AbstractIn brittle stars, the coelomic cavities that form during embryogenesis contribute to most of the internal organ systems of the juvenile. In the ancestral mode of development, the coelomic cavities begin with bilateral symmetry and play a minor role in the function of the ophiopluteus larva. However, the coelomic cavities undergo extensive changes during metamorphosis to set up the body systems of the juvenile brittle star. Many lineages of brittle stars have evolved life histories without the ophiopluteus larva. The non-feeding vitellaria larva has rapid development of juvenile structures. This work demonstrates the modifications to the origin and early development of the coelomic cavities in a vitellaria larva. Much of the archenteron forms an unpaired axocoel, hydrocoel, and somatocoel. The posterior-most portion of the archenteron forms the rudiment of the juvenile stomach. The right somatocoel and a portion of the left somatocoel form as invaginations of the lateral ectoderm. Later morphogenesis of the axocoel, the hydrocoel, and the two somatocoels is similar to what has been shown for brittle stars with an ophiopluteus larva. Confocal microscopy and three-dimensional modeling were used to show new details for the later morphogenesis of the coelomic cavities. The stone canal originates as an outgrowth of the hydrocoel between lobes 4 and 5. The hydrocoel lobes have minimal migration after they meet to complete the ring canal. The right somatocoel contributes to a component of the axial complex and perihemal system. A detailed description is given for how the left somatocoel contributes to multiple organ systems.