Clitellata
Encyclopedia
Clitellata is a class
of Annelid
worm
s, 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.
s.
During reproduction, the clitellum secretes a coat which hardens. The worm then creeps out backward from the coat and deposits either fertilized zygote
s 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).
of the polychaete
s.
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.
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:
Class (biology)
In biological classification, class is* a taxonomic rank. Other well-known ranks are life, domain, kingdom, phylum, order, family, genus, and species, with class fitting between phylum and order...
of Annelid
Annelid
The annelids , formally called Annelida , are a large phylum of segmented worms, with over 17,000 modern species including ragworms, earthworms and leeches...
worm
Worm
The term worm refers to an obsolete taxon used by Carolus Linnaeus and Jean-Baptiste Lamarck for all non-arthropod invertebrate animals, and stems from the Old English word wyrm. Currently it is used to describe many different distantly-related animals that typically have a long cylindrical...
s, characterized by having a clitellum
Clitellum
The clitellum is a thickened glandular section of the body wall in earthworms and leeches, that secretes a viscid sac in which the eggs are deposited...
- the 'collar' that forms a reproductive cocoon during part of their life cycle. The clitellates comprise around 8,000 species
Species
In biology, a species is one of the basic units of biological classification and a taxonomic rank. A species is often defined as a group of organisms capable of interbreeding and producing fertile offspring. While in many cases this definition is adequate, more precise or differing measures are...
. 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.Reproduction
All clitellata are hermaphroditeHermaphrodite
In biology, a hermaphrodite is an organism that has reproductive organs normally associated with both male and female sexes.Many taxonomic groups of animals do not have separate sexes. In these groups, hermaphroditism is a normal condition, enabling a form of sexual reproduction in which both...
s.
During reproduction, the clitellum secretes a coat which hardens. The worm then creeps out backward from the coat and deposits either fertilized zygote
Zygote
A zygote , or zygocyte, is the initial cell formed when two gamete cells are joined by means of sexual reproduction. In multicellular organisms, it is the earliest developmental stage of the embryo...
s or both ovae
Ovum
An ovum is a haploid female reproductive cell or gamete. Both animals and embryophytes have ova. The term ovule is used for the young ovum of an animal, as well as the plant structure that carries the female gametophyte and egg cell and develops into a seed after fertilization...
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).
Systematics
According to modern phylogenetic analyses the clitellata are considered to be a monophyletic subcladeSubclade
In genetics, subclade is a term used to describe a subgroup of a subgenus or haplogroup. It is commonly used today in describing genealogical DNA tests of human mitochondrial DNA haplogroups and human Y-chromosome DNA haplogroups....
of the polychaete
Polychaete
The Polychaeta or polychaetes are a class of annelid worms, generally marine. Each body segment has a pair of fleshy protrusions called parapodia that bear many bristles, called chaetae, which are made of chitin. Indeed, polychaetes are sometimes referred to as bristle worms. More than 10,000...
s.
Historically, the group was classified into the subclasses oligochaetes and the hirudinea. The oligochetes contained the tubificids, lumbricidae
Lumbricidae
The Lumbricidae is a family of earthworms which includes most of the earthworm species well known to Europeans. About 33 lumbricid species have become naturalized around the world, but the bulk of the species are in Holarctic: from Canada and the United States The Lumbricidae is a family of...
and the lumbriculidae
Lumbriculidae
The Lumbriculidae are a family of microdrile oligochaetes common in fresh-water environments, including streams, lakes, marshes, wells and ground-water. They should not be confused with the earthworm family Lumbricidae. Many species and even genera are highly endemic, mainly in Siberia and the...
- 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
Lumbriculidae
The Lumbriculidae are a family of microdrile oligochaetes common in fresh-water environments, including streams, lakes, marshes, wells and ground-water. They should not be confused with the earthworm family Lumbricidae. Many species and even genera are highly endemic, mainly in Siberia and the...
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)
- OligochaetaOligochaetaOligochaeta is a subclass of animals in the biological phylum Annelida, which is made up of many types of aquatic and terrestrial worms, and this includes all of the various earthworms...
(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
Oligochaeta
Oligochaeta is a subclass of animals in the biological phylum Annelida, which is made up of many types of aquatic and terrestrial worms, and this includes all of the various earthworms...
", which are actually an evolutionary grade
Evolutionary grade
In alpha taxonomy, a grade refers to a taxon united by a level of morphological or physiological complexity. The term was coined by British biologist Julian Huxley, to contrast with clade, a strictly phylogenetic unit.-Definition:...
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:
- 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.