less 

 fruitful in good result than a parrot-like acquaintance 

 with what is only a compressed epitome of the more 

 certain results of zoological enquiries, but which, it is 

 to be remembered, may at any time be profoundly 

 modified by further investigation. What is called a 

 classification of the animal kingdom is nothing more 

 or less than a precis of our knowledge at a given mo- 

 ment, and, at its best, can never be more than rela- 

 tively correct. 



On the other hand, the sketch that follows may be 

 of use as indicating the general course of development, 

 taken along different lines by different kinds of 

 animals. 



The simplest animals essentially resemble an 



