Worms from the other Articulates as a separate division. But the
structural plan of this type is a body divided by transverse constrictions
or joints; and whether those joints are uniformly arranged from one end of
the body to the other, as in the Worms, or whether the front joints are
soldered together so as to form two regions of the body, as in Crustacea,
or divided so as to form three regions of the body, as in winged Insects,
does not in the least affect the typical character of the structure, which
remains the same in all. Branches or types, then, are natural groups of
the animal kingdom, founded on plans of structure or structural ideas.

What now are classes? Are they lesser divisions, differing only in extent,
or are they founded on special characters? I believe the latter view to be
the true one, and that class characters have a significance quite
different from that of their mere range or extent. These divisions are
founded on certain categories of structure; and were there but one animal
of a class in the world, if it had those characters on which a class is
founded, it would be as distinct from all other animals as if its kind
were counted by thousands. Baer approached the idea of the classes when he
discriminated between plan of structure or type and the degree of
perfection in the structure. But while he understands the distinction
between a plan and its execution, his ideas respecting the different
features of structure are not quite so precise. He does not, for instance,
distinguish between the complication of a given structure and the mode of
execution of a plan, both of which are combined in what he calls degrees
of perfection. And yet, without this 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 significance. 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