six-jointed abdomen (the so-called tail), 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.
Now, these various appendages are distinct and different
enough as we 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.
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 others, that they all receive a common name—the ribs. There also (i.e. in the skeleton) we find a still more remarkable series of similar parts, the joints of the spine or backbone (vertebræ), which are admitted by all to possess a certain community of structure.