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The standard trilobite limb is segmented into three distinct portions — a walking leg, or endopodite, and a gill structure, the exopodite, are connected to the body by a spiny food-processing section, the protopodite.

Instead of having a spiny, triangular protopodite for processing food, they had a smooth, rounded structure attached to a short, flexible fingerlike endopodite that was just half the length of the creature’s other walking legs.

The jaws have the gnathobasic endites developed at the expense of the rest of the limb, the endopodite and exopodite persisting only as sensory “palps” or disappearing altogether.

General Morphology of Appendages.—Amid the great variety of forms assumed by the appendages of the Crustacea, it is possible to trace, more or less plainly, the modifications of a fundamental type consisting of a peduncle, the protopodite, bearing two branches, the endopodite and exopodite.

In many cases, one of the branches, generally the endopodite, is more strongly developed than the other.