these
three classes, the Worms, presents the typical structure of that branch
in the most uniform manner, with little individualization of parts. The
body is a long cylinder divided through its whole length by movable
joints, while the head is indicated only by a difference in the
front-joint. There is here no concentration of vitality in special parts
of the structure, as in the higher animals, but the nervous force is
scattered through the whole body,--every ring having, on its lower side,
either two nervous swellings, one on the right, the other on the left
side, connected by nervous threads with those that precede and those
that follow them, or these swellings being united in the median line.
It is this equal distribution of nervous force through the whole system
that gives to these animals such an extraordinary power of repairing
any injured part, so that, if cut in two, the front part may even
reconstruct a tail for itself, while the hind part produces a new
head, and both continue to live as distinct animals. This facility of
self-repair, after a separation of the parts, which is even a normal
mode of multiplication in some of them, does not indicate, as may at
first appear, a greater intensity of vital energy, but, on the contrary,
arises from an absence of any one nervous centre such as exists in
all the higher animals, and is the key to their whole organization. A
serious injury to the brain of a Vertebrate destroys vitality at once,
for it holds the very essence of its life; whereas in many of the lower
animals any part of the body may be destroyed without injury to the
rest. The digestive cavity in the Worms runs the whole length of the
body; and the respiratory organs, wherever they are specialized, appear
as little vesicles or gill-like appendages either along the back or
below the sides, connected with the locomotive appendages.

This class includes animals of various degrees of complication of
structure, from those with highly developed organizations to the lowest
Worms that float like long threads in the water and hardly seem to be
animals. Yet even these creatures, so low in the scale of life, are
not devoid of some instincts, however dim, of feeling and affection. I
remember a case in point that excited my own wonder at the time, and may
not be uninteresting to my readers. A gentleman from Detroit had had
the kindness to send me one of those long thread-like Worms (_Gordius_)
found often in brooks an