The difference of the sexes which, in the Gammarinæ is usually expressed chiefly in the structure of the anterior feet (gnathopoda, Sp. Bate) and in the Hyperinæ in the structure of the antennæ, is often so great that males and females have been described as distinct species, and even repeatedly placed in different genera (_Orchestia_ and _Talitrus, Cerapus_ and _Dercothoë, Lestrigonus_ and _Hyperia_) or even families (_Hypérines anormales_ and _Hypérines ordinaires_). Nevertheless it is only developed when the animals are nearly full-grown. Up to this period the young resemble the females in a general way, even in some cases in which these differ more widely than the males from the “Type” of the order. Thus in the male Shore-hoppers (_Orchestia_) the second pair of the anterior feet is provided with a powerful hand, as in the majority of the Amphipoda, but very differently constructed in the females. The young, nevertheless, resemble the female. Thus also,—and this is an extremely rare case,[6]—the females of _Brachyscelus_ are destitute of the posterior (or inferior) antennæ; the male possesses them like other Amphipodæ; in the young I, like Spence Bate, can find no trace of them. It is, however, to be particularly remarked, that the development of the sexual peculiarities does not stand still on the attainment of sexual maturity. For example, the younger sexually mature males of _Orchestia Tucurauna,_ n. sp., have slender inferior antennæ, with the joints of the flagellum not fused together, the clasping margin (“palm,” Sp. Bate) of the hand in the second pair of feet is uniformly convex, the last pair of feet is slender and similar to the preceding. Subsequently the antennæ become thickened, two, three, or four of the first joints of the flagellum are fused together, the palm of the hand acquires a deep emargination near its inferior angle, and the intermediate joints of the last pair of feet become swelled into a considerable incrassation. No museum-zoologist would hesitate about fabricating two distinct species, if the oldest and youngest sexually mature males were sent to him without the uniting intermediate forms. In the younger males of _Orchestia Tucuratinga,_ although the microscopic examination of their testes showed that they were already sexually mature, the emargination of the clasping margin of the hand (represented in Fig. 50) and the corresponding process of the finger, are still entirely wanting. The same may be observed in _Cerapus_ and _Caprella,_ and probably in all cases where hereditary sexual differences occur.