Aplacophora is a class of small, deep-water, exclusively

benthic The benthic zone is the ecological region at the lowest level of a body of water such as an ocean, lake, or stream, including the sediment surface and some sub-surface layers. The name comes from the Ancient Greek word (), meaning "the depths". ...

, marine

molluscs Mollusca is a phylum of protostome, protostomic invertebrate animals, whose members are known as molluscs or mollusks (). Around 76,000 extant taxon, extant species of molluscs are recognized, making it the second-largest animal phylum ...

found in all

ocean The ocean is the body of salt water that covers approximately 70.8% of Earth. The ocean is conventionally divided into large bodies of water, which are also referred to as ''oceans'' (the Pacific, Atlantic, Indian Ocean, Indian, Southern Ocean ...

s of the world. MolluscaBase eds. (2021). MolluscaBase. Aplacophora. Accessed through: World Register of Marine Species at: http://www.marinespecies.org/aphia.php?p=taxdetails&id=411 on 2021-04-13 All known modern forms are shell-less: only some extinct primitive forms possessed valves. The group comprises the two classes

Solenogastres The Solenogastres (less often referred to as Neomeniomorpha), common name, commonly known as solenogasters, are a class of small, worm-like, shell-less Mollusca, molluscs (Aplacophora), the other class being the Caudofoveata (Chaetodermomorpha).M ...

(Neomeniomorpha) and

Caudofoveata Caudofoveata is a small class within the phylum Mollusca, also known as Chaetodermomorpha. The class is often combined with Solenogastres and termed Aplacophora, but some studies have cast doubt on the monophyly of this group. Anatomy Caudof ...

(Chaetodermomorpha), which between them contain 28 families and about 320

species A species () is often defined as the largest group of organisms in which any two individuals of the appropriate sexes or mating types can produce fertile offspring, typically by sexual reproduction. It is the basic unit of Taxonomy (biology), ...

. 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 ; some species, however, can reach a length of .

Habitat

Caudofoveates generally burrow into the substrate while solenogasters are usually epibenthic. Both taxa are most common in water regions deeper than where some species may reach densities up to four or five specimens per square metre (three or four per square yard). Solenogasters are typically

carnivore A carnivore , or meat-eater (Latin, ''caro'', genitive ''carnis'', meaning meat or "flesh" and ''vorare'' meaning "to devour"), is an animal or plant Plants are the eukaryotes that form the Kingdom (biology), kingdom Plantae; they ar ...

s feeding on

cnidaria Cnidaria ( ) is a phylum under kingdom Animalia containing over 11,000 species of aquatic invertebrates found both in fresh water, freshwater and marine environments (predominantly the latter), including jellyfish, hydroid (zoology), hydroids, ...

ns or sometimes

annelid The annelids (), also known as the segmented worms, are animals that comprise the phylum Annelida (; ). The phylum contains over 22,000 extant species, including ragworms, earthworms, and leeches. The species exist in and have adapted to vario ...

s or other taxa while caudofoveates are mostly detritovores or feed on

foraminifera Foraminifera ( ; Latin for "hole bearers"; informally called "forams") are unicellular organism, single-celled organisms, members of a phylum or class (biology), class of Rhizarian protists characterized by streaming granular Ectoplasm (cell bio ...

ns.

Description

Aplacophorans are worm-like animals, with little resemblance to most other molluscs. They have no

shell Shell may refer to: Architecture and design * Shell (structure), a thin structure ** Concrete shell, a thin shell of concrete, usually with no interior columns or exterior buttresses Science Biology * Seashell, a hard outer layer of a marine ani ...

, although small calcified spicules are embedded in the skin; these spicules are occasionally coated with an organic pellicle that is presumably secreted by microvilli. Caudofoveates lack a foot while solenogasters have a narrow foot which lacks intrinsic musculature. The mantle cavity is reduced into a simple

cloaca A cloaca ( ), : cloacae ( or ), or vent, is the rear orifice that serves as the only opening for the digestive (rectum), reproductive, and urinary tracts (if present) of many vertebrate animals. All amphibians, reptiles, birds, cartilagin ...

, into which the

anus In mammals, invertebrates and most fish, the anus (: anuses or ani; from Latin, 'ring' or 'circle') is the external body orifice at the ''exit'' end of the digestive tract (bowel), i.e. the opposite end from the mouth. Its function is to facil ...

and excretory organs empty, and is located at the posterior of the animal. The head is rudimentary, and has no eyes or tentacles. The cuticle of both subclasses is chitinous, and has an irregular texture. Spines bear an organic matrix.

Sclerite A sclerite (Greek language, Greek , ', meaning "hardness, hard") is a hardened body part. In various branches of biology the term is applied to various structures, but not as a rule to vertebrate anatomical features such as bones and teeth. Instea ...

s can be hollow or solid; the void within the sclerites of some species fills during growth. Sclerites generally form within the cuticle, protruding through when they are fully grown. They probably start life as amorphous calcium carbonate, which the organic matrix coaxes into an aragonitic habit as the spines mature. Sclerites can be shaped as simple spines, straight, curved, keeled, striated or hooked; or as cupped blades; more complex arrangements are common in copulatory spicules. In several species of solenogaster, sclerites change morphology during growth; young specimens might bear flat, solid, scale-like sclerites, to be replaced with longer hollow spine-like sclerites in adults. The relationship with other molluscs, however, is apparent from some features of the digestive system; aplacophorans possess both a radula and a

style Style, or styles may refer to: Film and television * ''Style'' (2001 film), a Hindi film starring Sharman Joshi, Riya Sen, Sahil Khan and Shilpi Mudgal * ''Style'' (2002 film), a Tamil drama film * ''Style'' (2004 film), a Burmese film * '' ...

. A variety of radular forms and functions exist. Solenogasters are hermaphroditic and assumed to have internal fertilization, in contrast to caudofoveates which 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. Caudofoveates also differ from Solenogasters in having a head shield and a body that is differentiated into three sections.

Taxonomy and evolution

This class was once classified as

sea cucumber Sea cucumbers are echinoderms from the class (biology), class Holothuroidea ( ). They are benthic marine animals found on the sea floor worldwide, and the number of known holothuroid species worldwide is about 1,786, with the greatest number be ...

s in the

echinoderm An echinoderm () is any animal of the phylum Echinodermata (), which includes starfish, brittle stars, sea urchins, sand dollars and sea cucumbers, as well as the sessile sea lilies or "stone lilies". While bilaterally symmetrical as ...

s. In 1987, they were officially recognized as molluscs and given their own class. It consists of two

clade In biology, a clade (), also known as a Monophyly, monophyletic group or natural group, is a group of organisms that is composed of a common ancestor and all of its descendants. Clades are the fundamental unit of cladistics, a modern approach t ...

s: the

and the

. It has been considered polyphyletic but more recent molecular evidence supports it as a monophyletic clade. The affinities of Aplacophorans have long been uncertain. Molecular and fossil evidence seemed to put Aplacophorans in the clade Aculifera, as a sister group to Polyplacophora. For a summary, see The discovery of '' Kulindroplax'' in 2012, a fossil aplacophoran with a polyplacophoran-like armour, strongly supports this hypothesis, and shows that aplacophorans evolved from progenitors that bore valves.

References

{{Taxonbar, from=Q213228 Aplacophorans Mollusc classes