constantly footless abdomen of Insects. The appendages of the middle-body (_pereiopoda_) seem never, even in their youngest form, to possess two equal branches, a peculiarity which usually characterises the appendages of the hind-body. This is a circumstance which renders very doubtful the equivalence of the middle-body of the Malacostraca with the section of the body which in the Copepoda bears the swimming feet and in the Cirripedia the cirri. The comprehension of the feet of the hind-body and tail in a single group (as “fausses pattes abdominales,” or as “pleopoda”) seems not to be justifiable. When there is a metamorphosis, they are probably always produced at different periods, and they are almost always quite different in structure and function. Even in the Amphipoda, in which the caudal feet usually resemble in appearance the last two pairs of abdominal feet, they are in general distinguished by some sort of peculiarity, and whilst the abdominal feet are reproduced in wearisome uniformity throughout the entire order, the caudal feet are, as is well-known, amongst the most variable parts of the Amphipoda. [2] Mecznikow has recently found Naupliiform shrimp-larvæ in the sea near Naples. CHAPTER IV. SEXUAL PECULIARITIES AND DIMORPHISM. Our _Tanais,_ which in nearly all the particulars of its structure is an extremely remarkable animal, furnished me with a second fact worthy of notice in connection with the theory of the origin of species by natural selection. When hand-like or cheliform structures occur in the Crustacea, these are usually more strongly developed in the males than in the females, often becoming enlarged in the former to quite a disproportionate size, as we have already seen to be the case in _Melita._ A better known example of such gigantic chelæ is presented by the males of the Calling Crabs ( _Gelasimus_ ), which are said in running to carry these claws “elevated, as if beckoning with them”—a statement which, however, is not true of all the species, as a small and particularly large-clawed one, which I have seen running about by thousands in the cassava-fields at the mouth of the Cambriú, always holds them closely pressed against its body.