absorbing its whole
substance through the under side, thus forming a massive close body
instead of a hollow one. In the Articulate, the germ is turned in a
position exactly opposite to that of the Mollusk, and absorbs the yolk
upon the back. In the Vertebrate, the germ divides in two folds, one
turning upward, the other turning downward, above and below the central
backbone. These four modes of development seem to exhaust the
possibilities of the primitive sphere, which is the foundation of all
animal life, and therefore I believe that Cuvier and Baer were right in
saying that the whole animal kingdom is included under these four
structural ideas.

Leuckart proposed to subdivide the Radiates into two groups: the
Coelenterata, including Polyps and Acalephs or Jelly-Fishes,--and
Echinoderms, including Star-Fishes, Sea-Urchins, and Holothurians. His
reason for this distinction is the fact that in the latter the organs are
inclosed within walls of their own, distinct from the body-wall; whereas
in the former the organs are formed by internal folds of the outer wall of
the body, as in the Polyps, or are hollowed out of the substance of the
body, as in Jelly-Fishes. This implies no difference in the plan, but
merely a difference in the execution of the plan. Both are equally radiate
in their structure; and when Leuckart separated them as distinct primary
types, he mistook a difference in the material expression of the plan for
a difference in the plan itself. So some naturalists have distinguished
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
foun