According to Sars, the 1 In Huxley's terminology the first two or three joints of the stem constitute a "protopodite," from which spring the "endopodite" and "exopodite."
- 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.
Very often the basal segment of the protopodite bears, on the outer side, a lamellar appendage (more rarely, two), the epipodite, which may function as a gill.
Protopodite, endopodite, exopodite, and epipodite were considered to be the morphological units of the crustacean limb.