[Illustration: FIG. 109. Metamorphosis of one of the higher Crustacea, a Shrimp (_Peneus potimirim_), after Fritz Müller. _A_, the nauplius larva with the three pairs of appendages: _I_, the antennæ; _II_ and _III_, the biramose swimming-feet. _Au_, the single eye. _B_, first Zoæa stage, with six pairs of appendages (_I-VI_). _Skn_, area where new segments are being formed.]
That the development has in general followed those lines is made clear chiefly by the fact that the members of all these different orders of Crustaceans still arise from nauplius larvæ, even in those cases in which the perfect animal possesses a structure differing widely from the usual Crustacean form. _All_ Crustaceans arise from the _nauplius form_, even those of the higher orders, though they may not arise from a nauplius _larva_. But this very circumstance, that in most of the higher and many of the lower Crustaceans, the young animal, when it emerges from the egg, already possesses more numerous segments and limbs than a nauplius larva, again points to the connexion between phylogeny and ontogeny, for in these cases the nauplius stage _is gone through within the ovum_. The whole difference between this and the forms we considered first lies in the fact that, in the latter, the development is greatly shortened, condensed, as we might say, so that the nauplius stage forms a part of the _embryonic_ development, and that new segments and limbs develop in the embryo nauplius within the egg, so that the young animal leaves the egg in a more advanced state, nearer to that of the perfect animal, to which it can, therefore, attain in a shorter time.
[Illustration: FIG. 109. _C_, second Zoæa stage. The thorax is now divided into cephalothorax (_Cph_) and abdomen (_Abd_); seven pairs of appendages are developed, and five more (_VIII-XII_) are beginning to appear. _Au_, paired eyes.]
We should expect that this shortening of the larval period would be associated with a prolongation of embryogenesis, especially in those Crustaceans which possess a large number of segments and limbs, that is--in the higher forms--and in the main this is the case. But there are exceptions in two directions; in the first place there are some, even among the lower Crustaceans, which leave the egg not as a nauplius but in the perfect form of the adult, and secondly, there are, among the higher Crustaceans, certain species which emerge from the egg not in the more mature form but still in the primitive nauplius form. Fritz Müller was the first to furnish an example of this last case, a Brazilian shrimp, _Peneus potimirim_. Like the lowest Copepods or Branchiopods, this species, which belongs to the highest order of Crustaceans, goes through the whole long development, from the nauplius through a series of higher larval forms up to the perfect animal, and all _outside of the egg_, as an independent free-swimming larva (Fig. 109, _A-E_). This is in sharp contrast to its near relative, the freshwater crayfish, which goes through this whole development within the egg, and emerges perfectly formed.