23-467: Branchiobdellae Hirudinea " Oligochaeta " ( paraphyletic ) and see text 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. Clitellate annelids are segmented worms characterised by
46-481: A fixed number of segments, 33, but the segmentation is not visible externally because the cuticle is marked with annulations. Leeches do not bear chaetae. The front few segments or head have been modified into a sucker that usually surrounds the mouth. These segments usually bear several ocelli on the upper side. The clitellum occupies segments 9 to 11 but is only noticeable during breeding periods. The hindermost segments form another, larger, disc-shaped sucker located on
69-457: A polyphyletic grouping are not inherited from a common ancestor, but evolved independently. Monophyletic groups are typically characterised by shared derived characteristics ( synapomorphies ), which distinguish organisms in the clade from other organisms. An equivalent term is holophyly . The word "mono-phyly" means "one-tribe" in Greek. These definitions have taken some time to be accepted. When
92-412: A single river system or a single cave. An Appalachian brook crayfish was found to have one species of branchiobdellid in the gill chambers, one on the oral and ventral surfaces of the body and one on the chelae. Branchiobdellida feed on the micro-organisms and detritus that accumulate on their host; they will also feed on any soft tissues exposed where the host's exoskeleton is damaged. The relationship
115-456: Is generally symbiotic in that the host benefits from the cleaning activities of the branchiobdellids, and the latter benefit from a constant supply of food and a surface on which to deposit their cocoons ; the worms have been maintained for months in the laboratory in the absence of a host, but the cocoons must be attached to a living host in order for normal development of the embryos to occur. Monophyletic In biological cladistics for
138-483: Is now acknowledged to be sister to leeches and acanthobdellidans . The order is monotypic and contains only one family, the Branchiobdellidae . Branchiobdellida vary in length between 1 mm (0.04 in) and 10 mm (0.4 in). The body is formed from fifteen segments. There is no prostomium , and the peristomium and next three segments are modified into a sucker, surrounded by small tentacles, with
161-443: The clitellum or girdle which is located near the head end of mature individuals. The mouth is on the ventral surface and is overhung by the prostomium (proboscis). The brain is not located in the head but in one of the body segments. The clitellum is formed by a modification of several segments, and either includes the female gonopores or is located just behind them. During copulation, this glandular tissue secretes mucus that keeps
184-472: The Clitellata are considered to be a monophyletic clade embedded deep in the polychaetes . The group is divided into the subclasses Oligochaeta and Hirudinea . The oligochaetes contain the tubificids ( Naididae , Lumbricidae , and Lumbriculidae - commonly the tube worms and the earthworms. Hirudinea contains leeches (Hirudinida), Acanthobdellida, and Branchiobdellida. Hirudinea is monophyletic , but
207-473: The Euro/Mediterranean region, however, crayfish are exclusively used as hosts. Some branchiobdellids are generalists , but a few are limited to association with a single host species. Several different species of branchiobdellid are sometimes found on a single crayfish. Their hosts include open-water crustaceans, cave-dwellers and burrowers, but many branchiobdellida have a very limited range, occurring in
230-650: The clade is embedded among the Oligochaeta . Two approaches are possible: Branchiobdellae Branchiobdellida is an order of freshwater leech -like clitellates that are obligate ectosymbionts or ectoparasites, mostly of astacoidean crayfish . They are found in the Northern Hemisphere and have a holarctic distribution in East Asia, the Euro-Mediterranean region and North and Central America, with
253-404: The cladistics school of thought became mainstream in the 1960s, several alternative definitions were in use. Indeed, taxonomists sometimes used terms without defining them, leading to confusion in the early literature, a confusion which persists. The first diagram shows a phylogenetic tree with two monophyletic groups. The several groups and subgroups are particularly situated as branches of
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#1732852439667276-689: The classification of organisms , monophyly is the condition of a taxonomic grouping being a clade – that is, a grouping of taxa which meets these criteria: Monophyly is contrasted with paraphyly and polyphyly as shown in the second diagram. A paraphyletic grouping meets 1. but not 2., thus consisting of the descendants of a common ancestor, excepting one or more monophyletic subgroups. A polyphyletic grouping meets neither criterion, and instead serves to characterize convergent relationships of biological features rather than genetic relationships – for example, night-active primates, fruit trees, or aquatic insects. As such, these characteristic features of
299-552: The clitellum secretes a yolk (albumen) and a proteinaceous sheath 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). According to modern phylogenetic analyses,
322-408: The conclusion that the group should be raised from a family to an order , a sister group to the oligochaetes and leeches . Forty years later, molecular data from rDNA and mitochondrial DNA studies has shown that he was correct, and that Oligochaeta , Branchiobdellida, Acanthobdellida and Hirudinea form a monophyletic group and that each should be considered an order. Branchiobdellida
345-451: The earthworms as the largest members of the group, mostly live on land, burrowing in damp soil. Smaller freshwater species burrow in mud or live among aquatic vegetation. The marine species are mostly tiny and live in the interstices between sand grains, from the intertidal zone to the deep sea. All clitellata are hermaphrodites . During copulation, the clitellum produces a mucus that holds worms in place whilst they mate. During reproduction,
368-413: The fact that a monophyletic group includes organisms (e.g., genera, species) consisting of all the descendants of a unique common ancestor. Conversely, the term polyphyly , or polyphyletic , builds on the ancient Greek prefix πολύς ( polús ), meaning "many, a lot of", and refers to the fact that a polyphyletic group includes organisms arising from multiple ancestral sources. By comparison,
391-603: The greatest species diversity being in North and Central America. The exact relationship of this group of clitellates with other groups has been the subject of debate for a century or more. The American zoologist Perry C. Holt wrote his doctoral thesis on branchiobdellids and devoted his 45 years of research to the taxon. With his colleague, the American zoologist Richard L. Hoffman , he described 8 new genera and 75 new species , and re-described various other species. In 1965, he came to
414-622: The mouth at its centre. The buccal cavity has a single dorsal and a single ventral tooth. Segment 14 has the anus on its dorsal surface, and segment 15 forms a sucker. The majority of branchiobdellida use crayfish as hosts, usually living on their heads, carapaces and chelae (claws), but in some instances living inside their gill cavities. In East Asia, some species live on freshwater shrimp, and in Northern and Central America, freshwater crabs , shrimps and isopods host branchiobdellids, and some have even been found on Chesapeake blue crabs . In
437-428: The ocean. The subclass Hirudinea (leeches) contains three orders with various habitat preferences. Branchiobdellida are commensal with freshwater crayfish , grazing algae from their exoskeletons. Acanthobdellida are parasitic on freshwater fishes such as grayling . Leeches can be found in nearly every part of the world, in freshwater, terrestrial, and marine habitats. The subclass Oligochaeta , which includes
460-441: The paired individuals together while they exchange sperm. Afterwards it secretes material that forms a cocoon that encircles the animal's body and encloses the eggs and sperm. The animal works this cocoon forward and over its head end, whereupon the ends of the cocoon become sealed, with fertilisation and development taking place inside. Earthworms and their kin, in the subclass Oligochaeta, lack eyes but have photoreceptor cells in
483-564: The skin, especially in the dorsal portion of the anterior end. They also lack parapodia and appendages on the prostomium, the body and the periproct (terminal segment on which the anus is located). The gonads are located in a few segments near the clitellum, with the testes being anterior to the ovaries . There are four bundles of one to twenty-five chaetae on each segment; these have muscles attached to their bases and can be extended or retracted. Leeches, order Hirudinida, mostly have flattened bodies, usually tapered at both ends. They have
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#1732852439667506-493: The tree to indicate ordered lineal relationships between all the organisms shown. Further, any group may (or may not) be considered a taxon by modern systematics , depending upon the selection of its members in relation to their common ancestor(s); see second and third diagrams. The term monophyly , or monophyletic , derives from the two Ancient Greek words μόνος ( mónos ), meaning "alone, only, unique", and φῦλον ( phûlon ), meaning "genus, species", and refers to
529-402: The underside of the body. The anus is on the dorsal surface just in front of the posterior sucker. The body wall includes strong transverse, longitudinal and diagonal muscles which give the animal great flexibility and extensibility. Genetic studies have shown the class' genome have gone through an extreme rearrangement, more than any other animals. Clitellates live on land, in freshwater or in
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