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.

References

• 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.