Clitellata

Source: Wikipedia, the free encyclopedia.

Clitellata
Temporal range: Triassic–Recent[1]
Earthworm
Scientific classification Edit this classification
Domain: Eukaryota
Kingdom: Animalia
Phylum: Annelida
Clade: Pleistoannelida
Clade: Sedentaria
Class: Clitellata
Subclasses

Branchiobdellae

Hirudinea

"
paraphyletic
)
and see text

The Clitellata are a

parapodia
and their heads are less developed.

Characteristics

Clitellate annelids are segmented worms characterised by the clitellum or girdle which is located near the head end of mature individuals. The mouth is on the ventral surface and is overhung by the prostomium (proboscis). The brain is not located in the head but in one of the body segments. The clitellum is formed by a modification of several segments, and either includes the female gonopores or is located just behind them. During copulation, this glandular tissue secretes mucus that keeps the paired individuals together while they exchange sperm. Afterwards it secretes material that forms a cocoon that encircles the animal's body and encloses the eggs and sperm. The animal works this cocoon forward and over its head end, whereupon the ends of the cocoon become sealed, with fertilisation and development taking place inside.[2][3]

Earthworms and their kin, in the subclass Oligochaeta, lack eyes but have photoreceptor cells in the skin, especially in the dorsal portion of the anterior end. They also lack parapodia and appendages on the prostomium, the body and the periproct (terminal segment on which the anus is located). The gonads are located in a few segments near the clitellum, with the testes being anterior to the ovaries. There are four bundles of one to twenty-five chaetae on each segment; these have muscles attached to their bases and can be extended or retracted.[2]

Leeches, order Hirudinida,

ocelli on the upper side. The clitellum occupies segments 9 to 11 but is only noticeable during breeding periods. The hindermost segments form another, larger, disc-shaped sucker located on the underside of the body. The anus is on the dorsal surface just in front of the posterior sucker. The body wall includes strong transverse, longitudinal and diagonal muscles which give the animal great flexibility and extensibility.[2]

Distribution and habitat

Clitellates live on land, in freshwater or in the ocean. The subclass

Hirudinea (leeches) contains three orders with various habitat preferences. Branchiobdellida are commensal with freshwater crayfish, grazing algae from their exoskeletons. Acanthobdellida are parasitic on freshwater fishes such as grayling. Leeches
can be found in nearly every part of the world, in freshwater, terrestrial, and marine habitats.

The subclass Oligochaeta, which includes the earthworms as the largest members of the group, mostly live on land, burrowing in damp soil. Smaller freshwater species burrow in mud or live among aquatic vegetation. The marine species are mostly tiny and live in the interstices between sand grains, from the intertidal zone to the deep sea.[2]

Reproduction

All clitellata are

ovae and sperm into the coat, which is then packed into a cocoon. The zygotes then develop directly in the cocoon without passing through a larval stage (as opposed to other annelids, e.g. Polychaeta.) This mechanism is considered to be apomorphic (a newly derived characteristic rather than an evolutionarily ancestral one).[5]

Systematics

See also:
List of Annelid families

According to modern phylogenetic analyses, the Clitellata are considered to be a

monophyletic clade embedded deep in the polychaetes.[6][7][8]

The group is divided into the subclasses

Hirudinea. The oligochaetes contain the tubificids (Naididae, Lumbricidae, and Lumbriculidae - commonly the tube worms and the earthworms. Hirudinea contains leeches (Hirudinida), Acanthobdellida, and Branchiobdellida.[9]

Hirudinea is

monophyletic, but the clade is embedded among the Oligochaeta. Two approaches are possible:[10]

Footnotes

  1. doi:10.1111/j.1463-6409.1991.tb00300.x
    .
  2. ^
    ISBN 978-81-315-0104-7.{{cite book}}: CS1 maint: multiple names: authors list (link
    )
  3. ^ "Animal reproductive system: Mechanisms that aid in the union of gametes: Annelids and mollusks". Encyclopedia Britannica. Retrieved 25 October 2017.
  4. doi:10.1016/j.ympev.2018.05.001
    .
  5. ^ Reichardt (2006): pp.63, 67–68
  6. PMID 19660115
    .
  7. S2CID 4428998
    .
  8. PMID 24567512
    .
  9. doi:10.1016/j.ympev.2018.05.001
    .
  10. ^ a b c Erséus et al. (2008)

References

  • Erséus, Christer; Wetzel, Mark J. & Gustavsson, Lena (2008): ICZN rules – a farewell to Tubificidae (Annelida, Clitellata). Zootaxa 1744: 66–68. PDF fulltext
  • Reichardt, Anna Katharina (2006): Systematische Zoologie.

External links

Wikimedia Commons has media related to Clitellata.
Retrieved from "https://en.wikipedia.org/w/index.php?title=Clitellata&oldid=1188077032"