INTRODUCTION. II 
plans. These plans, then, give us the primary basis for a classi- 
fication of the animal kingdom—all the animals formed upon 
one plan being grouped together so as to forma 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 éxamples of each. 
We have to understand, then, that all the animals belonging 
to each sub-kingdom agree with one another in their morpho- 
logical 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 morphologically, there 
is only one other way in which they cam differ, and that is phy- 
stologically, in the manner in which they discharge 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 had separated 
from the mass of machines before us all those which were in- 
tended to mark the lapse of time, and had in this way assem- 
bled a large collection of hour-glasses, watches, timepieces, 
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 peculiar to itself. 
The hour-glasses, as the most simple, would form one division ; 
the timepieces and clocks, possessing pendulums, 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 accord- 
ing to the value we might see fit to place upon the differences 
between them. But we must further suppose that we wished 
to divide one of these groups—say the watches—into still 
smaller groups. If they were all standing, we should probably 
find this a matter of very great difficulty. The moment, how- 
ever, that they commenced to go—or, in other words, to per- 
