Morphogenesis of Adult Traits during the Early Development of Mespilia globulus Linnaeus, 1758 (Echinodermata: Echinoidea)

We observed the development of Mespilia globulus L. (Temnopleuridae) and Hemicentrotus pulcherrimus A. Agassiz, 1863 (Strongylocentrotidae) and focused on the formation of adult traits by light and scanning electron microscopy. In M. globulus, a primary pore canal constructing a part of the adult water vascular system appeared at the left and right coelomic sacs in the prism stage. The left and right primary pore canals respectively elongated to the left lateral and right dorsal ectoderm and opened to the outside, and then the right one disappeared until the 2-armed larval stage. On the other hand, the primary pore canal of prisms of H. pulcherrimus formed on the left side only. We first confirmed the existence of different types of asymmetrical primary pore canal formation at the prism stage in sea urchins. In 2-armed larvae of M. globulus, a cell mass was formed by invagination and pinching off of the left oral ectoderm. At the 6-armed larval stage, the developing cell mass attached to the hydrocoel, and together they formed an adult rudiment. In contrast, the left larval ectoderm of 6-armed larvae of H. pulcherrimus began to invaginate between the postoral and postdorsal arms and formed an amniotic cavity. The amniotic cavity had attached to the hydrocoel by the 8-armed larval stage for adult rudiment formation. This is the 1st report describing how adult rudiment formation depends on the cell mass in the early larval stage of M. globulus larvae. From these results, we propose that asymmetrical primary pore canal formation is initiated in the embryonic stage with some modifications in formational modes among species, and that the heterochronical start of adult rudiment formation by the cell mass may allow a sufficient period to develop and protect the rudiment.