[Illustration: FIG. 13.--Early Stages of _Balanus_. (After Spence Bate.)
A, Nauplius. e, Eye. B, _Cypris_-larva with a bivalve shell and just before becoming attached (represented feet upwards for comparison with E, where it is attached). C, After becoming attached, side views. D, Later stage, viewed from above. E, Side view, later stage and with cirri extended.
[Illustration: FIG. 14.--Zoea of Common Shore-Crab in its second stage. (Spence Bate.)
r, Rostral spine. s, Dorsal spine. m, Maxillipeds. t, Buds of thoracic feet. a, Abdomen.]
In most Crustacea, however, this primitive scheme is more or less modified. The earlier stages may be suppressed or passed through within the egg (or within the maternal brood-chamber), so that the larva, on hatching, has reached a stage more advanced than the nauplius. Further, the gradual appearance and differentiation of the successive somites and appendages may be accelerated, so that comparatively great advances take place at a single moult. In the Cirripedia, for example, the latest nauplius stage (fig. 13, A) gives rise directly to the so-called _Cypris_-larva (fig. 13, B), differing widely from the nauplius in form, and possessing all the appendages of the adult. Another very common modification of the primitive method of development is found in the accelerated appearance of certain somites or appendages, disturbing the regular order of development. This modification is especially found in the Malacostraca. Even in those which have most fully retained the primitive order of development, as in the _Penaeidea_ and _Euphausiidae_, the last pair of abdominal appendages make their appearance in advance of those immediately in front of them. The same process, carried further, leads to the very peculiar larva known as the _Zoea_, in the typical form of which, found in the Brachyura (fig. 14), the posterior five or six thoracic somites have their development greatly retarded, and are still represented by a short unsegmented region of the body at a time when the abdominal somites are fully formed and even carry appendages. The _Zoea_ was formerly regarded as a recapitulation of an ancestral form, but there can be no doubt that its peculiarities are the result of secondary modification. It is most typically developed in the most specialized Decapoda, the Brachyura, while the more primitive groups of Malacostraca, the _Euphausiidae_, _Penaeidea_ and Stomatopoda, retain the primitive order of appearance of the somites, and, for the most part, of the limbs. At the same time, the tendency to a retardation in the development of the posterior thoracic somites is very general in Malacostracan larvae, and may perhaps be correlated with the fact that in the primitive Phyllocarida the whole thoracic region is very short and the limbs closely crowded together.