Aplacophora

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Aplacophora
Head region of the caudofoveate Falcidens
Scientific classification Edit this classification
Domain: Eukaryota
Kingdom: Animalia
Phylum: Mollusca
Class: Aplacophora
Included groups

Aplacophora

benthic, marine molluscs found in all oceans of the world. [1]

All known modern forms are shell-less: only some extinct primitive forms possessed valves. 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 in); some species, however, can reach a length of 30 centimetres (12 in).

Habitat

Caudofoveates generally burrow into the

detritovores or feed on foraminiferans.[2]

Description

Epimenia babai, a member of the Solenogastres

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

aragonitic habit as the spines mature.[6] 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.[8][9] 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.[10]

The relationship with other molluscs, however, is apparent from some features of the digestive system; aplacophorans possess both a radula and a style. A variety of radular forms and functions exist.[11] 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.[4] Caudofoveates also differ from Solenogasters in having a head shield and a body that is differentiated into three sections.[12]

Taxonomy and evolution

This class was once classified as

sea cucumbers in the echinoderms. In 1987, they were officially recognized as molluscs and given their own class. It consists of two clades: the Solenogastres and the Caudofoveata. It has been considered polyphyletic[13] but more recent molecular evidence supports it as a monophyletic clade.[14][15]

The affinities of Aplacophorans have long been uncertain. Molecular and fossil evidence seemed to put Aplacophorans in the clade

Polyplacophora.[14][15][16][17][18] 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.[19]

References

  1. ^ 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
  2. S2CID 85911879
    .
  3. ^ Haas, Winfried (1981). "Evolution of calcareous hardparts in primitive molluscs". Malacologia. 21: 403–418.
  4. ^
    ISBN 978-0-03-056747-6
    .
  5. doi:10.1016/0305-0491(72)90117-4
    .
  6. ^
    S2CID 15935925
    .
  7. S2CID 135492113
    .
  8. ^ "The Aplacophoran Family Prochaetodermatidae in the North American Basin, Including Chevroderma N.G. and Spathoderma N.G. (Mollusca; Chaetodermomorpha)*". Retrieved 7 June 2013. {{cite journal}}: Cite journal requires |journal= (help)
  9. S2CID 28029055
    .
  10. PMID 20181015
    .
  11. doi:10.1111/j.1744-7410.2005.00023.x
    .
  12. ^ Biology and Evolution of the Mollusca
  13. PMID 16675549
    .
  14. ^
    PMID 21892190
    .
  15. ^
    S2CID 4321076
    .
  16. PMID 17652065. For a summary, see "The Mollusca"
    . University of California Museum of Paleontology. Retrieved 2 October 2008.
  17. PMID 29300613
    .
  18. PMID 21976685
    .
  19. S2CID 4422386
    .
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