CATEGORIES OF CLASSIFICATION. 39 
distinction, the difference between classes and 
orders cannot be understood; for classes and 
orders rest upon a just appreciation of these two 
categories, which are quite distinct from each 
other, and have by no means the same signifi- 
cance. 
Again, quite distinct from both of these is the 
character of form, not to be confounded either 
with complication of structure, on which orders 
are based, or with the execution of the plan, on 
which classes rest. An example will show that 
form is no guide for the determination of classes 
or orders. Take, for instance, a Beche-de-Mer, 
a member of the highest class of Radiates, and 
compare it with a Worm. They are both long 
cylindrical bodies ; but one has parallel divisions 
along the length of the body, the other has the 
body divided by transverse rings. Though in 
external form they resemble each other, the one 
is a worm-like Radiate, the other is a worm-like 
Articulate, each having the structure of its own 
type; so that they do not even belong to the 
same great division of the animal kingdom, much 
less to the same class. We have a similar in- 
stance in the Whales and Fishes, — the Whales 
having been for a long time considered as Fishes, 
on account of their form, while their structural 
complication shows them to be a low order of the 
class of Mammalia, to which we ourselves belong, 
