In the recently discovered Archivolva cheripodia Lorenz, 2024 from the Maldives, an organ at the tip of the foot was observed, which aids in navigating across the branches of the host.
It had never been recognized in other species of the family, but closer scrutiny revealed that it does exist also in other genera and species.

Sequences and movies showing the organ:

A. cheripoda 1

A. cheripoda 2

A. cheripoda 3

D. masaoi 1

D. masaoi 2

Extracts from the description of the new species and the cheripodium:

The foot is translucent and pale reddish brown, with darker brown stripes above the edge in the anterior half. The posterior half shows fainter and finer branching hairlines. When resting on its host, the foot is visible only as a narrow band encircling the shell. While crawling below the surface of water in a container, it can be extended to almost twice the length of the shell. It is elongated and not split in a propodial and metapodial section. A whitish area at the anterior end of the foot is well-discernible. It is embedded in the tissue of the foot, above the pedal fold running across the crawling surface in the frontsection.

 

When kept in a container, animals were observed to lift their shell from the ground, balancing on the posterior portion of their foot. A white structure protruded from the pedal fold, extending to a bell-shaped, muscular, tube with six dark colored ridges dorsally, and a contractible canal basally, which could be extended beyond the area bearing dark striped ridges. The white structure was joined to the darker striped foot by a narrow transparent flange, and at first sight, it reminded of a gigantic proboscis. When fully extended, the upper part appeared somewhat detached from the foot. It could be extended to an length of more than 3 mm, rapidly moved up, down, and sideways, with its canal contracting and expanding, while the animal itself was remaining in the same spot, balancing on the posterior half of its foot. The movements and contractions of the tube reminded of a hand reaching about in search of a holdfast. When partly retracted, the dorsal ridges became visible as short digits along the frame of that structure when viewing the animal through a glass. When the animal got disturbed, or returned to resting on all of its foot, the structure retracted into the pedal fold within seconds and was then only visible through the tissue of the foot as the whitish area described above.

This remarkable hand-like structure emerging from above the pedal fold has never been described before. Its function apparently is that of a grasping hand reaching out for holfast when the animal is moving between branches of the host. The foot of an ovulid itself is not split in a propodium and metapodium as in other gastropods. As the cheripodium is not homologous to a propodium, it deserves recognition as a distinct organ.