centipedes, crustaceans, insects, spiders,--we find these
numbers greatly reduced, down to twenty-two, thirteen, and even fewer;
and accompanying this there is a shortening or integration of the whole
body, reaching its extreme in crabs and spiders. Similarly with the
development of an individual crustacean or insect. The thorax of a
lobster, which, in the adult, forms, with the head, one compact box
containing the viscera, is made up by the union of a number of segments
which in the embryo were separable. The thirteen distinct divisions seen
in the body of a caterpillar, become further integrated in the
butterfly: several segments are consolidated to form the thorax, and the
abdominal segments are more aggregated than they originally were. The
like truth is seen when we pass to the internal organs. In the lower
annulose forms, and in the larvae of the higher ones, the alimentary
canal consists either of a tube that is uniform from end to end, or else
bulges into a succession of stomachs, one to each segment; but in the
developed forms there is a single well-defined stomach. In the nervous,
vascular, and respiratory systems a parallel concentration may be
traced. Again, in the development of the _Vertebrata_ we have sundry
examples of longitudinal integration. The coalescence of several
segmental groups of bones to form the skull is one instance of it. It is
further illustrated in the _os coccygis_, which results from the fusion
of a number of caudal vertebrae. And in the consolidation of the sacral
vertebrae of a bird it is also well exemplified.

That which we may distinguish as _transverse integration_, is well
illustrated among the _Annulosa_ in the development of the nervous
system. Leaving out those simple forms which do not present distinct
ganglia, it is to be observed that the lower annulose animals, in common
with the larvae of the higher, are severally characterized by a double
chain of ganglia running from end to end of the body; while in the more
advanced annulose animals this double chain becomes a single chain. Mr.
Newport has described the course of this concentration in insects; and
by Rathke it has been traced in crustaceans. In the early stages of the
_Astacus fluviatilis_, or common cray-fish, there is a pair of separate
ganglia to each ring. Of the fourteen pairs belonging to the head and
thorax, the three pairs in advance of the mouth consolidate into one
mass to form the brain, or cephalic ganglion