Protoconch

The teleoconch starts forming when the larval gastropod becomes a juvenile, and the protoconch may dissolve.

In some gastropod groups (such as the Architectonicidae), the teleoconch whorls spiral in the opposite direction to the protoconch.

In species which have a veliger or swimming larval stage which hatches out of egg capsules, there are two parts to the protoconch.

The structure of the protoconch has been widely used as a discriminating feature in gastropod systematics.

[1] The homologous structure in bivalves (clams) is called the prodissoconch.

An SEM image of the trochophore of Haliotis asinina 11 hours post fertilization, with calcified protoconch (pc) prior to torsion .
Protoconch of Haliotis asinina when the torsion is complete.
Apical view of the whole shell.
Detail of apical view of the same shell showing clearly visible protoconch, that has 2¼ whorls
Growth series of Caecum glabrum showing protoconch in bottom image.