The larva hatches out as a Zoaea[148] (Fig. 124, A) very similar to that of the Anomura; it is furnished with an anterior and posterior spine on the carapace. It is characteristic of the Brachyuran Zoaea that the third maxillipede is fashioned from the beginning in its definitive expanded form, and is never a biramous swimming organ as in the Anomura. The only exception to this rule is found in the Dromiacea, the most primitive of the Brachyura, to be soon considered, in which not only the third maxillipede, but also the first pair of pereiopods may be developed as biramous oars, a condition taking one back to the Mysis stage of the Macrura. The Metazoaea (Fig. 124, B) has the rudiments of the thoracic limbs developed and crowded together at the back of the carapace; they are all laid down in their definitive forms, and the abdomen has the pleopods precociously developed. These Zoaeal stages are of course pelagic, but the Metazoaea next passes into the Megalopa stage (Fig. 125), in which the little crab forsakes its pelagic life and assumes the ground-habits of the adult; the Megalopa, which corresponds exactly to the Glaucothoe of the Pagurids, resembles a small _Galathea_ or _Porcellana_, the abdomen being still large and unflexed and furnished with normal pleopods. From this stage the adult structure is soon achieved, though, owing to the continued growth of the Crustacea even after maturity is reached, there is often a slight progressive change in structure, especially in the male, at each successive moult of the individual. The Megalopa of _Corystes cassivelaunus_ is peculiar in the immense production of the second antennae, which act as a respiratory tube (Fig. 125). The Brachyura must be considered under the following subdivisions:— =Tribe 1. Dromiacea.=