Far more profound is the difference of the youngest brood from the sexually mature animal in by far the greater majority of the Podophthalma, which quit the egg in the form of _Zoëa._ This young form occurs, so far as our present observations go, in all the Crabs, with the sole exception of the single species investigated by Westwood. I say _species,_ and not _ genus,_ for in the same genus, _Gecarcinus,_ Vaughan Thompson found Zoëa-brood,[2] which is also met with in other terrestrial Crabs (_Ocypoda, Gelasimus,_ etc.). All the Anomura seem likewise to commence their lives as Zoëæ: witness the _ Porcellanæ,_ the Tatuira (_Hippa emerita_) and the Hermit Crabs. Among the Macrura we are acquainted with the same earliest form principally in several Shrimps and Prawns, such as _Crangon_ (Du Cane), _Caridina_ (Joly), _Hippolyte, Palæmon, Alpheus,_ etc. Lastly, it is not improbable, that the youngest brood of the Mantis-Shrimps (_Squilla_) is also in the same case. The most important peculiarities which distinguish this Zoëa-brood from the adult animal, are as follows:— The middle-body with its appendages, those five pairs of feet to which these animals owe their name of Decapoda, is either entirely wanting, or scarcely indicated; the abdomen and tail are destitute of appendages, and the latter consists of a single piece. The mandibles, as in the Insecta, have no palpi. The maxillipedes, of which the third pair is often still wanting, are not yet brought into the service of the mouth, but appear in the form of biramose natatory feet. Branchiæ are wanting, or where their first rudiments may be detected as small verruciform prominences, these are dense cell-masses, through which the blood does not yet flow, and which therefore have nothing to do with respiration. An interchange of the gases of the water and blood may occur all over the thin-skinned surface of the body; but the lateral parts of the carapace may unhesitatingly be indicated as the chief seat of respiration. They consist, exactly as described by Leydig in the _Daphniæ,_ of an outer and inner lamina, the space between which is traversed by numerous transverse partitions dilated at their ends; the spaces between these partitions are penetrated by a more abundant flow of blood than occurs anywhere else in the body of the Zoëa. To this may be added that a constant current of fresh water passes beneath the carapace in a direction from behind forwards, maintained as in the adult animal, by a foliaceous or linguiform appendage of the second pair of maxillæ (Fig. 18). The addition of fine coloured particles to the water allows this current of water to be easily detected even in small Zoëæ. Fig. 17. Zoëa of a Marsh Crab (Cyclograpsus ?), magnified. Fig. 18. Maxilla of the second pair in the same species, magnified. Fig. 17. Zoëa of a Marsh Crab (_Cyclograpsus ?_), magnified. Fig. 18. Maxilla of the second pair in the same species, magnified.