Brachiopods formed another of his classes;
but these differ from the other Bivalves only in having a net-work of
blood-vessels in the place of the free gills, and this is merely a
complication of structure, not a difference in the general mode
of execution, for their position and relation to the rest of the
organization are exactly the same in both. Pteropods constituted another
class in his division of the type of Mollusks; but these animals, again,
form only an order in the class of Gasteropoda, as Brachiopods form an
order in the class of Acephala.
In the third division of the Animal Kingdom, the Articulates, we have
again three classes: Worms, Crustacea, and Insects. The lowest of 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.
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