Amongst the Decapoda, instances of direct development like those just described are exceptional, but in some of the other orders of the Malacostraca direct development is the rule. In the great division Peracarida, as we have already seen, the females are provided with a pouch, or marsupium (from which the name of the division is derived), in which the eggs are carried. Within this pouch the young undergo the whole of their development, and they only leave it, as a rule, when they have attained the structure of the adults. Among the more familiar representatives of this division, the Sand-hoppers (Amphipoda), the Woodlice (Isopoda), and the Opossum Shrimps (Mysidacea), may be mentioned as examples of this mode of development. The Woodlice and their immediate allies differ a little from the other members of the division in the fact that the young leave the brood-pouch with the last pair of legs still undeveloped, though in other respects they are like miniature adults.
In those Crustacea which have a direct development without free-swimming larval stages, it is sometimes possible to find traces of such stages in the early development of the embryo. This is shown most clearly, perhaps, in the Opossum Shrimps (Mysidacea). In these the embryo becomes free from the egg-membrane (or may, in a sense, be said to "hatch") at a very early stage, and lies free within the brood-pouch as a maggot-shaped body, on which three pairs of rudimentary limbs can be made out. The later development shows that these three rudiments correspond to the antennules, antennæ, and mandibles, so that the maggot-shaped embryo is, in fact, a disguised nauplius without the power of swimming or of leading an independent existence. In other cases—as, for instance, in the Crayfish, where the earlier stages are confined within the egg-membrane (or "egg-shell")—the nauplius stage, although more difficult to examine, is quite as well marked.
Of the other groups of the Malacostraca, the Syncarida and Leptostraca are hatched in nearly the adult form, but the Stomatopoda have a long series of larval stages. These larvæ ([Fig. 32]) are all distinguished by the large size of the carapace, which in some cases envelops the greater part of the body. Some Stomatopod larvæ, in the warmer seas, attain to a relatively great size, sometimes exceeding 2 inches in length, and their glass-like transparency gives them a very striking appearance.
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).
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
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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.