As we have seen, it is exceptional to find a free-swimming nauplius larva among the Malacostraca, but it is the commonest larval stage in the other subclasses of Crustacea. Most of the Branchiopoda are hatched in this form (Fig. 33), and reach the adult state by a very gradual series of changes in which new somites and appendages are added in regular order from before backwards till the full number is reached. The Water-fleas (Cladocera), however, differ from most of the other Branchiopoda in having a direct development. The eggs are carried in a brood-pouch under the back of the carapace, and in this the embryos undergo their development. In the common _Daphnia_, for instance, numerous eggs or young can generally be seen through the transparent carapace (see Fig. 12, p. 37). [Illustration: FIG. 33--LARVAL STAGES OF THE BRINE SHRIMP (_Artemia salina_). (After Sars.) A, Nauplius, just hatched; B-E, later stages, showing progressive increase in number of somites and appendages. The adult form of this species is shown in Fig. 55, p. 164] Many of the Ostracoda have a direct development, but in some cases the young animal, on hatching, has only the first three pairs of appendages, and is therefore regarded as a nauplius, although it possesses a bivalved shell like that of the adult, and is very unlike the nauplius larvæ of other Crustacea. [Illustration: FIG. 34--EARLY NAUPLIUS LARVA OF A COPEPOD (_Cyclops_). MUCH ENLARGED. (From Lankester's "Treatise on Zoology.") _a´_, Antennule; _a´´_, antenna; _gn_, jaw-spine of antenna; _lbr_, upper lip; _md_, mandible] Most of the Copepoda also leave the egg in the nauplius stage; and, indeed, it was to the young of the common fresh-water _Cyclops_ (Fig. 34) that the name of _Nauplius_ was first given by the Danish naturalist, O. F. Müller, in the eighteenth century, in the belief that it was an adult and independent species of Crustacea. In the Copepoda, the changes which transform the nauplius into the adult are gradual, and consist chiefly in the successive addition of new somites and appendages. [Illustration: FIG. 35--LARVAL STAGES OF THE COMMON ROCK BARNACLE (_Balanus balanoides_--SEE PLATE III.) A, Nauplius stage (after Hoek); B, cypris stage (after Spence Bate)]