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  • José H. Leal

Upside-down Baby!

In gastropod mollusks, the larval shell ("baby" shell, or protoconch), is usually retained in the apex (the "tip") of the adult shell. After metamorphosis from larva to adult, there may be differences in color, thickness, and texture between the protoconch and the adult shell, but usually the direction of coiling stays the same with growth. Gastropods with a right-handed larval shell continue to grow a right-handed adult shell, and vice versa.

An exception to this same‐coiling-direction "rule" is the transition from the larval to the adult shell in members of the sundial family Architectonicidae (see figures). In the Common Sundial, Architectonica nobilis (Linnaeus, 1758), the direction of shell coiling changes almost 180 degrees when the free‐living larva shifts to the adult stage. This causes the protoconch of the adult shell to appear "upside‐down" inside the umbilicus (the big round hole on the shell bottom, or base) in that species (arrow and detail in the illustration).


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