#aplacophora

Monoplacophora. This class is made up of deep sea molluscs with a single cap-like shell

Aplacophora. This class is made up of worm-like deep-sea benthic molluscs which lack shells.

Kulindroplax perissokomos, a mollusc from the mid-Silurian of England (~428-423 mya).

About 4cm long (1.6″), it had a wormlike body covered in spicules (tiny spines) which suggests it was a member of the aplacophoran molluscs -- but it also had a row of seven larger shells along its back resembling those of chitons.

While arthropods and roundworms exceed the phylum Mollusca in terms of species, molluscs hold their own when it comes to anatomical diversity. There may be well over a million species of arthropod, but crabs, spiders and bees are all obviously related, sharing the same multi-limbed body plan organised around a jointed exoskeleton. Molluscs are very different. Clams, snails and squid are all…

Aplacophorans are molluscs who have completely given up on the molluscan life.

They have no shell, no foot, no tentacles and no eyes. Some have a radula but a lot of them lost that, too.

It turns out they're not relics from an ancient time when molluscs had no shell. Rather they've adapted to a lifestyle burrowing in the seabed or hanging onto the corals some of them eat.

Epimenia australis

...is a species of aplacophoran mollusc that is distributed throughout the waters around Indonesia and Papua New Guinea. Like other Solenogasters E.australis possesses a radula and uses it to feed on soft corals and other cnidarians. E.australis is mostly active at night and during the day it can be found hiding from predators within rocks.

Phylogeny

Animalia-Mollusca-Aplacophora-Cavibelonia-Epimeniidae-Epimenia-australis