The Development and Larval Form of an Echinothurioid Echinoid, Asthenosoma ijimai, Revisited. The modified development from cleavage to late larval form of the echinothurioid echinoid, Asthenosoma ijimai, was re-examined using light microscopy and scanning electron microscopy of whole and sectioned stages. Although an original study (Amemiya and Tsuchiya, 1979) reported direct development without evidence of a pluteus larva, we found that the unusual development can be interpreted as a topologically reflected, reduced pluteus, with vestigial larval arms and a greatly reduced larval skeleton. This developmental pattern produces the third and most reduced pluteus form known among the six echinoid lineages with modified development that have been studied thus far. Features such as an equal fourth cleavage, extrusion of yolk into the blastocoel, and the presence of large numbers of cells within the blastocoel are convergent with traits reported for other species with modified development. Coelom formation is clearly modified from that of species with feeding larval development, but notably the hydrocoel begins to develop podial buds prior to separation from the archenteron. Echinothurioid sea urchins are considered to be the most primitive living euechinoids, and in A. ijimai the timing of mesenchyme cell ingression and the formation of epineural folds were similar to these features in other euechinoids. Indentation of the juvenile oral surface relatively late in larval development raises the possibility that the amniotic invagination (vestibule), common in all other euechinoids, may be a trait incorporated into the development of echinoids at the time of origin of the echinothurioids. The structural comparisons reported here show a need for further detailed morphological studies of developmental modifications in other echinoid species.