{{short description|Parasitic or predatory annelid worms}}{{Other uses}}{{pp-semi-indef}}{{pp-move-indef}}{{EngvarB|date = June 2020}}{{Use dmy dates|date=May 2020}}{{Automatic taxobox

Permianâ€“recent, {{Fossil_range >266

earliest=Kasimovian}} Possible Virgilian record| image = Sucking leech.jpg| image_caption = Hirudo medicinalis sucking blood| image2 = EuropÃ¤ischer-Platt-Egel cropped.jpg| image2_caption = Helobdella sp.| taxon = Hirudinea

Jean-Baptiste Lamarck>Lamarck 1818}}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 bodies that can lengthen and contract. Both groups are hermaphrodites and have a clitellum, but leeches typically differ from the oligochaetes in having suckers at both ends and in having ring markings that do not correspond with their internal segmentation. The body is muscular and relatively solid, and the coelom, the spacious body cavity found in other annelids, is reduced to small channels.The majority of leeches live in freshwater habitats, while some species can be found in terrestrial or marine environments. The best-known species, such as the medicinal leech, Hirudo medicinalis, are hematophagous, attaching themselves to a host with a sucker and feeding on blood, having first secreted the peptide hirudin to prevent the blood from clotting. The jaws used to pierce the skin are replaced in other species by a proboscis which is pushed into the skin. A minority of leech species are predatory, mostly preying on small invertebrates.The eggs are enclosed in a cocoon, which in aquatic species is usually attached to an underwater surface; members of one family, Glossiphoniidae, exhibit parental care, the eggs being brooded by the parent. In terrestrial species, the cocoon is often concealed under a log, in a crevice or buried in damp soil. Almost seven hundred species of leech are currently recognised, of which some hundred are marine, ninety terrestrial and the remainder freshwater.Leeches have been used in medicine from ancient times until the 19th century to draw blood from patients. In modern times, leeches find medical use in treatment of joint diseases such as epicondylitis and osteoarthritis, extremity vein diseases, and in microsurgery, while hirudin is used as an anticoagulant drug to treat blood-clotting disorders.The leech appears in the biblical Book of Proverbs as an archetype of insatiable greed.WEB, Proverbs 30:15 {{!, Ellicott's Commentary for English Readers |url=http://biblehub.com/commentaries/proverbs/30-15.htm |publisher=BibleHub |access-date=27 April 2018}} The term "leech" is used to characterise a person who takes without giving, living at the expense of others.WEB, Leech,weblink Merriam-Webster, 27 April 2018,

Diversity and phylogeny

File:Haemadipsa zeylanica japonica.jpg|thumb|alt=A terrestrial leech, Haemadipsa zeylanica|Haemadipsa zeylanica, a terrestrial leech]]File:Parasite180056-fig1 Placobdelloides siamensis (Glossiphoniidae).png|thumb|alt=Dorsal (upper) surface and ventral (lower) surface of Placobdelloides siamensis, ventral showing numerous young leeches|Placobdelloides siamensis, a parasite of turtles in Thailand ThailandSome 680 species of leech have been described, of which around 100 are marine, 480 freshwater and the remainder terrestrial.JOURNAL, Sket, Boris, Trontelj, Peter, 2008, Global diversity of leeches (Hirudinea) in freshwater, Hydrobiologia, 595, 1, 129â€“137, 10.1007/s10750-007-9010-8, 46339662, JOURNAL, Fogden, S., Proctor, J., Notes on the Feeding of Land Leeches (Haemadipsa zeylanica Moore and H. picta Moore) in Gunung Mulu National Park, Sarawak, Biotropica, 17, 2, 172â€“174, 1985, 10.2307/2388511, 2388511, 1985Biotr..17..172F, Among Euhirudinea, the true leeches, the smallest is about {{convert|1|cm|in|frac=2|abbr=on}} long, and the largest is the giant Amazonian leech, Haementeria ghilianii, which can reach {{convert|30|cm|in|0|abbr=on}}. Except for Antarctica, leeches are found throughout the world but are at their most abundant in temperate lakes and ponds in the northern hemisphere. The majority of freshwater leeches are found in the shallow, vegetated areas on the edges of ponds, lakes and slow-moving streams; very few species tolerate fast-flowing water. In their preferred habitats, they may occur in very high densities; in a favourable environment with water high in organic pollutants, over 10,000 individuals were recorded per square metre (over 930 per square foot) under rocks in Illinois. Some species aestivate during droughts, burying themselves in the sediment, and can lose up to 90% of their bodyweight and still survive. Among the freshwater leeches are the Glossiphoniidae, dorso-ventrally flattened animals mostly parasitic on vertebrates such as turtles, and unique among annelids in both brooding their eggs and carrying their young on the underside of their bodies.WEB, Siddall, Mark E., Glossiphoniidae,weblink American Museum of Natural History, 1 May 2018, 1998,weblink" title="web.archive.org/web/20190809160055weblink">weblink 9 August 2019, The terrestrial Haemadipsidae are mostly native to the tropics and subtropics,{{harvnb|Ruppert|Fox|Barnes|2004|p=480}} while the aquatic Hirudinidae have a wider global range; both of these feed largely on mammals, including humans.{{harvnb|Ruppert|Fox|Barnes|2004|p=471}} A distinctive family is the Piscicolidae, marine or freshwater ectoparasites chiefly of fish, with cylindrical bodies and usually well-marked, bell-shaped, anterior suckers.JOURNAL, Meyer, Marvin C., A Revision of the Leeches (Piscicolidae) Living on Fresh-Water Fishes of North America, Transactions of the American Microscopical Society, July 1940, 59, 3, 354â€“376, 3222552, 10.2307/3222552, Not all leeches feed on blood; the Erpobdelliformes, freshwater or amphibious, are carnivorous and equipped with a relatively large, toothless mouth to ingest insect larvae, molluscs, and other annelid worms, which are swallowed whole.JOURNAL, Oceguera, A., Leon, V., Siddall, M., Phylogeny and revision of Erpobdelliformes (Annelida, Arhynchobdellida) from Mexico based on nuclear and mitochondrial gene sequences, Revista Mexicana de Biodiversidad, 2005, 76, 2, 191â€“198, 10.22201/ib.20078706e.2005.002.307,weblink 2020-10-23, free, In turn, leeches are prey to fish, birds, and invertebrates.WEB, Leeches,weblink Australian Museum, 3 June 2020, 14 November 2019, The name for the subclass, Hirudinea, comes from the Latin hirudo (genitive hirudinis), a leech; the element -bdella found in many leech group names is from the Greek Î²Î´ÎÎ»Î»Î± bdella, also meaning leech.BOOK, Scarborough, John, Medical and Biological Terminologies: Classical Origins,weblink 1992, University of Oklahoma Press, 978-0-8061-3029-3, 58, The name Les hirudinÃ©es was given by Jean-Baptiste Lamarck in 1818.BOOK, Lamarck, Jean-Baptiste, Jean-Baptiste Lamarck, Histoire naturelle des animaux sans vertÃ¨bres ... prÃ©cÃ©dÃ©e d'une introduction offrant la dÃ©termination des caractÃ¨res essentiels de l'animal, sa distinction du vÃ©gÃ©tal et des autres corps naturels, enfin, l'exposition des principes fondamentaux de la zoologie. Volume 5, 1818, 5, VerdiÃ¨re, Paris,weblink Leeches were traditionally divided into two infraclasses, the Acanthobdellidea (primitive leeches) and the Euhirudinea (true leeches).WORMS, Kolb, JÃ¼rgen, 2018, Hirudinea, 2041, 7 May 2018, The Euhirudinea are divided into the proboscis-bearing Rhynchobdellida and the rest, including some jawed species, the "Arhynchobdellida", without a proboscis.The phylogenetic tree of the leeches and their annelid relatives is based on molecular analysis (2019) of DNA sequences. Both the former classes "Polychaeta" (bristly marine worms) and "Oligochaeta" (including the earthworms) are paraphyletic: in each case the complete groups (clades) would include all the other groups shown below them in the tree. The Branchiobdellida are sister to the leech clade Hirudinida, which approximately corresponds to the traditional subclass Hirudinea. The main subdivision of leeches is into the Rhynchobdellida and the Arhynchobdellida, though the Acanthobdella are sister to the clade that contains these two groups.JOURNAL, Phillips, Anna J., Dornburg, Alex, Zapfe, Katerina L., Anderson, Frank E., James, Samuel W., ErsÃ©us, Christer, Moriarty Lemmon, Emily, Lemmon, Alan R., Williams, Bronwyn W., 3, Phylogenomic Analysis of a Putative Missing Link Sparks Reinterpretation of Leech Evolution, Genome Biology and Evolution, 11, 11, 2019, 3082â€“3093, 1759-6653, 10.1093/gbe/evz120, 31214691, 6598468, {{clade|label1=Annelida|1={{clade

|1="Polychaeta" (exc. "Oligochaeta") (File:Nereis pelagica.jpg|70px)|state1=dashed
|label2=Clitellata
|2={{clade
|1="Oligochaeta" (exc. Lumbriculidae) (File:à®®à®£à¯à®ªà¯à®´à¯.jpg|70px) |state1=dashed
|2={{clade
|1=Lumbriculidae (blackworms) (File:Lumbriculidae unknown species (cropped).jpg|50px)
|2={{clade
|sublabel1=symbionts
|label1=Branchiobdellida
|1=(File:Signal crayfish branchiobdellid crop 2.jpg|120px)
|label2=Hirudinida
|sublabel2=parasitic
|2={{clade
|1=Acanthobdella (File:Acanthobdella 001 (detail).png|80px)
|label2=Euhirudinea
|2={{clade
|label1=Arhynchobdellida
|1={{clade
|1=Erpobdelliformes (File:ErpobdellaOctoculata wwalas 01.JPG|80px)
|2=Hirudiniformes (File:SvÃ¸mmende blodigle.JPG|80px)
}}
|label2=Rhynchobdellida
|2={{clade
|1=Glossiphoniidae (File:Parasite180056-fig5A Placobdelloides siamensis (Glossiphoniidae).png|80px)
|label2=Oceanobdelliformes
|2={{clade
|sublabel1=ectoparasitic
|1=Piscicolidae (File:CystobranchusRespiransRutilusRutilus.JPG|80px)
|2=Ozobranchidae (File:Ozobranchus jantseanus cropped.jpg|80px)
}}
}}
}}
}}
}}
}}
}}
}}

}}

Evolution

File:Leech-like_worm_from_Waukesha_Biota,_Telychian,_Waukesha_County,_Wisconsin,_USA.jpg|thumb|upright=0.6|alt=Fossil of a possible leech found in Wisconsin|Fossil of a worm that was once considered as leech but denied, from the Waukesha Biota, in the Silurian of Wisconsin WisconsinThe most ancient annelid group consists of the free-living polychaetes that evolved in the Cambrian period, being plentiful in the Burgess Shale about 500{{nbs}}million years ago. Oligochaetes evolved from polychaetes and the leeches branched off from the oligochaetes.BOOK, Margulis, Lynn, Lynn Margulis, Chapman, Michael J., Kingdoms and Domains: An Illustrated Guide to the Phyla of Life on Earth,weblink 2009, Academic Press, 978-0-08-092014-6, 308, The oldest leech fossils are from the middle Permian period around 266{{nbs}}million years ago,JOURNAL, Prevec, Rosemary, Nel, AndrÃ©, Day, Michael O., Muir, Robert A., Matiwane, Aviwe, Kirkaldy, Abigail P., Moyo, Sydney, Staniczek, Arnold, Cariglino, BÃ¡rbara, Maseko, Zolile, Kom, Nokuthula, Rubidge, Bruce S., Garrouste, Romain, Holland, Alexandra, Barber-James, Helen M., 3, 2022-10-30, South African LagerstÃ¤tte reveals middle Permian Gondwanan lakeshore ecosystem in exquisite detail, Communications Biology, en, 5, 1, 1154, 10.1038/s42003-022-04132-y, 36310243, 9618562, 2399-3642, there is also unpublished study about possible leech from Virgilian (Late Carboniferous) of New Mexico. Although fossil with external ring markings found from Silurian strata in Wisconsin is sometimes identified as leech,BOOK, Thorp, James H., Covich, Alan P., Ecology and Classification of North American Freshwater Invertebrates,weblink 2001, Academic Press, 978-0-12-690647-9, 466, JOURNAL, Mikulic, D. G., Briggs, D. E. G., Kluessendorf, J., 1985, A new exceptionally preserved biota from the Lower Silurian of Wisconsin, U.S.A., Philosophical Transactions of the Royal Society B, Philosophical Transactions of the Royal Society of London B, 311, 1148, 75â€“85, 10.1098/rstb.1985.0140, 1985RSPTB.311...75M, but assignment of fossil is still putative and contentious,JOURNAL, Wendruff, Andrew J., Babcock, Loren E., Kluessendorf, Joanne, Mikulic, Donald G., 3, 2020-05-15, Paleobiology and taphonomy of exceptionally preserved organisms from the Waukesha Biota (Silurian), Wisconsin, USA,weblink Palaeogeography, Palaeoclimatology, Palaeoecology, en, 546, 109631, 10.1016/j.palaeo.2020.109631, 2020PPP...54609631W, 212824469, 0031-0182, and the animal was also alternatively interpreted as a member of Cycloneuralia.JOURNAL, Shcherbakov, Dmitry, Tarmo, Timm, Tzetlin, Alexander B., Vinn, Olev, Zhuravlev, Andrey, 2020, A probable oligochaete from an Early Triassic LagerstÃ¤tte of the southern Cis-Urals and its evolutionary implications, Acta Palaeontologica Polonica, 65, 2, 219â€“233, 10.4202/app.00704.2019, 219097612, free, JOURNAL, Braddy, Simon J., Gass, Kenneth C., Tessler, Michael, 2023, Not the first leech: An unusual worm from the early Silurian of Wisconsin, Journal of Paleontology, 97, 4, 799â€“804, 10.1017/jpa.2023.47, free, 2023JPal...97..799B,

Anatomy and physiology

Leeches show a remarkable similarity to each other in morphology, very different from typical annelids which are cylindrical with a fluid-filled space, the coelom (body cavity). In leeches, most of the coelom is filled with botryoidal tissue, a loose connective tissue composed of clusters of cells of mesodermal origin.Ultrastructure and functional versatility of hirudinean botryoidal tissue The remaining body cavity has been reduced to four slender longitudinal channels. Typically, the body is dorso-ventrally flattened and tapers at both ends. Longitudinal and circular muscles in the body wall are supplemented by diagonal muscles, giving the leech the ability to adopt a large range of body shapes and show great flexibility. Most leeches have a sucker at both the anterior (front) and posterior (back) ends, but some primitive leeches have a single sucker at the back.File:Leech anatomy in cross-section.svg|thumb|left|upright=1.5|alt=Cross-section of a leech showing its anatomy|Leech anatomy in cross-section: the body is solid, the coelom coelomLike most annelids, with a few exceptions like Sipuncula, Echiura and Diurodrilus,Spiralian Phylogeny Informs the Evolution of Microscopic Lineages the leech is a segmented animal, but unlike other annelids, the segmentation is masked by secondary external ring markings (annuli).BOOK, Buchsbaum, Ralph, Buchsbaum, Mildred, Pearse, John, Pearse, Vicki, 3, Animals Without Backbones,weblink registration, 3rd, The University of Chicago Press, 1987, 978-0-226-07874-8, 312â€“317, The number of annulations varies, both between different regions of the body and between species.{{harvnb|Ruppert|Fox|Barnes|2004|pp=471â€“472}} In one species, the body surface is divided into 102 annuli.BOOK, Payton, Brian, Neurobiology of the Leech, Muller, Kenneth, Nicholls, John, Stent, Gunther, Cold Spring Harbor Laboratory Press, Cold Spring Harbor Laboratory, 1981, 978-0-87969-146-2, 35â€“50, All leech species, however, have 32 segments, called somites, (34 if two head segments, which have different organization, are counted).JOURNAL, Castle, W. E., 1900, The Metamerism of the Hirudinea,weblink Proceedings of the American Academy of Arts and Sciences, 35, 15, 285â€“303, 10.2307/25129933, 25129933, 0199-9818, Of these segments, the first five are designated as the head and include the anterior brain, several ocelli (eyespots) dorsally and the sucker ventrally. The following 21 mid-body segments each contain a nerve ganglion, and between them contain two reproductive organs, a single female gonopore and nine pairs of testes. The last seven segments contain the posterior brain and are fused to form the animal's tail sucker. The septa that separates the body segmentsâ€”and the mesenteries which in turn separates each segment into a left and right halfâ€”in the majority of annelids, have been lost in leeches except for the primitive genus Acanthobdella, which still have some septa and mesenteries.JOURNAL, Kuo, Dian-Han, Lai, Yi-Te, On the origin of leeches by evolution of development, Development, Growth & Differentiation, 61, 1, 4 November 2018, 10.1111/dgd.12573, 43â€“57, 30393850, 53218704, free, Reproductive Strategies and Developmental Patterns in AnnelidsThe body wall consists of a cuticle, an epidermis and a thick layer of fibrous connective tissue in which are embedded the circular muscles, the diagonal muscles and the powerful longitudinal muscles. There are also dorso-ventral muscles. In leeches the original blood vascular system has been lost and replaced by the modified coelom known as the haemocoelomic system, and the coelomic fluid, called the haemocoelomic fluid, has taken over the role as blood. The haemocoelomic channels run the full length of the body, the two main ones being on either side.Modern Text Book of Zoology: Invertebrates Part of the lining epithelium consists of chloragogen cells which are used for the storage of nutrients and in excretion. There are 10 to 17 pairs of metanephridia (excretory organs) in the mid-region of the leech. From these, ducts typically lead to a urinary bladder, which empties to the outside at a nephridiopore.{{harvnb|Ruppert|Fox|Barnes|2004|pp=474â€“475}}

Reproduction and development

{{Further|Leech embryogenesis}}Leeches are hermaphrodites, with the male reproductive organs, the testes, maturing first and the ovaries later. In hirudinids, a pair will line up with the clitellar regions in contact, with the anterior end of one leech pointing towards the posterior end of the other; this results in the male gonopore of one leech being in contact with the female gonopore of the other. The penis passes a spermatophore into the female gonopore and sperm is transferred to, and probably stored in, the vagina.{{harvnb|Ruppert|Fox|Barnes|2004|pp=477â€“478}}Some jawless leeches (Rhynchobdellida) and proboscisless leeches (Arhynchobdellida) lack a penis, and in these, sperm is passed from one individual to another by hypodermic injection. The leeches intertwine and grasp each other with their suckers. A spermatophore is pushed by one through the integument of the other, usually into the clitellar region. The sperm is liberated and passes to the ovisacs, either through the coelomic channels or interstitially through specialist "target tissue" pathways.Some time after copulation, the small, relatively yolkless eggs are laid. In most species, an albumin-filled cocoon is secreted by the clitellum and receives one or more eggs as it passes over the female gonopore. In the case of the North American Erpobdella punctata, the clutch size is about five eggs, and some ten cocoons are produced.JOURNAL, Sawyer, R. T., 1970, Observations on the Natural History and Behavior of Erpobdella punctata (Leidy) (Annelida: Hirudinea), The American Midland Naturalist, 83, 1, 65â€“80, 10.2307/2424006, 2424006, Each cocoon is fixed to a submerged object, or in the case of terrestrial leeches, deposited under a stone or buried in damp soil. The cocoon of Hemibdella soleae is attached to a suitable fish host.JOURNAL, Gelder, Stuart R., Gagnon, Nicole L., Nelson, Kerri, 2002, Taxonomic Considerations and Distribution of the Branchiobdellida (Annelida: Clitellata) on the North American Continent, Northeastern Naturalist, 9, 4, 451â€“468, 3858556, 10.1656/1092-6194(2002)009[0451:TCADOT]2.0.CO;2, 85774943, The glossiphoniids brood their eggs, either by attaching the cocoon to the substrate and covering it with their ventral surface, or by securing the cocoon to their ventral surface, and even carrying the newly hatched young to their first meal.BOOK, Rohde, Klaus, Marine Parasitology,weblink 2005, CSIRO Publishing, 978-0-643-09927-2, 185, When breeding, most marine leeches leave their hosts and become free-living in estuaries. Here they produce their cocoons, after which the adults of most species die. When the eggs hatch, the juveniles seek out potential hosts when these approach the shore. Leeches mostly have an annual or biannual life cycle.

Feeding and digestion

About three quarters of leech species are parasites that feed on the blood of a host, while the remainder are predators. Leeches either have a pharynx that they can protrude, commonly called a proboscis, or a pharynx that they cannot protrude, which in some groups is armed with jaws.{{harvnb|Ruppert|Fox|Barnes|2004|pp=475â€“477}}In the proboscisless leeches, the jaws (if any) of Arhynchobdellids are at the front of the mouth, and have three blades set at an angle to each other. In feeding, these slice their way through the skin of the host, leaving a Y-shaped incision. Behind the blades is the mouth, located ventrally at the anterior end of the body. It leads successively into the pharynx, a short oesophagus, a crop (in some species), a stomach and a hindgut, which ends at an anus located just above the posterior sucker. The stomach may be a simple tube, but the crop, when present, is an enlarged part of the midgut with a number of pairs of ceca that store ingested blood. The leech secretes an anticoagulant, hirudin, in its saliva which prevents the blood from clotting before ingestion. A mature medicinal leech may feed only twice a year, taking months to digest a blood meal.(File:Leach bite site.jpg|thumb|alt=Magnified reddish leech bites on a cow's udder|Leech bites on a cow's udder)The bodies of predatory leeches are similar, though instead of a jaw many have a protrusible proboscis, which for most of the time they keep retracted into the mouth. Such leeches are often ambush predators that lie in wait until they can strike prey with the proboscises in a spear-like fashion.WEB, All about leeches, Govedich, Fredric R., Bain, Bonnie A., 14 March 2005,weblink 19 January 2010,weblink" title="web.archive.org/web/20100821165542weblink">weblink 21 August 2010, dead, Predatory leeches feed on small invertebrates such as snails, earthworms and insect larvae. The prey is usually sucked in and swallowed whole. Some Rhynchobdellida however suck the soft tissues from their prey, making them intermediate between predators and blood-suckers.(File:Egel als Schneckenparasit 04.JPG|thumb|left|alt=A leech attacking a slug's underside|Leech attacking a slug)Blood-sucking leeches use their anterior suckers to connect to hosts for feeding. Once attached, they use a combination of mucus and suction to stay in place while they inject hirudin into the hosts' blood. In general, blood-feeding leeches are non host-specific, and do little harm to their host, dropping off after consuming a blood meal. Some marine species however remain attached until it is time to reproduce. If present in great numbers on a host, these can be debilitating, and in extreme cases, cause death.Leeches are unusual in that they do not produce certain digestive enzymes such as amylases, lipases or endopeptidases. A deficiency of these enzymes and of B complex vitamins is compensated for by enzymes and vitamins produced by endosymbiotic microflora. In Hirudo medicinalis, these supplementary factors are produced by an obligatory mutualistic relationship with the bacterial species, Aeromonas veronii. Non-bloodsucking leeches, such as Erpobdella octoculata, are host to more bacterial symbionts.WEB, Sawyer, Roy T., Leech biology and behaviour, biopharm-leeches.com,weblink dead,weblink" title="web.archive.org/web/20110910161114weblink">weblink 10 September 2011, In addition, leeches produce intestinal exopeptidases which remove amino acids from the long protein molecules one by one, possibly aided by proteases from endosymbiotic bacteria in the hindgut.JOURNAL, DziekoÅ„ska-Rynko, Janina, Bielecki, Aleksander, PaliÅ„ska, Katarzyna, 2009, Activity of selected hydrolytic enzymes from leeches (Clitellata: Hirudinida) with different feeding strategies, Biologia, 64, 2, 370â€“376, 10.2478/s11756-009-0048-0, free, 2009Biolg..64..370D, This evolutionary choice of exopeptic digestion in Hirudinea distinguishes these carnivorous clitellates from oligochaetes, and may explain why digestion in leeches is so slow.

Nervous system

A leech's nervous system is formed of a few large nerve cells. Their large size makes leeches convenient as model organisms for the study of invertebrate nervous systems. The main nerve centre consists of the cerebral ganglion above the gut and another ganglion beneath it, with connecting nerves forming a ring around the pharynx a little way behind the mouth. A nerve cord runs backwards from this in the ventral coelomic channel, with 21 pairs of ganglia in segments six to 26. In segments 27 to 33, other paired ganglia fuse to form the caudal ganglion.{{harvnb|Ruppert|Fox|Barnes|2004|pp=472â€“474}} Several sensory nerves connect directly to the cerebral ganglion; there are sensory and motor nerve cells connected to the ventral nerve cord ganglia in each segment.Leeches have between two and ten pigment spot ocelli, arranged in pairs towards the front of the body. There are also sensory papillae arranged in a lateral row in one annulation of each segment. Each papilla contains many sensory cells. Some rhynchobdellids have the ability to change colour dramatically by moving pigment in chromatophore cells; this process is under the control of the nervous system but its function is unclear as the change in hue seems unrelated to the colour of the surroundings.Leeches can detect touch, vibration, movement of nearby objects, and chemicals secreted by their hosts; freshwater leeches crawl or swim towards a potential host standing in their pond within a few seconds. Species that feed on warm-blooded hosts move towards warmer objects. Many leeches avoid light, though some blood feeders move towards light when they are ready to feed, presumably increasing the chances of finding a host.

Gas exchange

Leeches live in damp surroundings and in general respire through their body wall. The exception to this is in the Piscicolidae, where branching or leaf-like lateral outgrowths from the body wall form gills. Some rhynchobdellid leeches have an extracellular haemoglobin pigment, but this only provides for about half of the leech's oxygen transportation needs, the rest occurring by diffusion.

Movement

Leeches move using their longitudinal and circular muscles in a modification of the locomotion by peristalsis, self-propulsion by alternately contracting and lengthening parts of the body, seen in other annelids such as earthworms. They use their posterior and anterior suckers (one on each end of the body) to enable them to progress by looping or inching along, in the manner of geometer moth caterpillars. The posterior end is attached to the substrate, and the anterior end is projected forward peristaltically by the circular muscles until it touches down, as far as it can reach, and the anterior end is attached. Then the posterior end is released, pulled forward by the longitudinal muscles, and reattached; then the anterior end is released, and the cycle repeats.BOOK, Elder, H. Y., Elder, H. Y., Trueman, E. R., Peristaltic Mechanisms, Society for Experimental Biology, Seminar Series: Volume 5, Aspects of Animal Movement,weblink 1980, CUP Archive, 978-0-521-29795-0, 84â€“85, BOOK, Brusca, Richard, Hirudinoidea: Leeches and Their Relatives, Invertebrates, Sinauer Associates, 2016, 978-1-60535-375-3, 591â€“597, Leeches explore their environment with head movements and body waving.BOOK, Sawyer, Roy, Neurobiology of the Leech, Muller, Kenneth, John, Nicholls, Gunther, Stent, Cold Spring Harbor Laboratory Press, Cold Spring Harbor Laboratory, 1981, 978-0-87969-146-2, 7â€“26, The Hirudinidae and Erpobdellidae can swim rapidly with up-and-down or sideways undulations of the body; the Glossiphoniidae in contrast are poor swimmers and curl up and fall to the sediment below when disturbed.BOOK, Smith, Douglas Grant, Pennak's Freshwater Invertebrates of the United States: Porifera to Crustacea,weblink 2001, John Wiley & Sons, 978-0-471-35837-4, 304â€“305, File:Leech looping locomotion.jpg|alt=Sketches of the looping movement of a leech using its front and back suckers|Leeches move by looping using their front and back suckers.File:20100214 Leech climbing door at Lake Leake, Tasmania.ogv|Video of looping movement

Interactions with humans

(File:Leech Removal.JPG|upright|thumb|alt=Hand preparing to grip a leech and remove it from the top of a foot|Leeches can be removed by hand, since they do not burrow into the skin or leave the head in the wound.BOOK, Burke, Don, 2005, The complete Burke's backyard: the ultimate book of fact sheets, Murdoch Books, 978-1-74045-739-2,weblink BOOK, Fujimoto, Gary, Robin, Marc, Dessery, Bradford, 2003, The Traveler's Medical Guide, Prairie Smoke Press, 978-0-9704482-5-5,weblink )

Bites

Leech bites are generally alarming rather than dangerous, though a small percentage of people have severe allergic or anaphylactic reactions and require urgent medical care. Symptoms of these reactions include red blotches or an itchy rash over the body, swelling around the lips or eyes, a feeling of faintness or dizziness, and difficulty in breathing.Victorian Poisons Information Centre: Leeches Victorian Poisons Information Centre. Retrieved 28 July 2007 An externally attached leech will detach and fall off on its own accord when it is satiated on blood, which may take from twenty minutes to a few hours; bleeding from the wound may continue for some time. Internal attachments, such as inside the nose, are more likely to require medical intervention.JOURNAL, 15815064,weblink 2005, Chow, C. K., Wong, S. S., Ho, A. C., Lau, S. K., Unilateral epistaxis after swimming in a stream, 11, 2, 110â€“112, Hong Kong Medical Journal, See also: weblink" title="web.archive.org/web/20131228094723weblink">lay summary from Reuters, 11 April 2005.Bacteria, viruses, and protozoan parasites from previous blood sources can survive within a leech for months, so leeches could potentially act as vectors of pathogens. Nevertheless, only a few cases of leeches transmitting pathogens to humans have been reported.JOURNAL, Ahl-Khleif, A., Roth, M., Menge, C., Heuser, J., Baljer, G., Herbst, W., 3, Tenacity of mammalian viruses in the gut of leeches fed with porcine blood, Journal of Medical Microbiology, 60, 6, 787â€“792, 2011, 10.1099/jmm.0.027250-0, 21372183, free, JOURNAL, Nehili, Malika, Ilk, Christoph, Mehlhorn, Heinz, Ruhnau, Klaus, Dick, Wolfgang, Njayou, Mounjohou, 3, Experiments on the possible role of leeches as vectors of animal and human pathogens: a light and electron microscopy study, Parasitology Research, 80, 4, 1994, 0044-3255, 10.1007/bf02351867, 277â€“290, 8073013, 19770060, Leech saliva is commonly believed to contain anaesthetic compounds to numb the bite area, but some authorities disagree.JOURNAL, Meir, Rigbi, Levy, Haim, Eldor, Amiram, Iraqi, Fuad, Teitelbaum, Mira, Orevi, Miriam, Horovitz, Amnon, Galun, Rachel, 3, 1987, The saliva of the medicinal leech Hirudo medicinalisâ€”II. Inhibition of platelet aggregation and of leukocyte activity and examination of reputed anaesthetic effects, Comparative Biochemistry and Physiology C, 88, 1, 95â€“98, 10.1016/0742-8413(87)90052-1, 2890494, WEB, Myth Busters: Leech Anaesthetic, Mark, Siddall, BdellaNea, 7 July 2008,weblink JOURNAL, Singh, 2011, Efficacy of leech therapy in the management of osteoarthritis (Sandhivata), Ayu, 32, 2, 213â€“217, 10.4103/0974-8520.92589, 22408305, 3296343, free, Although morphine-like substances have been found in leeches, they have been found in the neural tissues, not the salivary tissues. They are used by the leeches in modulating their own immunocytes and not for anaesthetising bite areas on their hosts.JOURNAL, Laurent, V., Salzet, B., Verger-Bocquet, M., Bernet, F., Salzet, M., 2000, Morphine-like substance in leech ganglia. Evidence and immune modulation, European Journal of Biochemistry, 267, 8, 2354â€“2361, 10759861, 10.1046/j.1432-1327.2000.01239.x, free, Depending on the species and size, leech bites can be barely noticeable or they can be fairly painful.WEB,weblink Blood Lust II, Siddall, Mark, Borda, Liz, Burreson, Gene, Williams, Juli, 3, Laboratory of Phylohirudinology, American Museum of Natural History, 15 December 2013,weblink" title="web.archive.org/web/20200607045534weblink">weblink 7 June 2020, BOOK, Yi-Te Lai, Jiun-Hong Chen, è‡ºç£è›é¡žå‹•ç‰©å¿—: Leech Fauna of Taiwan-Biota Taiwanica, åœ‹ç«‹è‡ºç£å¤§å¸å‡ºç‰ˆä¸å¿ƒ, zh, 2010, 89, 978-986-02-2760-4,weblink

Medical use

{{Further|Hirudotherapy|Hirudo medicinalis#Current}}The medicinal leech Hirudo medicinalis, and some other species, have been used for clinical bloodletting for at least 2,500 years: Ayurvedic texts describe their use for bloodletting in ancient India. In ancient Greece, bloodletting was practised according to the theory of humours found in the Hippocratic Corpus of the fifth century{{nbs}}BC, which maintained that health depended on a balance of the four humours: blood, phlegm, black bile and yellow bile. Bloodletting using leeches enabled physicians to restore balance if they considered blood was present in excess.BOOK, Payton, Brian, Neurobiology of the Leech, Muller, Kenneth, Nicholls, John, Stent, Gunther, Cold Spring Harbor Laboratory Press, Cold Spring Harbor Laboratory, 1981, 978-0-87969-146-2, 27â€“34, JOURNAL, Mory, Robert N., Mindell, David, Bloom, David A., 2014, The Leech and the Physician: Biology, Etymology, and Medical Practice with Hirudinea medicinalis, World Journal of Surgery, 24, 7, 878â€“883, 10.1007/s002680010141, 10833259, 2027.42/42411, 18166996, free, Pliny the Elder reported in his Natural History that the horse leech could drive elephants mad by climbing up inside their trunks to drink blood.BOOK, Marren, Peter, Mabey, Richard, Richard Mabey, Bugs Britannica,weblink 2010, Chatto & Windus, 978-0-7011-8180-2, 45â€“48, Pliny also noted the medicinal use of leeches in ancient Rome, stating that they were often used for gout, and that patients became addicted to the treatment.BOOK, Pliny, Healy, John F., Pliny the Elder, Natural History: A Selection, 1991, Penguin Books, 978-0-14-044413-1, 283, In Old English, lÇ£ce was the name for a physician as well as for the animal, though the words had different origins, and lÇ£cecraft, leechcraft, was the art of healing.JOURNAL, Mory, Robert N., Mindell, David, Bloom, David A., The Leech and the Physician: Biology, Etymology, and Medical Practice with Hirudinea medicinalis, World Journal of Surgery, 24, 7, 2014, 878â€“883, 0364-2313, 10.1007/s002680010141, 10833259, 2027.42/42411, 18166996, free, File:Leech Finders Costume of Yorkshire 1814.jpg |alt=Print from Costumes of Yorkshire 1814 showing three women searching for leeches in a pond|"Leech finders" from The Costume of Yorkshire by George Walker, 1814, engraved by Robert HavellFile:Pharmacy leech jar, England, 1830-1870 Wellcome L0058506.jpg|alt=Photograph of white and blue ceramic English pharmacy leech jar with airholes in the lid|Pharmacy leech jar with airholes in the lid. England, 1830â€“1870.File:Three leeches in the role of physicians attend a grasshopper in the role of the patient and announce a course of bloodletting Wellcome V0011722.jpg|alt=Coloured lithograph cartoon showing three leech doctors treating a grasshopper patient|Three leech doctors decide on bloodletting for their grasshopper patient.{{efn|The caption below the lithograph reads "There's redundancy of blood and humours, we'll bleed you to-morrow, till then, very little food."}} Lithograph by F.-J.-V. Broussais from a cartoon by J. J. Grandville, {{circa|1832}}William Wordsworth's 1802 poem "Resolution and Independence" describes one of the last of the leech-gatherers, people who travelled Britain catching leeches from the wild, and causing a sharp decline in their abundance, though they remain numerous in Romney Marsh. By 1863, British hospitals had switched to imported leeches, some seven million being imported to hospitals in London that year. {{anchor|Hirudiculture}}In the nineteenth century, demand for leeches was sufficient for hirudiculture, the farming of leeches, to become commercially viable.JOURNAL, Jourdier, August, Coste, M., Hirudiculture (Leech-Culture) (from La Pisciculture et la Production des Sanguesues (Fish farming and leech production). Paris : Hachette et Cie, 641â€“648, The Journal of Agriculture, New Series, March 1859, 8 (July 1857â€“March 1859 ),weblink William Blackwood and Sons, Leech usage declined with the demise of humoral theory,BOOK, anon, Medicine: The Definitive Illustrated History,weblink 2016, Dorling Kindersley, 978-0-241-28715-6, 35, but made a small-scale comeback in the 1980s after years of decline, with the advent of microsurgery, where venous congestion can arise due to inefficient venous drainage. Leeches can reduce swelling in the tissues and promote healing, helping in particular to restore circulation after microsurgery to reattach body parts.WEB, Cho, Joohee, Some Docs Latching Onto Leeches,weblink ABC News, 27 April 2018, 4 March 2008, JOURNAL, Adams, Stephen L., The Medicinal Leech: A Page from the Annelids of Internal Medicine, Annals of Internal Medicine, 109, 5, 399â€“405, 1988, 10.7326/0003-4819-109-5-399, 3044211, Other clinical applications include varicose veins, muscle cramps, thrombophlebitis, and joint diseases such as epicondylitis and osteoarthritis.JOURNAL, Teut, M., Warning, A., 2008, Leeches, phytotherapy and physiotherapy in osteo-arthrosis of the kneeâ€”a geriatric case study, Forsch KomplementÃ¤rmed, 15, 5, 269â€“272, 10.1159/000158875, 19001824, 196365336, JOURNAL, 12233807, 2002, Michalsen, A., Moebus, S., Spahn, G., Esch, T., Langhorst, J., Dobos, G.J., Leech therapy for symptomatic treatment of knee osteoarthritis: Results and implications of a pilot study, 8, 5, 84â€“88, Alternative Therapies in Health and Medicine, JOURNAL, Sig, A. K., Guney, M., Uskudar Guclu, A., Ozmen, E., 3, 2017, Medicinal leech therapyâ€”an overall perspective, Integrative Medicine Research, 6, 4, 337â€“343, 10.1016/j.imr.2017.08.001, 29296560, 5741396, JOURNAL, Abdualkader, A. M., Ghawi, A. M., Alaama, M., Awang, M., Merzouk, A., Leech Therapeutic Applications, Indian Journal of Pharmacology, Indian Journal of Pharmacological Science, 2013, 75, 2 (Marchâ€“April), 127â€“137, 24019559, 3757849, Leech secretions contain several bioactive substances with anti-inflammatory, anticoagulant and antimicrobial effects. One active component of leech saliva is a small protein, hirudin.JOURNAL, 10.1098/rspl.1883.0135, IV. On the action of a secretion obtained from the medicinal leech on the coagulation of the blood, Proceedings of the Royal Society of London, 36, 228â€“231, 478â€“487, 1883, Haycraft, John B., 83910684, It is widely used as an anticoagulant drug to treat blood-clotting disorders, and manufactured by recombinant DNA technology.JOURNAL, Fischer, Karl-Georg, Van de Loo, Andreas, Bohler, Joachim, Recombinant hirudin (lepirudin) as anticoagulant in intensive care patients treated with continuous hemodialysis, Kidney International, 1999, 56, Suppl. 72, S46â€“S50, 10560805,weblink 10.1046/j.1523-1755.56.s72.2.x, free, JOURNAL, Sohn, J., Kang, H., Rao, K., Kim, C., Choi, E., Chung, B., Rhee, S., Current status of the anticoagulant hirudin: its biotechnological production and clinical practice, Applied Microbiology and Biotechnology, 57, 5â€“6, 2001, 606â€“613, 10.1007/s00253-001-0856-9, 11778867, 19304703, In 2012 and 2018, Ida Schnell and colleagues trialled the use of Haemadipsa leeches to gather data on the biodiversity of their mammalian hosts in the tropical rainforest of Vietnam, where it is hard to obtain reliable data on rare and cryptic mammals. They showed that mammal mitochondrial DNA, amplified by the polymerase chain reaction, can be identified from a leech's blood meal for at least four months after feeding. They detected Annamite striped rabbit, small-toothed ferret-badger, Truong Son muntjac, and serow in this way.JOURNAL, Schnell, Ida BÃ¦rholm, Thomsen, Philip Francis, Wilkinson, Nicholas, Rasmussen, Morten, Jensen, Lars R. D., Willerslev, Eske, Bertelsen, Mads F., Gilbert, M. Thomas P., 3, Screening mammal biodiversity using DNA from leeches, Current Biology, 22, 8, 2012, R262â€“R263, 0960-9822, 10.1016/j.cub.2012.02.058, 22537625, free, 2012CBio...22.R262S, JOURNAL, Schnell, Ida BÃ¦rholm, Bohmann, Kristine, Schultze, Sebastian E., et al, 2018, Debugging diversity â€“ a pan-continental exploration of the potential of terrestrial blood-feeding leeches as a vertebrate monitoring tool, Molecular Ecology Resources, 18, 6, 1282â€“1298, 10.1111/1755-0998.12912, 29877042, 46972335,weblink

Water pollution

Exposure to synthetic estrogen as used in contraceptive medicines, which may enter freshwater ecosystems from municipal wastewater, can affect leeches' reproductive systems. Although not as sensitive to these compounds as fish, leeches showed physiological changes after exposure, including longer sperm sacs and vaginal bulbs, and decreased epididymis weight.JOURNAL, Kidd, Karen A., Graves, Stephanie D., McKee, Graydon I., Dyszy, Katarzyna, Podemski, Cheryl L., 3, 2020, Effects of Whole-Lake Additions of Ethynylestradiol on Leech Populations,weblink Environmental Toxicology and Chemistry, 39, 8, 1608â€“1619, 10.1002/etc.4789, 32692460, 220669536,

Notes

References

General bibliography

BOOK, Ruppert, Edward E., Fox, Richard S., Barnes, Robert D., Invertebrate Zoology, 7th Edition, 2004, Cengage Learning, 978-81-315-0104-7,

External links

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