abdominal feet serve for respiration, these in our cheliferous Isopod (Fig. 2) are solely motory organs, into which no blood-corpuscle ever enters, and the chief seat of respiration is, as in the _Zoëae,_ in the lateral parts of the carapace, which are abundantly traversed by currents of blood, and beneath which a constant stream of water passes, maintained, as in _Zoëae_ and the adult Decapoda, by an appendage of the second pair of maxillæ, which is wanting in all other Edriophthalma. Tanais dubius Fig. 2. _Tanais dubius_ (?) Kr. hermaphrodite, magnified, showing the orifice of entrance (_x_) into the cavity overarched by the carapace, in which an appendage of the second pair of maxillæ (_f_) plays. On four feet (_i, k, l, m_) are the rudiments of the lamellæ which subsequently form the brood-cavity. For both these discoveries, it may be remarked in passing, science is indebted less to a happy chance than immediately to Darwin’s theory. Species of _Penëus_ live in the European seas, as well as here, and their _Nauplius_-brood has no doubt repeatedly passed unnoticed through the hands of the numerous naturalists who have investigated those seas, as well as through my own,[2] for it has nothing which could attract particular attention amongst the multifarious and often wonderful _Nauplius_-forms. When I, fancying from the similarity of its movements that it was a young _Penëus-Zoëa,_ had for the first time captured such a larva, and on bringing it under the microscope found a _Nauplius_ differing _toto cœlo_ from this _Zoëa,_ I might have thrown it aside as being completely foreign to the developmental series which I was tracing, if the idea of early Naupliiform stages of the higher Crustacea, which indeed I did not believe to be still extant, had not at the moment vividly occupied my attention. And if I had not long been seeking among the Edriophthalma for traces of the supposititious _Zoëa_-state, and seized with avidity upon everything that promised to made this refractory Order serviceable to me, Van Beneden’s short statement could hardly have affected me so much in the manner of an electric shock, and impelled me to a renewed study of the _Tanaides,_ especially as I had once before plagued myself with them in the Baltic, without getting any further than my predecessors, and I have not much taste for going twice over the same ground.