This image showing one of each of the 19 paired of appendages of a crayfish embedded in a plastic block nicely
illustrates the concept of serial homology in which a series of initially
similar, homologous body parts have been modified to perform different tasks.
The first two pairs of appendages called antennules (1) and antennae (2) are
involved primarily in the senses of touch and taste. The mandibles (3) are used
for biting and crushing food. The mandibles are covered by thin,
food-handling structures called the first maxillae (4) and second maxillae (5).
Next comes three pairs of maxillipeds (6-8) that are involved in touch, taste
and food handling.