ects. It is true that the Centipedes
have a long uniform body like Worms, and the Spiders have the body
divided into two regions like the Crustacea, while the body in true
Insects has three distinct regions, head, chest, and hind body; but
notwithstanding this difference, both the former share in the peculiar
class-character that places them with the Winged Insects in a separate
group, distinct from all the other Articulates. We have seen that in the
Worms the respiratory organs are mere vesicles, while in the Crustacea
they are more highly organized gills; but in Centipedes, Spiders,
and Winged Insects, the breathing-apparatus is aerial, consisting of
air-holes on the sides of the body, connected with a system of tubes and
vessels extending into the body and admitting air to all parts of it. In
the Winged Insects this system is very elaborate, filling the body with
air to such a degree as to render it exceedingly light and adapted to
easy and rapid flight. The general arrangement of parts is the same in
this class as in the two others, the typical character being alike in
all.

We come now to the highest branch of the Animal Kingdom, that to which
we ourselves belong,--the Vertebrates. This type is usually divided into
four classes, Fishes, Reptiles, Birds, and Mammalia; and though many
naturalists believe that it includes more, and I am myself of that
opinion, I shall allude here only to the four generally admitted
classes, as they are sufficient for my present purpose, and will serve
to show the characters upon which classes are based. In a former paper I
have explained in general terms the plan of structure of this type,--a
backbone, with a bony arch above and a bony arch below, forming two
cavities that contain all the systems of organs, the whole being
surrounded by the flesh and skin. Now whether a body so constructed lie
prone in the water, like a Fish,--or be lifted on imperfect legs, like
a Reptile,--or be balanced on two legs, while the front limbs become
wings, as in Birds,--or be raised upon four strong limbs terminating in
paws or feet, as in Quadrupeds,--or stand upright with head erect, while
the limbs consist of a pair of arms and a pair of legs, as in Man,--does
not in the least affect that structural conception under which they are
all included. Every Vertebrate has a backbone; every Vertebrate has a
bony arch above that backbone and a bony arch below it, forming two
cavities,--no matter whether these