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.