To review the developmental history of the different Malacostraca in detail would furnish no results at all correspondent to the time occupied by it,—if our knowledge was more complete it would be more profitable. I therefore abandon it, but will not omit to mention that in it many difficulties which cannot at present be satisfactorily solved would present themselves. To these isolated difficulties I ascribe the less importance, however, because even a little while ago, before the discovery of the Prawn-Nauplius, this entire domain of the development of the Malacostraca was almost inaccessible to Darwin’s theory. Nor will I dwell upon the contradictions which appear to result from the application of the Darwinian theory to this department. I leave it to our opponents to find them out. Most of them may easily be proved to be only apparent. There are two of these objections, however, which lie so much on the surface that they can hardly escape being brought forward, and these, I think, I must get rid of. “The peculiarities in which the Zoëæ of the Crabs, the _Porcellanæ,_ the Tatuira, the Hermit Crabs, and the Prawns with Zoëa-brood agree, and by which they are in common distinguished from the larvæ of _Penëus_ produced from Nauplii, forces us (it might be said) to the supposition that the common ancestor of these various Decapods quitted the egg in a similar Zoëa-form. But then neither _Penëus_ with its Nauplius-brood, nor even apparently the _Palinuri_ could be referred back to this ancestor. The mode of development of _Penëus_ and _Palinurus,_ as also several peculiar larvæ of unknown origin, but which are in all probability to be attributed to Macrurous Crustacea, necessitate on the contrary the opposite supposition, namely, that the different groups of the Macrura have passed from their original to their present mode of development independently of each other and also independently of the Crabs.” To this we may answer that the occurrence of the Zoëa-form in all the above-mentioned Decapoda, its existence in _Penëus_ during the whole of that period of life which is richest in progress and in which the wide gap between the Nauplius and the Decapod is filled up, its recurrence even in the development of the Stomapoda, the occurrence of a larval form closely approaching the youngest Zoëa of _Penëus_ in the Schizopod genus _Euphausia,_) and the reminiscence of the structure of Zoëa, which even the adult _Tanais_ has preserved in its mode of respiration,—all indicate Zoëa as one of those steps in development which persisted as a permanent form throughout a long period of repose, perhaps through a whole series of geological formations, and thus has also made a deeper impression upon the development of its descendants, and formed a firmer nucleus in the midst of other and more readily effaced young states. It cannot, therefore, surprise us that in transitions from the original mode of