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Molluscs, annelids and numerous smaller phyla typically share stereotyped spiral cleavage patterns, cell-fate assignments and characteristic ciliated trochophore larvae, features that originated in the Precambrian era.

Crassostrea gigas is also an interesting model for developmental biology owing to its mosaic development with typical molluscan stages, including trochophore and veliger larvae and metamorphosis.

It is an ancestral larval form, corresponding perhaps to the stages immediately succeeding the trochophore in the development of Annelids, but with some of the later-acquired Crustacean characters superposed upon it.

C, Trochophore of Polygordius. and D, later stage of the same, showing the development of the trunk.