We may dismiss the uniramous type with a few words: it is obviously secondarily derived from the biramous type; this can be proved in detail in nearly every case. Thus, the uniramous second antennae of some adult forms are during the Nauplius stage invariably biramous, a condition which is retained in the adult Cladocera. Similarly the uniramous walking legs of many Decapoda pass through a biramous stage during development, the outer branches or exopodites of the limbs being suppressed subsequently, while the primitively biramous condition of the thoracic limbs is retained in the adults of the Schizopoda, which doubtless own a common ancestry with the Decapoda. The only Crustacean limb which appears to be constantly uniramous both in larval and adult life is the first pair of antennae. We are reduced, therefore, to two types—the foliaceous and biramous. Sir E. Ray Lankester,[6] in one of his most incisive morphological essays, has explained how these two types are really fundamentally the same. He compares, for instance, the foliaceous first maxillipede (Fig. 1, A), or the second maxilla (Fig. 1, B) of a Decapod, _e.g._ _Astacus_, with the foliaceous thoracic limb of _Branchipus_ (Fig. 1, D), and with the typically biramous first maxillipede of a Schizopod (Fig. 1, F). In each case there is present, on the outer edge of the limb, one or more projections or epipodites which are generally specialised for respiratory purposes, and may carry the gills. The 6th and 5th “endites” in the foliaceous limb (Fig. 1, D) are compared with the exopodite and endopodite respectively of the biramous limb, while the endites 4–1 of the foliaceous limb are found in the basal joints of the biramous limb. Lankester presumes that the biramous type of limb throughout has been derived from the foliaceous type by the suppression of the endites 1–4, as discrete rami, and the exaggerated development of the endites 5 and 6, as above indicated. [Illustration: FIG. 1.—Appendages of Crustacea (=A-G=) and Trilobita (=H=). =A=, First maxillipede of _Astacus_; =B=, second maxilla of _Astacus_; =C=, second walking leg of _Astacus_; =D=, thoracic limb of _Branchipus_; =E=, first maxillipede of _Mysis_; =F=, first maxillipede of _Gnathophausia_; =G=, thoracic limb of _Nebalia_; =H=, thoracic limb of _Triarthrus_. _bp_, basipodite; _br_, bract; _cp_, carpopodite; _cxp_, coxopodite; _cx.s_, coxopoditic setae; _dp_, dactylopodite; _end_, endopodite; _ep_, epipodite; _ex_, exopodite; _ip_, ischiopodite; _mp_, meropodite; _pp_, propodite; 1–6, the six endites. ]