Scientific name Clitellata Rank Class | Higher classification Annelid | |
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Lower classifications Oligochaeta, Earthworms, Branchiobdellida, Leech | ||
Lineus viridis nemertea heteronemertea feeding on paranais litoralis annelida clitellata
The Clitellata are a class of annelid worms, 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. Unlike the class of Polychaeta, they do not have parapodia and their heads are less developed.
Contents
Habitats
Most clitellates live on land, in freshwater or in the ocean.
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 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 subclade of the polychaetes.
Historically, the group was classified into the subclasses Oligochaeta and the Hirudinea. The oligochaetes contained the tubificids (Naididae, Lumbricidae, and Lumbriculidae - commonly the tube worms and the earthworms. The Hirudinea contained the leeches and the branchiobdellids. Modern analysis has revealed Branchiobdella and Hirudinea are two sister groups to the lumbriculids and they are daughter groups to the tree of oligochaetes. Hence, the terms Oligochaeta 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 are embedded among the Oligochaeta, which are an evolutionary grade of lineages that are outwardly similar, but not very close relatives. In particular, the leeches and earthworms appear to be very close relatives. Two approaches are possible: