shall see that these four types, with their four modes of growth, seem to
fill out completely the plan or outline of the animal kingdom, and leave
no reason to expect any further development or any other plan of animal
life within these limits. The eggs of all animals are spheres, such as I
have described them; but in the Radiate the whole periphery is transformed
into the germ, so that it becomes, by the liquefying of the yolk, a hollow
sphere. In the Mollusks, the germ lies above the yolk, 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