Protostome versus Deuterostome Development
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State
In a protostome embryo, the first opening becomes the mouth; in a deuterostome embryo, that opening becomes the anus, with the mouth forming second.
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A mollusk's earliest opening becomes its mouth, protostome-style, while a sea star's earliest opening becomes its anus instead.
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This criterion sorts bilaterians by developmental sequence, not mouth position — the defining event happens before the adult body forms.