ABSTRACT
Direct development of the subantarctic sea-urchin Abatus cordatus: formation of the oral face.
At the end of gastrulation, the oral cavity develops within a thickening of the ectoderm. The latter becomes isolated within the mesoderm and extends under the ectoderm of the oral pole of the embryo. At this stage, the oral surface differentiates: the neural plate, water ring, five radial canals and five papillae arise from folds in the ectodermal floor of the cavity. The oral surface is exposed at the beginning of juvenile stage 1 when an opening develops in the ceiling of the cavity.
While the morphology of the oral surface is similar to that described for other echinoids during metamorphosis, no larval stage was observed. This is the first description of an actual direct development in echinoids.
