A third example of this disposition, in an
animal belonging to the same class of Crustaceans,
whereby the legs are reduced to soft
paddles, and combine the functions of respiration
with those of locomotion, is afforded by
the Branchipus stagnalis, (Cancer stagnalis,
Lin.), of our English ponds, (see PI. 45, Figs.
3, e. 4, e. 5, e.)
In the comparison here made between four
different families of Crustaceans, for the purpose
of illustrating the history of the long extinct
Trilobites, by the analogies we find in the Serolis,
Limulus, and Branchipus; we have a beautiful
example, taken from the extreme points of time
horny transverse plates (Fig. 2, e. 2, e'. and 2, e".) supporting
the fibres of the branchiae, and at the same time acting as paddles
for swimming. The same disposition of laminated branchiae is
found also in the Serolis, Fig. 7. e. Fig. 8, is a magnified
representation of these laminated branchiae, very similar to those
at Figs. 3, e. and 5, e.
Thus while the Serolis (Fig. 7) presents an union of antennae
and crustaceous legs with soft paddles bearing the Branchiae, we
have in the Limulus (Fig. 2), a similar disposition of legs and
paddles, and only slight traces of antennae ; in the Branchipus,
(Figs. 3 and 5,) we find antennae, but no crustaceous legs; while
the Trilobite, being without antennae, and having all its legs
represented by soft paddles, as in Branchipus, is by the latter
condition placed near Branchipus among the Entomostracous
Crustaceans, in the Order of Branchiopods, whose feet are represented
by ciliated paddles, combining the functions of respiration
and natation. At PI. 45, Fig. 3. e, Fig. 4. e, Fig. 5. e, represent
the soft branchiae of Branchipus, performing the double office
of feet and lungs.
of which Geology takes cognizance, of that
systematic and uniform arrangement of the
Animal Kingdom, under which every family is
nearly connected with adjacent and cognate families.
Three of the families under consideration
are among the present inhabitants of the
water, while the fourth has been long extinct,
and occurs only in a fossil state. When we see
the most ancient Trilobites thus placed in immediate
contact with our living Crustaceans, we
cannot but recognize them as forming part and
parcel of one great system of Creation, connected
through its whole extent by perfect unity of
design, and sustained in its minutest parts by
uninterrupted harmonies of organization.
We have in the Trilobites an example of that
peculiar, and, as it is sometimes called, rudimentary
development of the organs of locomotion
in the Class Crustaceans, whereby the legs are
made subservient to the double functions of
paddles and lungs. The advocate for the theory
of the derivation of existing more perfect species,
by successive changes from more simple ancient
forms, might imagine that he sees in the Trilobite
the extinct parent stock from which, by a
series of developments, consecutive forms of more
perfect Crustaceans may, during the lapse of ages,
have been derived; but according to this hypothesis,
we ought no longer to find the same simple
condition as that of the Trilobite still retained in