But let us stick to the Crustacea. In _Polyphemus_ Leydig finds the first traces of the intestinal tube even during segmentation. In _Mysis_ a provisional tail is first formed, and in _Ligia_ a maggot-like larva-skin. The simple median eye appears earlier, and would therefore be more important than the compound paired eyes; the scale of the antennæ in the Prawns would be more important than the flagellum; the maxillipedes of the Decapoda would be more important than the chelæ and ambulatory feet, and the anterior six pairs of feet in the Isopoda, than the precisely similarly formed seventh pair; in the Amphipoda the most important of all organs would be the “micropylar apparatus,” which disappears without leaving a trace soon after hatching; in _Cyclops_ the setæ of the tail would be more important than all the natatory feet; in the Cirripedia the posterior antennæ, as to which we do not know what becomes of them, would be more important than the cirri, and so forth. The most unimportant of all organs would be the sexual organs, and the most essential peculiarity would consist in colour, which is to be referred back to the ovarian egg. “_The embryos, or young states of different animals, resemble each other the more, the younger they are,_” or, as Johannes Müller expresses it, “_they approach the more closely to the common type._” Different as may be the ideas connected with the word “type,” no one will dispute that the typical form of the penultimate pair of feet in the Amphipoda is that of a simple ambulatory foot, and not that of a chela, for the latter occurs in no single adult Amphipod; we know it only in the young of the genus _Brachyscelus,_ which therefore in this respect undoubtedly depart more widely than the adults from the type of their order. This applies also to the young males of the Shore-hoppers (_Orchestia_) with regard to the second pair of anterior feet (_gnathopoda_). In like manner no one will hesitate to accept the possession of seven pairs of feet as a “typical” peculiarity of the Edriophthalma, which Agassiz, on this account, names Tetradecapoda; the young Isopoda, which are Dodecapoda, are also in this respect further from the “type” than the adults.