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Asexual reproduction in cestoda


Cestoda is a class of parasitic worms in the flatworm phylum Platyhelminthes. Most of the species—and the best-known—are those in the subclass Eucestoda ; they are ribbonlike worms as adults, known as tapeworms. Asexual reproduction in cestoda bodies consist of many similar units, known as proglottids, which are essentially packages of eggs that are regularly shed into the environment to infect other organisms.

Species of the other subclass, Cestodariaare mainly fish parasites. All cestodes are parasitic ; many have complex life historiesincluding a stage in a definitive main host in which the adults grow and reproduce, often for years, and one or two intermediate stages in which the larvae "Asexual reproduction in cestoda" in other hosts.

Typically the adults live in the digestive tracts of vertebrateswhile the larvae often live in the bodies of other animals, either vertebrates or invertebrates. For example, Diphyllobothrium has at least two intermediate hosts, a crustacean and then one or more freshwater fish; its definitive host is a mammal. Some cestodes are host-specific, while others are parasites of a wide variety of hosts.

Some six thousand species have been described; probably all vertebrates can host at least one species. The adult tapeworm has a scolex, or head, a short neck, and a strobila, or segmented body formed of proglottids. Tapeworms anchor themselves to the Asexual reproduction in cestoda of the intestine of their host using their scolex, which typically has hooks, suckers, or both.

They have no mouth, but absorb nutrients directly from the host's gut. The neck continually produces proglottids, each one containing a reproductive tract; mature proglottids are full of eggs, and fall off Asexual reproduction in cestoda leave the host, either passively in the feces or actively moving.

All tapeworms are hermaphrodites, with each individual having both male and female reproductive organs. Humans are subject to infection by several species of tapeworms if they eat undercooked meat such as pork Taenia soliumbeef T. The unproven concept of using tapeworms as a slimming aid has been touted since around All species of Cestoda are parasitesmainly intestinal; their definitive hosts are vertebrates, both terrestrial and marine, while their intermediate hosts include insects, crustaceans, molluscs, and annelids as well as other vertebrates.

Cestodes have no gut or mouth [6] and absorb nutrients from the host's alimentary tract through their specialised neodermal cuticle, or tegument[7] through which gas exchange also takes place. The body form of adult eucestodes is simple, with a scolex, or grasping head, adapted for attachment to the definitive hosta short neck, and a strobila, or segmented [a] trunk formed of proglottids, which makes up the worm's body.

Members of the subclass Cestodariathe Amphilinidea and Gyrocotylideaare wormlike but not divided into proglottids.

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Amphilinids have a muscular proboscis at the front end; Gyrocotylids have a sucker or proboscis which they can pull inside or push outside at the front end, and a holdfast rosette at the posterior end.

The Cestodaria have 10 larval hooks while Eucestoda have 6 larval hooks. The scolex, which attaches to the intestine of the definitive host, is often minute in comparison with the proglottids. It is typically a four-sided knob, armed with suckers or hooks or both.

Cyclophyllid cestodes can be identified Asexual reproduction in cestoda the presence of four suckers on their scolices. In the larval stage the scolex is similarly shaped and is known as the protoscolex. Circular and longitudinal muscles lie under the neodermis, beneath which further longitudinal, dorso-ventral and transverse muscles surround the central parenchyma.

Protonephridial cells drain into the parenchyma. There are four longitudinal collection Asexual reproduction in cestoda, two dorso-lateral and two ventro-lateral, running along the length of the worm, with a transverse canal linking the ventral ones at the posterior of each segment.

When the proglottids begin to detach, these canals open to the exterior through the terminal segment. The main nerve centre of a cestode is a cerebral ganglion in its scolex. Nerves emanate from the ganglion to supply the general body muscular and sensory endings, with two lateral nerve cords running the length of the strobila. Sensory function includes both tactoreception touch and chemoreception smell or taste. Once anchored to the host's intestinal wall, tapeworms absorb nutrients through their surface as their food flows past them.

A tapeworm can reproduce sexually,...

The tapeworm body is composed of a series of segments called proglottids. These are produced from the neck by mitotic growth which is followed by transverse constriction. Asexual reproduction in cestoda segments become larger and more mature as they are displaced backwards by newer segments.

The sum of the proglottids is called a strobila, which is thin and resembles a strip of tape; from this is derived the common name "tapeworm".

Proglottids are continually being produced by the neck region of the scolex, as long as the scolex is attached and alive. Mature proglottids are essentially bags of eggs, each of which is infective to the proper intermediate host.

They are released and leave the host in faeces, or migrate outwards as independent motile proglottids. Their layout comes in two forms: Cestodes are exclusively hermaphroditeswith both male and female reproductive systems in each body. The reproductive system includes Asexual reproduction in cestoda or more testes, cirri, vas deferensand seminal vesicles as male organs, and a single lobed or unlobed ovary with the connecting oviduct and uterus as female organs.

The common external opening for both male and female reproductive systems is known as the genital pore, which is situated at the surface opening of the cup-shaped atrium.

Cestodes are parasites of vertebrates, with each species infecting a single definitive host or group of closely related host species. All but amphilinids and gyrocotylids which burrow through the gut or body wall to reach the coelom [6] are intestinal, though some life-cycle stages rest in muscle or other tissues. The definitive host is always a vertebrate but in nearly all cases, one or more intermediate hosts are involved in the lifecycle, typically arthropods or other vertebrates.

Cestodes produce large numbers of eggs, but each one has a low probability of finding a host. To increase their chances, different species have adopted various strategies of egg release. In the Pseudophyllidea, many eggs are released in the brief period when their aquatic intermediate hosts are abundant semelparity. In contrast, in the terrestrial Cyclophyllidea, proglottids are released steadily over a period of years, or as long as their host lives "Asexual reproduction in cestoda." Another strategy is to have very long-lived larvae; for example, in Echinococcusthe hydatid larvae can survive for ten years or more in humans and other vertebrate hosts, giving the tapeworm an exceptionally long time window in which to find another host.

Many tapeworms have a two-phase lifecycle with two types of host. The adult Taenia saginata lives in the gut of a primate such as a human, its definitive host.

Proglottids leave the body through the anus and fall to the ground, where they may be eaten with grass by a grazing animal such as a cow. This animal then becomes an intermediate host, the oncosphere boring through the gut wall and migrating to another part of the body such as the muscle. Here it encysts, forming a cysticercus. The parasite completes its lifecycle when the intermediate host passes on the parasite to the definitive host, usually when the definitive host eats contaminated parts of the intermediate host, for example a human eating raw or undercooked meat.

Diphyllobothrium exhibits a more complex, three-phase lifecycle. If the eggs are laid in water, they develop into free-swimming oncosphere larvae. After ingestion by a suitable freshwater crustacean such as a copepodthe Asexual reproduction in cestoda intermediate host, they develop into procercoid larvae.

When the copepod is eaten by a suitable second intermediate host, typically a minnow or other small freshwater fish, the procercoid larvae migrate into the fish's flesh where they develop into plerocercoid larvae.

These are the infective stages for the mammalian definitive host.

If the small fish is eaten by a predatory fish, its muscles too can become infected. Schistocephalus solidus is another three-phase Asexual reproduction in cestoda. The intermediate hosts are copepods and small fish, and the definitive hosts are waterbirds. This species has been used to demonstrate that cross-fertilisation produces a higher infective success rate than self-fertilisation. Hosts can become immune to infection by a cestode if the lining, the mucosa, of the gut is damaged.

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This exposes the host's immune system to cestode antigensenabling the host to mount an antibody defence. Host antibodies can kill or limit cestode infection by damaging their digestive enzymes, which reduces their ability to feed and therefore to grow and to reproduce; by binding to Asexual reproduction in cestoda bodies; and by neutralising toxins that they produce.

When cestodes feed passively in the gut, they do not provoke an antibody reaction. Parasite fossils are rare, but recognizable clusters of cestode eggs, some with an operculum lid indicating that they had not erupted, one with a developing larva, have been discovered in Asexual reproduction in cestoda shark coprolites dating to the Permiansome million years ago. The position of the Cestoda within the Platyhelminthes and other Spiralian phyla based on genomic analysis is shown in the phylogenetic tree.

The non-parasitic flatworms, traditionally grouped as the " Turbellaria ", are paraphyleticas the parasitic Neodermata including the Cestoda arose within that grouping. The approximate times when major groups first appeared is shown in millions of years ago. The evolutionary history of the Cestoda has been studied using ribosomal RNAmitochondrial and other DNA, and morphological analysis and continues to be revised. Alternatives, generally for different species within an order, are shown in square brackets.

The Taeniidaeincluding species such as the pork tapeworm and the beef tapeworm that often infect humans, may be the most basal of the 12 orders of the Cyclophyllidea. In ancient Greecethe comic playwright Aristophanes and philosopher Aristotle described the lumps that form during cysticercosis as "hailstones".

Tapeworms have occasionally appeared in fiction. Peter Marren and Richard Mabey in Bugs Britannica write that Irvine Welsh 's sociopathic policeman in his novel Filth owns a talking tapeworm, which they call "the most attractive character in the novel"; it becomes the policeman's alter ego and better self.

There are unproven claims that, aroundtapeworm eggs were marketed to the public as slimming tablets. With sanitized tape worms. Like other species of mammal, humans can become infected with tapeworms. There may be few or no symptoms, and the first indication of the infection may be the presence of one or more proglottids in the stools. The proglottids appear as flat, rectangular, whitish objects about the size of a grain of rice, which may Asexual reproduction in cestoda size or move about.

Bodily symptoms which are sometimes present include abdominal pain, nausea, diarrhoea, increased appetite and weight loss. There are several classes of anthelminthic drugs, some effective against many kinds of parasite, others more specific; these can be used both preventatively [41] and to treat infections.