Aplacophora

Aplacophora
Scientific classification
Kingdom:	Animalia
Phylum:	Mollusca
Class:	"Aplacophora"
Included groups
Chaetodermomorpha (Caudofoveata) Chaetodermidae Falcidentidae Limifossoridae Metachaetodermatidae Prochaetodermatidae Scutopodidae Neomeniomorpha (Solenogastres) Aplotegmentaria Neomeniamorpha Hemimeniidae Neomeniidae Pholidoskepia Dondersiidae Gymnomeniidae Lepidomeniidae Macellomeniidae Meiomeniidae Sandalomeniidae Pachytegmentaria Cavibelonia Acanthomeniidae Amphimeniidae Drepanomeniidae Epimeniidae Notomeniidae Proneomeniidae Pruvotinidae Rhipidoherpiidae Rhopalomeniidae Simrothiellidae Strophomeniidae Syngenoherpiidae Sterrofustia Heteroherpiidae Imeroherpiidae Phyllomeniidae

Aplacophora is a monophyletic group of small, deep-water, exclusively benthic, shell-less marine mollusks found in all oceans of the world. The group comprises the two clades Solenogastres (Neomeniomorpha) and Caudofoveata (Chaetodermomorpha), which between them contain 28 families and about 320 species. The aplacophorans are traditionally considered ancestral to the other mollusc classes. However, the relationship between the two aplacophoran groups and to the other molluscan classes and to each other is as yet unclear.

Aplacophorans are cylindrical and worm-like in form, and most very small, being no longer than 5 centimetres (2.0 in); some species, however, can reach a length of 30 centimetres (12 in).

Habitat

Caudofoveates generally burrow into the substrate while solenogasters are usually epibenthic. Both taxa are most common in water regions deeper than 20 metres (66 ft) where some species may reach densities up to 4 or 5 specimens per m². Solenogasters are typically carnivores feeding on cnidarians or sometimes annelids or other taxa while caudofoveates are mostly detritovores or feed on foraminiferans.

Description

Aplacophorans are worm-like animals, with little resemblance to most other molluscs. They have no shell, although small calcified spicules are embedded in the skin. Caudofoveates lack a foot while solenogasters have a narrow foot which lacks intrinsic musculature. The mantle cavity is reduced in to a simple cloaca, into which the anus and excretory organs empty, and is located at the posterior of the animal. The head is rudimentary, and has no eyes or tentacles.

The relationship with other molluscs, however, is apparent from some features of the digestive system; aplacophorans possess both a radula and a style. Solenogasters are hermaphroditic, but caudofoveates have two sexes, and reproduce by external fertilization. During development, the mantle cavity of the larva curls up and closes, creating the worm-like form of the adult.

Taxonomy

This class was once classified as sea cucumbers in the echinoderms. In 1987, they were officially recognized as molluscs and given their own class. This class is paraphyletic or polyphyletic, and consists of two clades: the Solenogastres and the caudofoveata.