The relatively small but densely yolky eggs of balanomorph cirripedes undergo a bilaterally modified spiral cleavage, in which the yolk is confined within a single large cell, 4D, and the yolk cell becomes almost completely enclosed by yolk-free blastomeres as cleavage proceeds to the 33-cell stage. The cleavage sequence is similar in all four species, in spite of differences in the egg size. Presumptive areas are established at the 33-cell stage. The yolk cell 4D, exposed at the surface only postero-ventrally, is presumptive midgut. The cells 3A, 3B and 3C, lying at the surface ventrally in front of the exposed area of the presumptive midgut cell, are presumptive mesoderm. The remainder of the surface layer is presumptive ectoderm, comprising presumptive protocerebral ectoderm anteriorly, naupliar segmental ectoderm laterally, post-naupliar ectoderm posteriorly and temporarily attenuated ectoderm covering the yolk dorsally. A small area of presumptive stomodaeum lies midventrally at the presumptive antennal level. The presumptive midgut cell divides into a pair of yolky anterior midgut cells internally and a pair of small posterior midgut cells postero-ventrally at the surface. The latter migrate in, then posteriorly behind, the anterior midgut rudiment. The anterior midgut rudiment develops as the stomach of the nauplius, through cell division and gradual resorption of yolk. The posterior midgut rudiment develops through cell division as the intestine of the nauplius. The three mesoderm cells migrate inwards, then posteriorly and begin to divide. The products of their divisions concentrate as a posterior mass of mesoderm, from which paired mesodermal bands are proliferated forwards on either side of the anterior midgut. The bands concentrate as paired naupliar somites, which differentiate as naupliar muscles, and also as ventral stomodaeal and labral mesoderm. The residual posterior mass forms post-naupliar mesoderm, including paired groups of mesoteloblasts of the trunk somites. There are no preantennulary somites. The presumptive ectodermal areas develop directly into protocerebral and labral, antennulary, antennal, mandibular and post-naupliar surface epithelium, the latter forming the surface layer of a caudal papilla enclosing the residual post-naupliar mesoderm and the posterior end of the posterior midgut. The ventral naupliar ectoderm in front of, on either side of and behind the labrum and stomodaeum also proliferates the cells of the naupliar central nervous system. Only the mandibular proliferation can be recognized as a distinct ganglion. The dorsal ectoderm becomes temporarily attenuated over the mass of yolky anterior midgut cells, but is contracted and incorporated into the general dorsal epithelium of the naupliar region during later development. A comparison of the formation and fates of presumptive areas in Crustacea reveals that the cirripede mode of development exemplifies a basic developmental pattern for Crustacea. Modifications of development in other groups of crustaceans are all secondary functional adaptations related to the storage and exploitation of yolk during embryonic development. Radial cleavage in the small eggs of some cladocerans, copepods and penaeid malacostracans is a secondary modification of an ancestral spirally-based cleavage in Crustacea. The embryonic development of Crustacea, although based on spiral cleavage, differs fundamentally from the basic embryonic development of the onychophoran-myriapod-hexapod assemblage of arthropods, supporting the hypothesis of polyphyletic origin of the arthropods.