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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.

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.

As a rule the protopodite is composed of two segments, though one may be reduced or suppressed and occasionally three may be present.

The two distal endites are regarded as corresponding to the endopodite and exopodite of the higher Crustacea, the axis or corm of the Phyllopod limb representing the protopodite.