Intermediate Stages in the Embryonic Development of the Gastropod Ilyanassa obsoleta: a Scanning Electron Microscope Study

Summary

Scanning electron microscopy was used to study embryonic development of the mud snail, Ilyanassa obsoleta, from the fourth to the seventh day of development (70 to 165 hr post-trefoil at 20° C). At the beginning of this four-day interval, the embryo is more or less spherical and its only external features are the blastopore and a band of cilia in the median sagittal plane; by the end, it has become an early unhatched veliger. Developmental events observed include: closure of the blastopore and formation of the stomodeum; the differentiation of a characteristic ciliary pattern; a series of events related to the development of the shell field and mantle cavity; elongation of the embryo along the anterior-posterior axis; torsion; the formation of the larval kidneys, operculum, statocysts, foot, anus, excretory pore, and velum. Particular attention is given to shell field morphogenesis.

Keywords:

• Ilyanassa
• molluscan development
• organogenesis
• shell field