Phylum Aschelminthes-Characteristics, Classification, Examples

What are Aschelminthes?

Phylum Aschelminthes have entire digestive tubes and are pseudocoelomate, bilaterally symmetrical, triploblastic, unsegmented, vermiform, and organ-system grade of construction.

Phylum Aschelminthes characteristics

  • Most of them are aquatic, parasitic, or free-living.
  • They are unsegmented, bilaterally symmetrical worms.
  • The body is flattened or often worm-like, thin, and vermiform.
  • While many of them are tiny, even microscopic, others may grow to be a metre or longer.
  • They have organ system grade body organisation and are triploblastic and pseudocoelomates.
  • Body wall having syncytial or cellular epidermis and a thick scleroprotein cuticle on the outside.
  • Except for the front cilia of rotifers, cilia are missing.
  • Most of the fibres in muscles are longitudinal.
  • The digestive system is complete and straight, with a mouth, a muscular and highly specialised pharynx, a straight, non-muscular intestine, and anus.
  • Systems for breathing and moving blood are not present.
  • Protonephridia (in certain cases) are a series of canals in the excretory system that are used for osmoregulation. Some people have cloaca.
  • The circumenteric nerve ring and the anterior and posterior longitudinal nerves make up the simple nervous system.
  • In the form of pits, papillae, bristles, and eyespots, sense organs are present.
  • They generally have different sexes and are dioecious, single or twin gonads and ducts.
  • There is no asexual reproduction in them.
  • Eggs have a chitinous shell with spiral cleavage.
  • Typically, they have a simple or complex life cycle without any distinct larval stages.

Classification of the phylum Aschelminthes

Various zoologists have assigned distinct classifications to this phylum since it has a diverse range of different species. Although Storer and Usinger (1971) recognised alternative classifications of Aschelminthes as independent phyla and ranked Aschelminthes as a superphylum, this classification is based on and modified from L. H. Hymann’s (1951) work.

Class 1. Nematoda (Gr., nema=thread+ eidos= form)

  • Roundworm that is elongated and may live on land or in water.
  • Unsegmented, cylindrical, and lengthy body.
  • Body wall having cellular or syncytial epidermis, a thick cuticle, and longitudinal muscles arranged in four bands.
  • No cilia, no respiratory and cardiovascular systems.
  • The muscular pharynx and stomach complete the digestive system.
  • glandular canal or glandular organ excretory system.
  • Anterior and posterior nerves are part of the nervous system’s circumenteric ring.
  • Simple sense organs
  • Males are smaller than females and have penial spicules.
  • a single or two gonads. The female genital duct has a different entrance from the male genital ducts, which enter the cloaca.
  • Fertilization
  • Most development is straightforward. No regeneration or asexual reproduction.
  • Examples include Trichinella, Necator, Wuchereria, and Ascaris.

Order 1. Enploidea

  • Not ringed, it often has cuticular bristles.
  • 6 labial papillae on the anterior end.
  • One or two circlets of 10 to 12 bristles.
  • the anterior muscular and posterior glandular oesophagus.
  • cyathiform amphids and two cephalic slits.
  • Examples include Synonchus, Anticoma, and Enoplus.

Order 2, Dorylaimoidea

  • Zero bristles, smooth cuticle.
  • Two 6- and 10-papillae circlets are seen on the anterior end.
  • A spear-armed cavity in the buccal cavity.
  • The pharynx’s back portion became larger.
  • cycliform amphids.
  • Dorylamius, Tylencholaimus, and Actinolaimus are among examples.

Order 3. Mermithoidea

  • The juvenile stages of smooth, filiform nematodes, which mostly parasitize insects, allow them to live freely as adults in soil or water.
  • decreased to six or fewer papillae on the head.
  • blind and long pharynx.
  • It is a modified form of the blind gut that stores food.
  • reduced or cyathiform amphids.
  • For instance, there are Mermis, Paramermis, and Aproctonema.

Order 4: Chromadoroidea

  • the ringed or smooth cuticle.
  • Typically, the cuticle is extensively embellished with bristles, knobs, and punctures.
  • has a posterior bulb in the pharynx.
  • Amphids either originate from or are spirals.
  • Examples include Paracanthonchus and Pacytholamius.

Order 5, Araeolaimoidea

  • Sometimes bristles, sometimes smooth cuticles.
  • There are labial papillae.
  • 4 cephalic bristles on the anterior end.
  • loop- or spiral-shaped amphids.
  • Plectus, Wilsonema, and Odontophora, as examples.

Order 6. Monhysteroidea

  • Cuticles that are smooth or somewhat ringed and often have bristles.
  • bristles on the anterior end are 4, 6, or 8.
  • spherical amphids.
  • Examples include siphonolaimus and cyclolaimus.

Order 7. Desmoscalecoidea

  • Cuticle with a thick ring and noticeable bristles all over or in a small region.
  • 4 bristles on the anterior end.
  • Crescent-shaped amphids
  • marine nematodes
  • Desmoscolex, Tricoma, and Greeffiella, for instance.

Order 8. Rhabditoidea or Anguilluloidea

  • independent or parasitic.
  • the smooth or ringed cuticle.
  • The pharynx often has swelling anterior to the nerve ring and a posterior bulb.
  • Caudal glands are absent.
  • Pocket-sized amphids.
  • Rhabditis, Diploscapter, and Diplogaster, as examples.

Order 9. Rhabdiasoidea

  • Nematodes have a distinct pharyngeal bulb that is smooth.
  • Hermaphrodites are also capable of pathogenicity.
  • animal phases of parasitism.
  • Males and females may emerge from free-living stages.
  • Rhabdias and Entomelas are two examples.

Order 10. Oxyuroidea

  • pharynx with the posterior bulb valvulated.
  • Long, pointed tail; female; terminal elements of the female system often strongly muscled.
  • Male has one or two equally sized spicules.
  • Typically, a cuticular brusa is formed by caudal alae.
  • Examples include Enterobius or Oxyuris.

Order 11. Ascaroidea

  • three pronounced lips around the mouth.
  • The pharynx is not vulvulated whether the posterior bulb is present or absent.
  • Pharynx, intestine, or sometimes both with caeca.
  • zero buccal capsule.
  • Females have blunt tails, while males lack caudal alae and have two spicules that are almost equal in size.
  • For instance, Ascaris

Order 12. Strongyloidea

  • lips are absent, but the mouth often has a leaf crown.
  • No bulb in the pharynx.
  • Typically, females have an ovijector.
  • Male with a normally 13-rayed copulatory brusa supported by muscles.
  • Strongylus, Necator, and Ancylostoma, for instance.

Order 13: Spiruroidea

  • Typically, the mouth has two lateral lips; sometimes there are four or six tiny ones.
  • Without a bulb, the pharynx is anteriorly musculoskeletal and posteriorly glandular.
  • Males lack brays; spicules are uneven and different.
  • Examples include Thelazia, Oxyspirura, and Rictularia.

Order 14. Dracunculoidea

  • lacking cuticularized buccal capsules or distinct lips.
  • Without a bulb, the pharynx is anteriorly musculoskeletal and posteriorly glandular.
  • The vulva is normally found behind or close to the centre of the body, but it is not functioning.
  • Males lack brusa but have equal filiform spicules.
  • Examples include Micropleura, Philometra, and Dracunculus.

Order 15. Filarioidea

  • a lipless, worm-like shape.
  • buccal capsules that are small or simple.
  • The pharynx has posterior glands and anterior muscles.
  • There is no pharyngeal bulb.
  • Females have an anterior vulva.
  • Lacking brusa, spicules that are uneven and different.
  • Wuchereria and Loa Loa are two examples.

Order 16.Trichuroidea or Trichinelloidea

  • Front of the body is filiform.
  • sans lips, a mouth.
  • thin pharynx.
  • provided by a cirrus, and just one spicule if any are present.
  • Trichinella and Trichuris, for instance,

Order 17. Dioctophymoidea

  • moderately to very lengthy.
  • 6, 12, or 18 papillae surround an open mouth without lips.
  • Bulbless, elongated pharynx.
  • Male without rays but with a muscular brusa
  • Dictyophyme, Hystrichis, and

Class 2: Nematomorpha (Greek: nema=thread + morphe= form) or Gordiacea

  • Hair worm is a freshwater species. Nectonema is the only marine genus.
  • a very long, narrow, cylindrical body.
  • body wall with tiny papillae and a thick cuticle.
  • single-layered, cellular epidermis.
  • In larvae, the digestive system is fully developed; in non-feeding adults, it deteriorates. Cloaca is there.
  • Parenchyma mostly fills the pseudocoel.
  • no respiratory, digestive, or circulatory systems.
  • has a nervous system that includes a midventral nerve cord and the circumenteric nerve ring.
  • pairings of gonads and gonoducts. Also leading into the cloaca are oviducts.
  • Grasshoppers, crickets, and other insect parasitic juveniles
  • For instance, there are Gordius, Paragordius, and Nectonema.

Order 1. Cordioidea

  • Bristles for swimming are lacking.
  • Parenchyma fills the pseudocoel.
  • gonad pairs.
  • Gordius and Paragordius, for instance.

Order 2. Nectonematoidea

  • Two rows of swimming bristles are placed.
  • Pseudocoel that is empty
  • a single gonad.
  • Marine dress
  • Consider the nectomema.

Class 3. Rotifera (L., rota= wheel + ferre= to bear)

  • These are tiny creatures that may be found in streams, lakes, and ponds. Oceans seldom have what are known as “wheel bearers.”
  • Anterior end of the body wall with ciliated corona (wheel organ) utilised for eating and motility. Body wall hardened into stiff plates or lorica.
  • Toes and sticky glands are present on the post-anal foot.
  • Transverse and longitudinal muscle strands and bands make up the body’s musculature.
  • the interior cuticle of the mastax, the digestive system’s grinding organ,
  • two protonephridia and two protonephridial tubes, which drain into bladders, make up the excretory system.
  • three main ganglia and nerves make up the nervous system.
  • antennas, eyespots, and other sensory organs.
  • Females are larger than males. Common parthenogenesis
  • No larval stages; female oviparous.
  • For instance, Rotaria, Asplanchna, and Philodina.

Order 1. Seisonacea

  • Long neck and a lean body.
  • Corona is a little
  • gonad pairs.
  • found on crustaceans as commensals.
  • For instance, seasoning

Order 2: Budelloidea

  • Typically, a corona has two trochal discs.
  • More than two pedal glands.
  • Degenerate male; paired germovitellaria in female.
  • Forms that swarm or crawl
  • Other examples are Rotaria or Rotifera, Philodina, and Mniobia.

Order 3. Monogonontea

  • forms that swim or are sessile.
  • Degenerate or undersized males.
  • typically just one testis in males.
  • Females only have one germovitellaria.
  • Mytilina and Limnias, as examples.

Class 4. Gastrotricha (L., gaster= stomach + trichos= hair)

  • freshwater, marine, or microscopic.
  • Cuticle-covered body wall with small, curving dorsal spines.
  • The ventral surface of the corona is devoid of cilia for movement.
  • Forked posterior end with attachment glands and sticky tubes.
  • There are six pairs of longitudinal muscles in the body.
  • bristles around the mouth.
  • The pharynx is muscular and triradiate.
  • the two protonephridia of the excretory system.
  • The nervous system consists of two lateral nerves and a saddle-shaped ganglion.
  • Whether monoecious or dioecious, parthenogenetic females are common.
  • direct development Young and old are similar.
  • For instance, take Chaetonotus and Macrodasys.

Order 1. Macrodasyoidea

  • worm-like marine formations.
  • several tubes of adhesive.
  • Absence of protonophelia
  • Among them is Macrodasys.

Order 2. Chaetonotoidea

  • On vegetation, freshwater forms are the majority.
  • tubes for adhesive on the tail.
  • a protonephridia pair.
  • Parthenogenesis, or parthenogenesis,
  • As for Chaetonotus and Neodesys, as examples,

Class 5. Kinorhyncha (Greek: more + rhynchos = beak)

  • marine, worm-like in size.
  • body is superficially divided into 13 or 14 overlapping rings (Zonites).
  • There are spiny cuticles on the body’s surface but no cilia.
  • mouth cone or protruding, scalid-covered head.
  • a pair of sticky tubes on the ventral surface’s front side.
  • Pseudocoel with amoebocyte-containing fluid.
  • Each zonite has a nerve ring, a ventral cord, and a ganglion. in some eye spots.
  • Salivary glands finish the digestive system.
  • Gonads are described as two tubular sacs.
  • Male penial spicules.
  • intrinsic feminization. a transformation including numerous larval stages.
  • There are many juvenile phases in development.
  • Echinoderes and Pycnophyes, for instance.

Order 1. Homalorhagida

  • Both the head and the neck protrude.
  • Trachydemus, as an example.

Order 2. Cyclorhagida

  • Only the protruding head ring is present.
  • Consider echinoderms.


  • Kotpal RL. 2017. Modern Text Book of Zoology- Invertebrates. 11th Edition. Rastogi Publications.
  • Jordan EL and Verma PS. 2018. Invertebrate Zoology. 14th Edition. S Chand Publishing.
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