The Clitellata are a class of annelid

worm Worms are many different distantly related bilateral animals that typically have a long cylindrical tube-like body, no limbs, and no eyes (though not always). Worms vary in size from microscopic to over in length for marine polychaete wo ...

s, characterized by having a clitellum - the 'collar' that forms a reproductive cocoon during part of their life cycles. The clitellates comprise around 8,000

species In biology, a species is the basic unit of classification and a taxonomic rank of an organism, as well as a unit of biodiversity. A species is often defined as the largest group of organisms in which any two individuals of the appropriate s ...

. Unlike the class of

Polychaeta Polychaeta () is a paraphyletic class of generally marine annelid worms, commonly called bristle worms or polychaetes (). Each body segment has a pair of fleshy protrusions called parapodia that bear many bristles, called chaetae, which ar ...

, they do not have 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

gonopore A gonopore, sometimes called a gonadopore, is a genital pore in many invertebrates. Hexapods, including insects have a single common gonopore, except mayflies, which have a pair of gonopores. More specifically, in the unmodified female it is t ...

s 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. 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 The periproct is the final body segment in annelid worms. The anus is located on this segment. The term also refers to the small region surrounding the anus of the sea urchin. See also *Prostomium *Earthworm *Sea urchin Sea urchins () are s ...

(terminal segment on which the anus is located). The gonads are located in a few segments near the clitellum, with the

testes A testicle or testis (plural testes) is the male reproductive gland or gonad in all bilaterians, including humans. It is homologous to the female ovary. The functions of the testes are to produce both sperm and androgens, primarily testoste ...

being anterior to the

ovaries The ovary is an organ in the female reproductive system that produces an ovum. When released, this travels down the fallopian tube into the uterus, where it may become fertilized by a sperm. There is an ovary () found on each side of the body. T ...

. 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. Leeches, order Hirudinida, mostly have flattened bodies, usually tapered at both ends. They have a fixed number of segments, 33, but the segmentation is not visible externally because the cuticle is marked with annulations. Leeches do not bear chaetae. The front few segments or head have been modified into a sucker that usually surrounds the mouth. These segments usually bear several

ocelli A simple eye (sometimes called a pigment pit) refers to a form of eye or an optical arrangement composed of a single lens and without an elaborate retina such as occurs in most vertebrates. In this sense "simple eye" is distinct from a multi-l ...

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.

Distribution and habitat

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

Hirudinea Leeches are segmented parasitic or predatory worms that comprise the subclass Hirudinea within the phylum Annelida. They are closely related to the oligochaetes, which include the earthworm, and like them have soft, muscular segmented bod ...

(leeches) contains three orders with various habitat preferences.

Branchiobdellida Branchiobdellida is an order of freshwater leech-like clitellates that are obligate Ectosymbiosis, ectosymbionts or ectoparasites, mostly of astacoidean crayfish. They are found in the Northern Hemisphere and have a Holarctic realm, holarctic dis ...

are

commensal Commensalism is a long-term biological interaction (symbiosis) in which members of one species gain benefits while those of the other species neither benefit nor are harmed. This is in contrast with mutualism, in which both organisms benefit fro ...

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 Oligochaeta () is a subclass of animals in the phylum Annelida, which is made up of many types of aquatic and terrestrial worms, including all of the various earthworms. Specifically, oligochaetes comprise the terrestrial megadrile earthworm ...

, 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.

Reproduction

All clitellata are hermaphrodites. During copulation, the clitellum produces a mucus that holds worms in place whilst they mate. During reproduction, the clitellum secretes a yolk (albumen) and a proteinaceous sheath which hardens. The worm then creeps out backward from the coat and deposits either fertilized

zygote A zygote (, ) is a eukaryotic cell formed by a fertilization event between two gametes. The zygote's genome is a combination of the DNA in each gamete, and contains all of the genetic information of a new individual organism. In multicell ...

s or both 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).

Systematics

According to modern phylogenetic analyses, the Clitellata are considered to be a monophyletic clade embedded deep in the

polychaete Polychaeta () is a paraphyletic class of generally marine annelid worms, commonly called bristle worms or polychaetes (). Each body segment has a pair of fleshy protrusions called parapodia that bear many bristles, called chaetae, which are made ...

s. The group is divided into the subclasses

and

. The oligochaetes contain the tubificids (

Naididae The Naididae (including the former family Tubificidae) are a family of clitellate oligochaete worms like the sludge worm, '' Tubifex tubifex''. They are key components of the benthic communities of many freshwater and marine ecosystems. In fres ...

, Lumbricidae, and Lumbriculidae - commonly the tube worms and the earthworms. Hirudinea contains leeches (Hirudinida), Acanthobdellida, and Branchiobdellida. Hirudinea is monophyletic, but the clade is embedded among the

. Two approaches are possible:Erséus ''et al.'' (2008) * abolish Oligochaeta as traditionally delimited in favor of a number of smaller monophyletic lineages * treat Oligochaeta and Clitellata as synonymous while splitting up the traditional "oligochaetes" into monophyletic lineages.

Footnotes

References

* (2008): ICZN rules – a farewell to Tubificidae (Annelida, Clitellata). ''

Zootaxa ''Zootaxa'' is a peer-reviewed scientific mega journal for animal taxonomists. It is published by Magnolia Press (Auckland, New Zealand). The journal was established by Zhi-Qiang Zhang in 2001 and new issues are published multiple times a week ...

'' 1744: 66–68
PDF fulltext
* (2006): ''Systematische Zoologie''.

External links

A Series of Searchable Texts on Earthworm Biodiversity, Ecology and Systematics from Various Regions of the World
{{Authority control Annelids