ABSTRACT
The echinothurioid, Asthenosoma ijimai, is known to have an unusual, nonfeeding larval development. Morphological features during development from cleavage through metamorphosis were observed on stained sections (3–5µ m) with a light microscope and on whole and partially sectioned material with a scanning electron microscope (SEM). The fourth cleavage was equally giving rise to 16 blastomeres of similar size. Just after hatching (22 hrs at 20°C), the blastocoel of the wrinkled blastula, became filled with yolk granules released from the nuclear DNA indicated that some mesenchyme cells were scattered among the yolk granules at this time. Coelom formation was evident by 51 hrs after fertilization, when the tip of the archenteron projected toward the side of the blastocoel. By 63 hrs, hydrocoelic (podial) bulges were present, but this coelomic compartment had not yet separated from the archenteron. Both sectioned material and SEM observations demonstrate epineural folds were present at 101 hrs, and indentation of oral surface occurred after the formation of podial bulges suggesting the possibility of a delayed appearance of an amniotic invagination. These results indicate that morphological variation during development of echinoids with abbreviated development is greater than previously known. The interpretation of morphological differences in development that are due to abbreviated development may only be made in the phylogenetic context of the taxa being compared.
