Variations of the ectodermal morphology of the adult rudiment of sea urchins larvae. The schematic diagrams show morphological changes of the ectoderm during adult rudiment formation. In members of the most primitive order, the Cidaroida, the larvae form tube feet directly on the left larval surface (top diagram; Emlet 1988; Parks et al. 1989; Olson et al. 1993). In contrast, in other sea urchin species, an invagination of the ectoderm forms an amniotic cavity that is always in contact with the exterior via an amniotic opening (middle diagram; Runström 1912
1918; Czihak 1965
1996). In a few species belonging to the infraorder Temnopleuridea, the larva forms a cell mass and the adult rudiment develops internally from the epithelium derived from this cell mass (bottom diagram; Fukushi 1959
1960; Mortensen 1921; Ubisch 1959).