And, to return to our Class of the Crustacea, has any particular attention been paid in their classification to the distinctions prevailing in the “most essential organs”? For instance, to the nervous system? In the Corycæidæ, Claus found all the ventral ganglia fused together into a single broad mass, and in the Calanidæ a long ventral chain of ganglia,—the former, therefore, in this respect resembling the Spider Crabs and the latter the Lobster; but no one would dream on this account of supposing that there was a relationship between the Corycæidæ and the Crabs, or the Calanidæ and the Lobsters.—Or to the organs of circulation? We have among the Copepoda, the Cyclopidæ and Corycæidæ without a heart, side by side with the Calanidæ and Pontellidæ with a heart. And in the same way among the Ostracoda, the _Cypridinæ,_ which I find possess a heart, place themselves side by side with _Cypris_ and _Cythere_ which have no such organ.—Or to the respiratory apparatus? Milne-Edwards did this when he separated _Mysis_ and _Leucifer_ from the Decapoda, but he himself afterwards saw that this was an error. In one _Cypridina_ I find branchiæ of considerable size, which are entirely wanting in another species, but this does not appear to me to be a reason for separating these species even generically. On the other hand, what do we know of the physiological significance of the number of segments, and all the other matters which we are accustomed to regard as typical peculiarities of the different organs, and to which we usually ascribe the highest systematic value? “_Those peculiarities which first appear, should be more highly estimated than those which appear subsequently. A system, in order to be true and natural, must agree with the sequence of the organs in the development of the embryo._” If the earlier manifested peculiarities are to be estimated more highly than those which afterwards make their appearance, then in those cases in which the structure of the adult animal requires one position in the system, and that of the larva another, the latter and not the former must decide the point. As the _Lernææ_ and Cirripedes, on account of their Nauplius-brood, were separated from their previous connexions and referred to the Crustacea, we shall, for the same reason, have to separate _Penëus_ from the Prawns and unite it with the Copepoda and Cirripedia. But the most zealous embryomaniac would probably shrink from this course. A “true and natural system” of the Crustacea to be in accordance with the sequence of the phenomena would have to take into account in the first place the various modes of segmentation, then the position of the embryo, next the number of limbs produced within the egg and so forth, and might be represented somewhat as follows:— CLASSIS CRUSTACEA. Sub-class I. HOLOSCHISTA.—Segmentation complete. No primitive band. Nauplius-brood. Ord. 1. _Ceratometopa._)—Nauplius with frontal horns. (Cirripedia, Rhizocephala.)