Aculifera = Aplacophora + Polyplacophora (chitons)

The Aculifera hypothesis unites the molluscs that possess sclerites by placing Polyplacophora as the sister taxon of Aplacophora. Aculiferans are the sister group to all other living molluscs so this group is important to study in order to understand the evolution of Mollusca as a whole. Molluscs are interesting to researchers because many species are economically important and food, producers of pearls and shells, and as invasive species or biofoulers. Plus they're ace (ace is Australian slang for cool, I'm really not kidding about this cheesy crocodile hunter parody)!


The worm-like aplacophoran molluscs are exclusively marine animals characterized by a narrow or completely reduced foot, a unique dorsoterminal sensory organ, and a small mantle (=pallial) cavity restricted to the posterior-most part of the body. As the name suggests, aplacophorans completely lack a shell. Instead, they are covered in a dense coat of spiny and/or scale-like calcareous sclerites. There are two distinct lineages of aplacophorans: Caudofoveata (also called Chaetodermomorpha) and Solenogastres (also called Neomeniomorpha). Although unfamiliar to even many zoologists, many of the roughly 400 described species of aplacophoran molluscs are important members of many marine benthic communities.

Cavibelonia sp. (Aplacophora, Solenogastres) collected recently during

an expedition in Antarctica. Anterior is to the left.

Polyplacophorans, or chitons are slug-like animals with eight plates covering their backs. These plates overlap somewhat at the front and back margins and articulate with each other to form a flexible dorsal armor. This allows chitons to be flexible, presumably an adaptation for living in the rocky intertidal as many species do. The shell plates are surrounded by mantle tissue bearing sclerites like those found in aplacophorans. Currently around 1,000 species of chitons are described.

Lepidopleurida sp. (Polyplacophora) collected recently during an expedition
in Antarctica. I'm not sure if this is a juvenile or just a really tiny species.
Anterior is to the right.