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Introduction to Aschelminthes

The Aschelminthes (also known as Aeschelminthes, Nemathelminthes, and Nematodes) are an obsolete phylum of pseudocoelomate and other similar creatures that have been elevated to their phyla after being closely affiliated with the Platyhelminthes. The term Aschelminth is currently mostly used as a colloquial word for any of the roughly ten diverse invertebrate phyla that used to be classified as Aschelminthes. It belongs to the polyphyletic category.

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Phylum Aschelminthes

Phylum Aschelminthes, Nemathelminthes, a name referring to an obsolete phylum of wormlike invertebrates, mostly of microscopic size. These animals were first classified together because they all appeared to have a unique sort of body cavity termed a pseudocoel (a body cavity without a lining of mesoderm) that develops differently than other animals' body cavities. However, it has become evident that these species do not have tight evolutionary ties, and each group has been assigned to its phylum. Rotifers, on the other hand, appear to be closely related to acanthocephalans, and the two taxa may eventually be lumped together in the same phylum.

Previously, phylum Aschelminthes included seven classes of animals Nematoda, Rotifera, Acanthocephala, Gastrotricha, Kinorhyncha, Nematomorpha, and Gnathostomulida.

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Classification of Aschelminthes

1. Acanthocephala

Acanthocephala is a parasitic worm phylum that includes acanthocephalans, thorny-headed worms, and spiny-headed worms. It is characterized by an eversible proboscis loaded with spines that it uses to pierce and hold the gut wall of its host.

Acanthocephalans have complicated life cycles that involve at least two hosts, which might include invertebrates, fish, amphibians, birds, or mammals. A total of 1420 species have been discovered. The Acanthocephala were once considered a separate phylum. According to a recent genomic study, they are descended from and should be classified as highly modified rotifers. Syndermata is the name given to this unified taxon.

Acanthocephalans will not have a mouth or a digestive system.

They share this trait with the Cestoda (tapeworms), even though the two families are not closely related. Adult stages dwell in their host's intestines and directly absorb nutrients that have been digested by the host through their body surface. Although certain species of acanthocephalans have been shown to contain flame cells, they lack an excretory system (protonephridia).

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These species range in size from a few millimetres in length to Gigantorhynchus Gigas, which ranges between 10 and 65 centimetres in length (3.9 to 25.6 in).

The huge size of many of the cells, such as nerve cells and cells composing the uterine bell, is a peculiar trait shared by both larva and adult. Polyploidy is frequent, with some species having up to 343n chromosomes.

The Acanthocephala body surface is peculiar. Externally, the epidermis is covered by a thin tegument, which is made up of a syncytium with no cell walls.

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2. Chaetognatha

The Chaetognatha are a phylum of predatory marine worms that make up a significant portion of plankton all around the world. About 20% of Chaetognatha species are benthic, meaning they may adhere to algae and rocks and are sometimes referred to as arrow worms. They may be found in many types of marine habitats, from shallow tidal pools and surface tropical seas to deep oceans and polar areas. Although the majority of chaetognaths are translucent and torpedo-shaped, certain deep-sea species are orange. They are available in sizes ranging from 2 to 120 centimetres (0.1 to 4.7 in). All species are hermaphroditic, meaning they contain both eggs and sperm in their bodies. Chaetognaths are epidermal layer clear or translucent dart-shaped animals.

There is a distinct head, trunk, and tail on the body. On either side of its head, there are four to fourteen hooked, gripping spines surrounding a hollow vestibule enclosing the mouth. When the animal is swimming, the spines are covered by a flexible hood that arises from the neck area. Chaetognaths are carnivores that feed on planktonic organisms. Some species have been described as omnivores, eating algae and detritus.

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Each animal has a pair of testes in the tail and a pair of ovaries in the main body cavity's posterior area. Immature sperm are discharged from the testes and mature inside the tail cavity before swimming via a small channel to a seminal vesicle and being bundled into a spermatophore. After the seminal vesicle fractures, each individual deposits a spermatophore on the neck of its mate during mating. The sperm depart the spermatophore quickly and travel along the animal's midline until they reach a pair of tiny pores directly in front of the tail.

3. Cycliophora

Symbion is the name of a genus of commensal aquatic animals living connected to the mouthparts of cold-water lobsters and is smaller than 0.5 mm broad. They have sac-like bodies and three distinct forms at various stages of their two-stage life cycle. They stand out so much from other creatures that they were given their phylum, Cycliophora, immediately after their discovery in 1995. Since the Loricifera in 1983, this is the first new phylum of multicellularity to be found. Symbion reproduces both asexually and sexually and has a complicated reproductive cycle that developed to generate as many children as possible that can survive and find a new host when the lobster on which they dwell loses its shell.

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The asexual people are the most numerous. Sexual folks do not consume food. They produce duplicates of themselves during the fall, where a new person grows within the parent body, one offspring at a time. The new progeny attach themselves to an accessible area on the lobster, begin feeding and eventually begin reproducing. The asexual animals begin to produce males in the early winter.

4. Gastrotricha

The gastrotrichs (phylum Gastrotricha), often known as hairy bellies or hairybacks, are a group of tiny (0.06-3.0 mm) worm-like pseudocoelomate animals that are found in both freshwater and marine environments.

They are mostly benthic, living in the periphyton, a layer of microscopic creatures and debris found on the seafloor and in the beds of other bodies of water. The majority of species reside on and between sediment particles or on other submerged surfaces, however, a few species are terrestrial, living on the land in the water film around soil grains. 

Gastrotrichs are divided into two categories, the Macrodasyida which are marine (except for two species), and the Chaetonotida, some of which are marine and some freshwater. Nearly 800 species of gastrotrich have been described.

Gastrotrichs have a simple body plan with a head region, a brain and sensory organs, and a trunk with a simple gut and reproductive organs. They contain sticky glands that allow them to stick to the substrate and cilia that allow them to move around. They feed on detritus, sucking up organic particles with their muscular pharynx. They are hermaphrodites, the marine species producing eggs that develop directly into miniature adults. 

5. Kinorhyncha

As part of the meiobenthos, Kinorhyncha is a phylum of tiny marine invertebrates that may be found in mud or sand at all depths. Mud dragons are another name for them. Modern species are just a few millimetres long, whereas Cambrian forms could grow to be 4 centimetres long. Kinorhynchs are limbless animals having a segmented body that includes a head, neck, and eleven-segmented trunk. They lack external cilia, unlike some other invertebrates, and instead contain a lot of spines around the body, as well as up to seven rings of spines around the head. These spines are employed for movement, with the head being pushed forward and the spines grasping the substrate as the body is drawn up. 

Although some sexual dimorphism in allometry has been reported, the two sexes appear identical. The gonads are located in the trunk's mid-region and open to pores in the last segment. The sperm duct in most species has two or three spiky features that are thought to help in copulation, however, the details remain unclear. Spermatozoa can grow to be a fourth of the length of the human body. The larvae are free-living, but nothing is known about how they reproduce. After laying an egg, the female envelops it in a protective mud and organic material envelope.

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6. Nematoda

The phylum Nematoda (also known as Nemathelminthes) is mainly composed of nematodes or roundworms, with plant-parasitic nematodes being known as eelworms. They are included in the group Ecdysozoa, together with insects and other moulting animals, and, unlike flatworms, have tubular digestive tracts with openings on both ends. They contain fewer Hox genes than tardigrades, but their sister phylum Nematomorpha has retained the original protostome Hox genotype, indicating that the reduction occurred inside the worm phylum. 

Nematodes are worms that are very small and thin, typically about 5 to 100 micrometre thick and 0.1 to 2.5 mm long. The smallest nematodes are microscopic, but free-living species may grow up to 5 cm (2 in) in length, while parasitic species can grow up to 1 m (3 ft) in length. 270 Ridges, rings, bristles, and other distinctive structures are frequently found on the body.

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7. Priapulida

The phylum Priapulida, sometimes known as penis worms, is made up of unsegmented sea worms. Because its basic form and extended spiny introvert (eversible) proboscis mimic the shape of a human penis, the phylum is named after the Greek god of fertility. They reside in the dirt and shallow water up to 90 metres (300 feet) deep. Some species are highly resistant to hydrogen sulphide and anoxia. In some regions, they can be fairly abundant. There have been reports of as many as 85 adult Priapulus caudatus per square metre in an Alaskan bay, with a larval density as high as 58,000 per square metre. 

Priapulids are cylindrical worm-like organisms that range in size from 0.2–0.3 centimetres (0.08–0.12 in) to 39 millimetres (0.08–0.12 in) in length, with no armature or tentacles in the median anterior mouth. The body is separated into two sections: a primary trunk or abdomen and a bulging proboscis with longitudinal ridges. The body is ringed with spines, which are frequently extended into the somewhat protruding throat. Tails or a pair of caudal appendages may be present in some animals. The animal's body contains a chitinous cuticle that is shed as it develops.

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8. Rotifera

The rotifers, also known as wheel animals or wheel animalcules, are a phylum of pseudocoelomate animals that are microscopic or near-microscopic. Rev. John Harris was the first to describe them in 1696, and Antonie van Leeuwenhoek characterised them in 1703. The majority of rotifers are between 0.1 and 0.5 millimetres in length (but they can range from 50 millimetres to over 2 millimetres) and are found in freshwater habitats across the world, with a few saltwater species. Some rotifers are planktonic and swim freely, while others inchworm their way over a substrate, and yet others are sessile, dwelling inside tubes or gelatinous holdfasts connected to the substrate. Rotifers are symmetrical on both sides and come in a variety of forms.

A rotifer's body is separated into a head, trunk, and foot, and is usually a cylindrical tail. The animal has a well-developed cuticle that can be thick and hard, giving it a box-like shape, or flexible, giving it a worm-like form; these rotifers are named loricate and illoricate, respectively. Multiple plates make up rigid cuticles, which may include spines, ridges, or other ornamentation. Their cuticle is formed from sclerotized proteins and is non-chitinous.

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Phylum Aschelminthes Characteristics

There are various types of different characteristics of aschelminthes are as follows,

  • These animals have triploblastic, unsegmented body.

  • They have a pseudocoelom, which means that the mesodermal layer does not border the body cavity.

  • They are symmetric on both sides.

  • The body is cylindrical or thread-like in appearance, with a long, thin worm-like appearance and tapering at both ends.

  • The epidermis, muscular layer, and cuticle cover the body wall.

  • These animals range in size from microscopic to many centimetres in length.

  • Most of these organisms are parasitic, with a few outliers.

  • They are organised at the organ system level.

  • There is minimal distinction between the anterior and posterior sections from the outside. Internal cephalization, on the other hand, is present.

  • There isn't a distinct head to speak of. The mouth, on the other hand, is visible in the front.

  • With a mouth and anus, the digestive system is complete.

  • In these species, the mouth is terminal and is encircled by sensory organ-bearing lips.

  • The principal sensory organs are amphids and papillae.

  • The oesophagus is encircled by a nerve ring, which is part of the nervous system. 

  • Nerves branch out anteriorly and posteriorly from it.

  • There are no respiratory organs.

  • Respiration takes place throughout the whole body surface. In free-living species, it is aerobic; parasitic species are anaerobic.

  • Canals and gland-like structures make up the excretory system.

  • The sexes are distinct and unisexual, with sexual dimorphism.

  • Internal fertilisation occurs.

  • They can be oviparous, viviparous, or ovo-viviparous.

  • These organisms have a life cycle. It can be done with or without the use of an intermediate host.

Aschelminthes Examples

Some phylum aschelminthes examples are as follows,

  • Ascaris lumbricoides – Roundworm

  • Enterobius vermicularis – Pinworm

  • Ancylostoma duodenale – Hookworm

  • Wuchereria bancrofti – Filarial worm

  • Loa-Loa – Eye Worm


The Aschelminthes are an obsolete phylum of pseudocoelomate and other similar creatures. They have been elevated to their phylum after being closely affiliated with the Platyhelminthes. The term Aschelminth is currently mostly used as a colloquial word for any of the roughly ten diverse invertebrate phyla that used to be classified as Aschel Minthes. These animals were first classified together because they all appeared to have a unique sort of body cavity. It has become evident that these species do not have tight evolutionary ties, and each group has been assigned to its phylum.

The Aschelcylindricalminthes are wormlike invertebrates, mostly of microscopic size. Oesophagus is encircled by a nerve ring, which is part of the nervous system. Nerves branch out anteriorly and posteriorly from it. Respiration takes place throughout the whole body surface. The sexes are distinct and unisexual, with sexual dimorphism.

FAQs on Aschelminthes

Q.1) Where are Aschelminthes Found?

Answer: Aschelminths can be parasitic or free-living. The free-living organisms, which feed on bacteria, are common in soils and sediments. Others, on the other hand, are plant parasites that can cause disease in economically important crops.

Q.2) What Do Aschelminthes Eat?

Answer: Dead plants and tails animals are eaten by earthworms. They eat soil and tiny pebbles as well as food. Microorganisms in the material they eat provide nutrients to earthworms. The waste is then excreted in the form of casts.

Q.3) Why are Segmented Worms Helpful to Humans?

Answer: Earthworms, despite their boring appearance, are extremely important organisms in the human ecosystem.

We sometimes take for granted their persistent labour in the earth beneath us, yet they are responsible for the rich soils that we needed to grow our food.