The section of the Branchiopoda includes two groups differing even in their development,—the Phyllopoda and the Cladocera. The latter minute animals, provided with six pairs of foliaceous feet, which chiefly belong to the fresh waters, and are diffused under similar forms over the whole world, quit the egg with their full number of limbs. The Phyllopoda, on the contrary, in which the number of feet varies between 10 and 60 pairs, and some of which certainly live in the saturated lie of salterns and natron-lakes, but of which only one rather divergent genus (_Nebalia_) is found in the sea,[1] have to undergo a metamorphosis. Mecznikow has recently observed the development of _Nebalia,_ and concludes from his observations “that _Nebalia,_ during its embryonal life, passes through the Nauplius- and Zoëa-stages, which in the Decapoda occur partly (in _Penëus_) in the free state.” “Therefore,” says he, “I regard Nebalia as a Phyllopodiform Decapod.” The youngest larvæ [of the Phyllopoda] are Nauplii, which we have already met with exceptionally in some Prawns, and which we shall now find reproduced almost without exception. The body-segments and feet, which are sometimes so numerous, are formed gradually from before backwards, without the indication of any sharply-discriminated regions of the body either by the time of their appearance or by their form. All the feet are essentially constructed in the same manner and resemble the maxillæ of the higher Crustacea.[2] We might regard the Phyllopoda as Zoëæ which have not arrived at the formation of a peculiarly endowed abdomen or thorax, and instead of these have repeatedly reproduced the appendages which first follow the Nauplius-limbs.