INTRODUCTION. H 



really constructed upon no more than some half-dozen primary- 

 plans of structure or morphological types. These types are 

 all different from one another, but there is no animal yet 

 known to us, living or extinct, which cannot be referred to 

 one or other of these six plans. These plans, then, give us the 

 primary basis for a classification of the animal kingdom — all 

 the animals formed upon one plan being grouped together so 

 as to form a single division. The animal kingdom, therefore, 

 is primarily divided into six great sections corresponding to 

 the six morphological types, and these sections are known to 

 naturalists under the name of the " sub-kingdoms." Each of 

 these sub-kingdoms has its special name, and it is the object 

 of the present work to describe the leading characters and 

 more important examples of each. 



We have to understand, then, that all the animals belong- 

 ing to each sub-kingdom agree with one another in their mor- 

 phological type, or, in other words, in the plan upon which 

 they are constructed ; and the question now arises how they 

 can be separated from each other. If they agree morphologi- 

 cally, there is only one other way in which they can differ, 

 and that '\b physiologically, in the manner in which they dis- 

 charge their vital functions. Consequently, all animals which 

 agree with one another in their plan of structure, and which 

 are therefore placed in the same sub-kingdom, are separated 

 from one another solely by their physiological perfection. In 

 other words, as machines, they are constructed of the same 

 fundamental parts, but they do their work in a different way 

 and with different instruments. 



Returning to our old illustration, suppose we bad sepa- 

 rated from the mass of machines before us all those which 

 were intended to mark the lapse of time, and had in this way 

 assembled a large collection of hour-glasses, watches, time- 

 pieces, and clocks, and suppose that we wanted to arrange 

 these more minutely, we should soon discover that each of 

 these different time-keepers was formed upon a principle pe- 

 culiar to itself. The hour-glasses, as the most simple, would 

 form one division ; the timepieces and clocks, possessing pen- 

 dulums, would form another ; and the watches would form a 

 third. These, as being constructed upon different plans, would 

 constitute three distinct groups, which we should call classes 

 or sub-kingdoms according to the value we might see fit to 

 place upon the differences between them. But we must fur- 

 ther suppose that we wished to divide one of these groups — ■ 

 say the watches — into still smaller groups. If they were all 



