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. [Illustration: FIG. 109. _D_, Mysis-stage. Thirteen pairs of appendages are now formed: _I_ and _II_, antennæ; _III_, mandibles; _IV_ and _V_, maxillæ; _VI-XIII_, swimming appendages with one branch or with two. _Abd_, abdomen. _Sfl_, tail-fin. _E_, the fully-formed Shrimp, with thirteen pairs of appendages on the cephalothorax (_Cph_); _I_ and _II_, the two pairs of antennæ; then follow the maxillæ and maxillipedes (_III-VIII_), the last of which is visible in the figure, and the five pairs of walking-legs (_IX-XIII_) of which the third bears a long chela. On the abdomen there are now six pairs of appendages (_XIV-XIX_).]