Most land snail species that have teeth or lamellae in the apertures of their shells develop them as they near sexual maturity. In a handful of species, such apertural formations are present in young snails, but diminish in number and size or disappear completely in adults. The North American snail Ventridens suppressus is in the latter group.
The apertures of the shells of newly hatched Ventridens suppressus are unobstructed. Up to 5 lamellae develop in their apertures as the snails go thru their ontogeny. But as they approach maturity, the lamellae get resorbed one by one and the adults end up with 1 small tooth. Sometimes even that disappears completely.
Here is one individual that I found recently (shell diameter was ~5 mm). It has 2 lamellae in its aperture. This is Pilsbry's 3rd neanic substage (Fig. 235 in Land Mollusca of North America, vol. II:1).
In a paper that came out about a year ago, I hypothesized that in the aperture of the shell of the semi-terrestrial snail Pedipes ovalis, one long lamellae functions to protect the penumostome (breathing hole) from the movements of the foot.
I have long been puzzled by the lack of apertural formations in young shells and, in the case of species like Ventridens suppressus, in old shells. If they have a function, why are they not present at all life stages?