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Clitellates - Class: Clitellata
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“It may be doubted whether there are many other animals which have played so important part in the history of the world,
as played by these less organized animals." - Charles Darwin (1809-1882) in his book:
"The Formation of Vegetable Mould, through the Actions of Worms, with Observations on their Habits", (1881), pp 313.
as played by these less organized animals." - Charles Darwin (1809-1882) in his book:
"The Formation of Vegetable Mould, through the Actions of Worms, with Observations on their Habits", (1881), pp 313.
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The Class: Clitellata is one of two classes within the Phylum: Annelida, the other being the Polychaeta. The difference between the two being that the Clitellates do not have parapodia, paired fleshy growths protruding from both sides of the body and which can serve any one or more of a number of functions. The Polychaeta also possess more developed heads; in Clitellates the head end is often indistinguishable from the tail end. Clitellates do have a clitellum once they are mature. The clitellum (= girdle or collar) is a glandular swelling of a number of segments in the forward half of the body. It is here that cocoons or egg capsules are formed.
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Clitellates, being soft-bodied, do not appear much in the fossil records. More commonly, traces of these animals are found, such as the mineralized tubes some species produce. The oldest Clitellate fossils known appear to be leech cocoons from the Triassic (252 - 201 Mya) which suggests that they probably originated as a group in the Paleozoic. The earliest oligochaete fossils are from the Tertiary which started some 66 Mya. There may well be a link between the evolution of the Oligochaetes (as well as a myriad of insects) evolving at about the same time as a boom in diversity amongst flowering plants occurred starting in the early Cretaceous (145 - 66 Mya).
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Image 1. Fragment of a fossilized worm spermatozoa. Image credit: Swedish Museum of Natural History (Dept. of Palaeobiology)
Scale: the bar = 1 micron.
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The division of the Class: Clitellata into the two subclasses Hirudinea and Oligochaeta is still acceptable to many taxonomists but it should be borne in mind that this is likely to change before too long. The Catologue of Life no longer recognises the division; whereas the Subclass: Oligochaeta remains, the Subclass: Hirudinea has been downgraded to family level. In its' place is now a subclass without a name assigned to it.
Currently some 10,000 species of species of Oligochaeta are known which is about half of the Phylum: Anellida. In contrast there are but a mere 680 or so species of leeches. About 100 of these are marine species, 480 are freshwater species and about 100 are terrestrial. |
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References and links:
> Image 1. Fragment of a fossilized worm spermatozoa. Image credit: Swedish Museum of Natural History (Dept. of Palaeobiology)
> Briones, Maria J.I., (2023). 'Earthworms: Essential ecosystem engineers providing vital ecosystem services.' Editor(s): M. J. Goss, M. Oliver, Encyclopedia of Soils in the Environment (Second Edition), Academic Press, 2023, Pages 112-119, https://doi.org/10.1016/B978-0-12-822974-3.00076-8.
> Kumar, R. et al., (2023). 'The Earthworms: Charles Darwin’s Ecosystem Engineer.' Organic Fertilizers - New Advances and Applications [Working Title]. IntechOpen,10-5-2023.
doi: 10.5772/intechopen.1001339.
> Selvan, C.J.D., Azhagesan, A., Ganesan, M., Chelladurai, K.S.N., Paulraj, V.D. and Christyraj, J.R.S.S., (2019). 'Understanding the Role of the Clitellum in the Regeneration Events of the Earthworm Eudrilus eugeniae.' Cells Tissues Organs. 2019;208(3-4):134-141. doi: 10.1159/000507243
> Sket, B. and Trontelj, P., (2008). 'Global diversity of leeches (Hirudinea) in freshwater.' Hydrobiologia. 595. 129-137. doi: 10.1007/s10750-007-9010-8.
https://www.britannica.com/biography/Charles-Darwin
https://www.britannica.com/science/Cretaceous-Period
https://www.britannica.com/science/Paleozoic-Era
https://www.britannica.com/science/Triassic-Period
https://www.catalogueoflife.org/
https://www.livescience.com/29231-cretaceous-period.html
https://www.theguardian.com/science/2015/jul/15/fossilised-sperm-found-in-antarctica-is-worlds-oldest-say-scientists
https://en.wikipedia.org/wiki/Clitellum
https://en.wikipedia.org/wiki/Leech
https://en.wikipedia.org/wiki/Parapodium
> Image 1. Fragment of a fossilized worm spermatozoa. Image credit: Swedish Museum of Natural History (Dept. of Palaeobiology)
> Briones, Maria J.I., (2023). 'Earthworms: Essential ecosystem engineers providing vital ecosystem services.' Editor(s): M. J. Goss, M. Oliver, Encyclopedia of Soils in the Environment (Second Edition), Academic Press, 2023, Pages 112-119, https://doi.org/10.1016/B978-0-12-822974-3.00076-8.
> Kumar, R. et al., (2023). 'The Earthworms: Charles Darwin’s Ecosystem Engineer.' Organic Fertilizers - New Advances and Applications [Working Title]. IntechOpen,10-5-2023.
doi: 10.5772/intechopen.1001339.
> Selvan, C.J.D., Azhagesan, A., Ganesan, M., Chelladurai, K.S.N., Paulraj, V.D. and Christyraj, J.R.S.S., (2019). 'Understanding the Role of the Clitellum in the Regeneration Events of the Earthworm Eudrilus eugeniae.' Cells Tissues Organs. 2019;208(3-4):134-141. doi: 10.1159/000507243
> Sket, B. and Trontelj, P., (2008). 'Global diversity of leeches (Hirudinea) in freshwater.' Hydrobiologia. 595. 129-137. doi: 10.1007/s10750-007-9010-8.
https://www.britannica.com/biography/Charles-Darwin
https://www.britannica.com/science/Cretaceous-Period
https://www.britannica.com/science/Paleozoic-Era
https://www.britannica.com/science/Triassic-Period
https://www.catalogueoflife.org/
https://www.livescience.com/29231-cretaceous-period.html
https://www.theguardian.com/science/2015/jul/15/fossilised-sperm-found-in-antarctica-is-worlds-oldest-say-scientists
https://en.wikipedia.org/wiki/Clitellum
https://en.wikipedia.org/wiki/Leech
https://en.wikipedia.org/wiki/Parapodium
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