Oligyra orbiculata

This little snail is an evolutionary oddity, a fish out of water. I found this particular individual clinging to a tree in the upper reaches of Shakerag Hollow. The common name is “globular drop,” quite appropriate given its pea-sized shell and rotund shape. The shell is very thick and feels like a pebble.

oligyra treeUnlike all other land snails and slugs in our region, this species belongs to the family Helicinidae, a group of mostly tropical land snails that are more closely related to the marine snails than they are to other terrestrial snails. The animal’s body reveals this kinship: it has an operculum (a “trap door”) with which it seals its shell when withdrawn, it has just one pair of antennae (other land snails have two pairs), and its eyes are located at the base of the tentacles instead of at their tips. These seemingly minor differences indicate a deep divergence, analogous perhaps to the difference between a bird and a bat. Like birds and bats, the two lineages of snails independently evolved the ability to live in a new habitat — creeping onto land. Not quite as impressive as taking wing, but then what use is a radula in the air?

The following photographs are from another individual, one that lived in the limestone at the bottom of the mountain.

Oligyra orbiculata with operculum protecting the shell aperture.

Oligyra orbiculata with operculum protecting the shell aperture.

One pair of tentacles...

One pair of tentacles…

4 thoughts on “Oligyra orbiculata

  1. Jacob Ham

    Thank you. I enjoyed learning about this snail. It makes me feel like I’m out enjoying nature on a wonderful walk with a learned docent.

    Jacob

    Reply

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