Aplacophora
Encyclopedia
Aplacophora is a monophyletic group of small, deep-water, exclusively benthic, shell-less marine mollusks found in all ocean
s of the world. The group comprises the two clades Solenogastres (Neomeniomorpha) and Caudofoveata (Chaetodermomorpha), which between them contain 28 families and about 320 species
. However, the relationship of these groups to the other molluscan classes and to each other is as yet unclear.
Aplacophorans are cylindrical and worm-like, and most very small, being no longer than 5 centimetres (2 in); some species, however, can reach a length of 30 centimetres (11.8 in).
while solenogasters are usually epibenthic. Both taxa are most common in water regions deeper than 20 metres (65.6 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.
, although small calcified spicule
s 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.
s. In 1987, they were officially recognized as molluscs and given their own class. This class is polyphyletic, and consists of two clade
s: the Solenogastres and the caudofoveata
.
Ocean
An ocean is a major body of saline water, and a principal component of the hydrosphere. Approximately 71% of the Earth's surface is covered by ocean, a continuous body of water that is customarily divided into several principal oceans and smaller seas.More than half of this area is over 3,000...
s of the world. The group comprises the two clades Solenogastres (Neomeniomorpha) and Caudofoveata (Chaetodermomorpha), which between them contain 28 families and about 320 species
Species
In biology, a species is one of the basic units of biological classification and a taxonomic rank. A species is often defined as a group of organisms capable of interbreeding and producing fertile offspring. While in many cases this definition is adequate, more precise or differing measures are...
. However, the relationship of these groups to the other molluscan classes and to each other is as yet unclear.
Aplacophorans are cylindrical and worm-like, and most very small, being no longer than 5 centimetres (2 in); some species, however, can reach a length of 30 centimetres (11.8 in).
Habitat
Caudofoveates generally burrow into the substrateSubstrate (marine biology)
Stream substrate is the material that rests at the bottom of a stream. There are several classification guides. One is:*Mud – silt and clay.*Sand – Particles between 0.06 and 2 mm in diameter.*Granule – Between 2 and 4 mm in diameter....
while solenogasters are usually epibenthic. Both taxa are most common in water regions deeper than 20 metres (65.6 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 shellMollusc shell
The mollusc shell is typically a calcareous exoskeleton which encloses, supports and protects the soft parts of an animal in the phylum Mollusca, which includes snails, clams, tusk shells, and several other classes...
, although small calcified spicule
Spicule
Spicules are tiny spike-like structures of diverse origin and function found in many organisms, such as the copulatory spicules of certain nematodes or the grains on the skin of some frogs.In sponges, spicules perform a structural function....
s 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
Cloaca
In zoological anatomy, a cloaca is the posterior opening that serves as the only such opening for the intestinal, reproductive, and urinary tracts of certain animal species...
, into which the anus
Anus
The anus is an opening at the opposite end of an animal's digestive tract from the mouth. Its function is to control the expulsion of feces, unwanted semi-solid matter produced during digestion, which, depending on the type of animal, may be one or more of: matter which the animal cannot digest,...
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
Radula
The radula is an anatomical structure that is used by molluscs for feeding, sometimes compared rather inaccurately to a tongue. It is a minutely toothed, chitinous ribbon, which is typically used for scraping or cutting food before the food enters the esophagus...
and a style
Digestive system of gastropods
The digestive system of gastropods has evolved to suit almost every kind of diet and feeding behavior. Gastropods as the largest taxonomic class of the mollusca are very diverse indeed: the group includes carnivores, herbivores, scavengers, filter feeders, and even parasites.In particular, the...
. Solenogasters are hermaphroditic
Hermaphrodite
In biology, a hermaphrodite is an organism that has reproductive organs normally associated with both male and female sexes.Many taxonomic groups of animals do not have separate sexes. In these groups, hermaphroditism is a normal condition, enabling a form of sexual reproduction in which both...
, but caudofoveates have two sexes, and reproduce by external fertilization
External fertilization
External fertilization is a form of fertilization in which a sperm cell is united with an egg cell external to the bodies of the reproducing individuals. In contrast, internal fertilization takes place inside the female after insemination through copulation....
. 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 echinodermEchinoderm
Echinoderms are a phylum of marine animals. Echinoderms are found at every ocean depth, from the intertidal zone to the abyssal zone....
s. In 1987, they were officially recognized as molluscs and given their own class. This class is polyphyletic, and consists of two clade
Clade
A clade is a group consisting of a species and all its descendants. In the terms of biological systematics, a clade is a single "branch" on the "tree of life". The idea that such a "natural group" of organisms should be grouped together and given a taxonomic name is central to biological...
s: the Solenogastres and the caudofoveata
Caudofoveata
Caudofoveata is a small class within the phylum Mollusca, also known as Chaetodermomorpha. The class is often combined with Solenogastres and termed Aplacophora. However, such a grouping is not monophyletic; molecular data suggests that the Caudofoveata are a sister group to the cephalopods.-...
.