e. The latter animal is a striking
example of serial homology. The body (except at its two ends) consists of a
longitudinal series of similar segments. Each segment supports a pair of
limbs, and the appendages of all the segments (except as before) are
completely alike.

[Illustration: SQUILLA.]

A less complete case of serial homology is presented by Crustacea (animals
of the crab class), notably by the squilla and by the common lobster. In
the latter animal we have a six-jointed abdomen (the so-called tail), {161}
in front of which is a large solid mass (the cephalo-thorax), terminated
anteriorly by a jointed process (the rostrum). On the under surface of the
body we find a quantity of moveable appendages. Such are, _e.g._, feelers
(Fig. 9), jaws (Figs. 6, 7, and 8), foot-jaws (Fig. 5), claws and legs
(Figs. 3 and 4), beneath the cephalo-thorax; and flat processes (Fig. 2),
called "swimmerets," beneath the so-called tail or abdomen.

[Illustration: PART OF THE SKELETON OF THE LOBSTER.]

Now, these various appendages are distinct and different enough as we {162}
see them in the adult, but they all appear in the embryo as buds of similar
form and size, and the thoracic limbs at first consist each of two members,
as the swimmerets always do.

This shows what great differences may exist in size, in form, and in
function, between parts which are developmentally the same, for all these
appendages are modifications of one common kind of structure, which becomes
differently modified in different situations; in other words, they are
serial homologues.

The segments of the body, as they follow one behind the other, are also
serially alike, as is plainly seen in the abdomen or tail. In the
cephalo-thorax of the lobster, however, this is disguised. It is therefore
very interesting to find that in the other crustacean before mentioned, the
squilla, the segmentation of the body is more completely preserved, and
even the first three segments, which go to compose the head, remain
permanently distinct.

[Illustration: SPINE OF GALAGO ALLENII.]

Such an obvious and unmistakeable serial repetition of parts does not
obtain in the highest, or backboned animals, the Vertebrata. Thus in man
and other mammals, nothing of the kind is _externally_ visible, and we have
to penetrate to his skeleton to find such a series of homologous parts.

There, indeed, we discover a number of pairs of bones, each pair so
obviously resembling the ot