Clitellata

Clitellata
Earthworm
Scientific classification
Kingdom:	Animalia
Phylum:	Annelida
Class:	Clitellata
Subclasses
Branchiobdellae Hirudinea "Oligochaeta" (paraphyletic) and see text

Clitellata is a class of Annelid worms, characterized by having a clitellum - the 'collar' that forms a reproductive cocoon during part of their life cycle. The clitellates comprise around 8,000 species. Unlike the class of Polychaeta, they do not have parapodia and their heads are less developed.

Habitats

Most clitellates live on land, in freshwater and in the ocean.^{[citation needed]}

Reproduction

All clitellata are hermaphrodites. During reproduction, the clitellum secretes a coat which hardens. The worm then creeps out backward from the coat and deposits either fertilized zygotes or both ovae and sperms into the coat, which is then packed into a cocoon. The zygotes then evolve further directly in the cocoon without passing through a larva stadium (as opposed to other annelids, e.g. polychaeta.) This mechanism is considered to be apomorphic (newer in evolution).^[1]

Systematics

According to modern phylogenetic analyses the clitellata are considered to be a monophyletic subclade of the polychaetes.

Historically, the group was classified into the subclasses oligochaetes and the hirudinea. The oligochetes contained the tubificids, lumbricidae and the lumbriculidae - commonly the tube worms and the earthworms. The hirudinea contained the leeches and the branchiobdellids. Modern analysis has revealed that the branchiobdella and hirudinea are two sister groups to the lumbriculidae and that they are daughter groups to the tree of oligochaetes. Hence in currently, the terms oligochaete and clitellata are considered synonymous.

• Branchiobdella - formerly in Hirudinea
• Hirudinea (leeches)
• Oligochaeta (earthworms - aquatic microdriles + terrestrial megadriles)

The Acanthobdellidea, a sister group to Hirudinea are sometimes moved out of the Hirudinea as a distinct subclass too. Overall. clitellate phylogeny is not well resolved.

Namely, the Acanthobdellidea, Branchiobdella and Hirudinea are monophyletic but actually embedded among the "Oligochaeta", which are actually an evolutionary grade of lineages that are outwardly similar but not actually very close relatives. In particular, the leeches and earthworms appear to be very close relatives. Two approaches are possible:^[2]

• 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

1. ^ Reichardt (2006): pp.63, 67-68
2. ^ 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

• Brief description
• A Series of Searchable Texts on Earthworm Biodiversity, Ecology and Systematics from Various Regions of the World