- These are pseudocoelomate, bilaterally symmetrical, triploblastic, unsegmented, vermiform, organ- system grade of construction with complete digestive tubes. These are commonly known as roundworms.
- These are generally known as roundworms.
- Aschelminthes are described by the presence of a pseudocoelom.
- Aschelminthes is now an obsolete phylum of invertebrates and the animals grouped under this are now kept in ten different phyla.
- The Aschelminthes are also known as Aeschelminthes, Nemathelminthes, Nematodes, closely associated with the Platyhelminthes.
Characteristics of Phylum Aschelminthes
- Most of them are aquatic, free-living or parasitic.
- Aschelminthes are bilaterally symmetrical and unsegmented worms.
- They has a slender, vermiform and usually worm-like or flattened body.
- They are small, even microscopic, while some of them reach a meter or more in length.
- Aschelminthes are triploblastic and pseudocoelomate with organ system grade of body organization.
- It does not contain any Respiratory and circulatory systems.
- The Excretory system of Aschelminthes comprises a system of canals, protonephridia (in some) for osmoregulation. Cloaca can be found in some.
- The Body wall contains syncytial or cellular epidermis, It is externally coated with the solid cuticle of scleroprotein.
- They don’t have Cilia except anterior cilia of rotifers.
- Most of them are dioecious e.g.. sexes separate. Gonads and ducts single or double.
- The Muscles of Aschelminthes carry mostly longitudinal fibers.
- It has a straight digestive canal which is complete with a mouth, straight non-muscular intestine, and anus, pharynx muscular and highly specialized.
- It has a simple nervous system which is made up of circumenteric nerve ring with anterior and posterior longitudinal nerves.
- They have several sense organs such as pits, papillae, bristles, and eyespots. The papillae acts as a tango-receptors (sensitive to touch) and the amphids (pits) are chemo or olfacto-receptors.
- They don’t follow Asexual reproduction.
- The Eggs of Aschelminthes contain a chitinous shell, cleavage determinate and spiral.
- There are no special larval stages in their life cycle.
- The Male is smaller than the female. In male, testis is single (monodelphic) whereas in female, a pair of ovary are present (didelphic).
Phylum Aschelminthes Classification
Phylum Aschelminthes contain a heterogenous collection of various animals, therefore it has been divided differently by many zoologists. This classification of Aschelminthes is based and modification from L. H. Hymann (1951) but Storer and Usinger (1971) have seen different classification of Aschelminthes as separate phyla and rank Aschelminthes as superphylum.
Class 1: Nematoda (Gr., nema=thread+ eidos= form)
- These are aquatic or terrestrial, free-living or parasitic, elongated roundworm.
- The body of Nematoda is elongated, cylindrical and unsegmented. The body wall contains a thick cuticle, cellular or syncytial epidermis and longitudinal muscles in four bands.
- The cilia, circulatory and respiratory system is absent.
- The digestive system contains the muscular pharynx and the Excretory system of glandular organs or canals or both.
- The Nervous system of Nematoda contains a circumenteric ring and anterior and posterior nerves.
- It has a simple sense organ.
- The Male with penial spicules and are smaller than females.
- Gonads one or two. Male genital ducts lead into the cloaca, female genital duct with a separate opening.
- Asexual reproduction or regeneration is absent. The Development process is usually direct.
- Some examples of Nematoda are; Ascaris, Necator, Wuchereria, Trichinella
Order 1: Enploidea
- These are not ringed cuticles, often contain cuticular bristles.
- The Anterior portion of Enploidea contains 6 labial papillae.
- It has 10 to 12 bristles in one or two circlets.
- The Esophagus has two distinct parts such as anterior muscular and posterior glandular.
- It has a pair of cephalic slits and cyathiform amphids.
- Some examples of Enploidea are; Enoplus, Anticoma, Synonchus.
Order 2: Dorylaimoidea
- These are Smooth cuticles without bristles.
- The Anterior portion of Dorylaimoidea contains two circlets of papillae of 6 and 10 each.
- The Buccal cavity of Dorylaimoidea is armed with a protrusible spear.
- Cyathiform amphids.
- The rear part of the pharynx was enlarged.
- Some examples of Dorylaimoidea are; Dorylamius, Tylencholaimus, Actinolaimus.
Order 3: Mermithoidea
- These are Smooth, filiform nematodes, parasitic in invertebrates (mainly insects) in juvenile stages of free-living as an adult in soil or water.
- The main sense organs are reduced to papillae, usually six.
- It is with a long, blind pharynx.
- The Blind intestine of Mermithoidea is altered into a food-storing organs.
- Cyathiform or reduced amphids.
- Some examples of Mermithoidea are; Mermis, Paramermis, Aproctonema.
Order 4: Chromadoroidea
- It has a smooth or ringed cuticle. The cuticle is heavily ornamented with bristles, knobs, punctation.
- The Pharynx has a posterior bulb.
- The Amphids of Chromadoroidea are spiral or derivable from a spiral.
- Some examples of Chromadoroidea are; Paracytholamius, Paracanthonchus.
Order 5: Araeolaimoidea
- It has a Smooth cuticle, sometimes bristles.
- It contain Labial papillae.
- The Anterior portion contains 4 cephalic bristles.
- It is Spiral or loop-like amphids.
- Some examples of Araeolaimoidea are; Plectus, Wilsonema, Odontophora.
Order 6: Monhysteroidea
- It has a smooth or slightly ringed cuticle, often furnished with bristles.
- The Anterior portion of Monhysteroidea contain 4, 6 or 8 bristles.
- It is Circular amphids.
- Some examples of Monhysteroidea are; Cylindrolaimus, Siphonolaimus.
Order 7: Desmoscalecoidea
- It contains a heavily ringed cuticle, with prominent bristles completely or in a restricted area.
- The Anterior end of Desmoscalecoidea contains 4 bristles.
- It is a Crescentic amphids.
- Marine nematodes.
- Some examples of Desmoscalecoidea are; Desmoscolex, Tricoma, Greeffiella.
Order 8: Rhabditoidea or Anguilluloidea
- These are Free-living or parasitic.
- It has a ringed or smooth cuticle.
- The Pharynx contains the posterior bulb and also usually with swelling anterior to the nerve ring.
- The caudal glands are absent.
- It is a Small pockets amphids.
- Some examples of Rhabditoidea or Anguilluloidea are; Rhabditis, Diploscapter, Diplogaster.
Order 9: Rhabdiasoidea
- It is a Smooth nematode with a definite pharyngeal bulb.
- Hermaphrodites; pathogenesis also occurs.
- It has Parasitic stages in animals.
- The Free-living stages of Rhabdiasoidea may occur in males and females.
- Some examples of Rhabdiasoidea are; Rhabdias, Entomelas.
Order 10: Oxyuroidea
- The Valvulated pharynx contain posterior bulb.
- The Female contains a long pointed tail; terminal parts of the female system are often heavily muscularised.
- The Male contains one spicule or two equal spicules.
- Usually, the caudal alae forming a cuticular brusa.
- Some examples of Oxyuroidea are; Oxyuris or Enterobius.
Order 11: Ascaroidea
- The Mouth of Ascaroidea is surrounded by 3 prominent lips.
- Pharynx doesn’t contain any posterior bulb or of present it is not Valvulated.
- Pharynx or intestine or both often with caeca.
- The buccal capsule is absent.
- The female tail is bunted; the male lacks caudal alae and holds two equal or nearly equal spicules.
- Examples of Ascaroidea is: Ascaris.
Order 12: Strongyloidea
- The Mouth of Strongyloidea lacks conspicuous lips but often with leaf crowns.
- The Pharynx lacks the bulb.
- Females usually with ovijector.
- Male with copulatory brusa assisted by muscular rays; typically, 13 in number.
- Some examples of Strongyloidea are; Strongylus, Necator, Ancylostoma.
Order 13: Spiruroidea
- The mouth usually carries 2 lateral lips; often 4 or 6 small ones.
- The Pharynx lacks the bulb; anteriorly muscular and posteriorly glandular.
- The Males lack the brays; spicules are unequal and dissimilar.
- Some examples of Spiruroidea are; Oxyspirura, Rictularia, Thelazia.
Order 14: Dracunculoidea
- Without definite lips or cuticularized buccal capsules.
- Pharynx without bulb; anteriorly muscular and posteriorly glandular.
- The vulva is not functioning normally located near or behind the middle of the body.
- Males with equal filiform spicules; Brusa wanting.
- Some examples of Dracunculoidea are; Dracunculus, Philometra, Micropleura.
Order 15: Filarioidea
- These are Filiform worms without any lips.
- The buccal capsules are small or rudimentary.
- The pharynx of Filarioidea is anteriorly muscular and posteriorly glandular.
- It lacks the Pharyngeal bulb.
- Vulva anterior in females.
- Brusa wanting, spicules unequal and dissimilar.
- Some examples of Filarioidea are; Wuchereria, Loa loa.
Order 16: Trichuroidea or Trichinelloidea
- The Body of Trichuroidea or Trichinelloidea is filiform anteriorly.
- The Mouth is without lips.
- Slender pharynx. Provided with a cirrus, spicule if present one only.
- Some examples of Trichuroidea or Trichinelloidea are; Trichinella, Trichuris.
Order 17: Dioctophymoidea
- Moderate to very long-sized.
- Mouth without lips surrounded by 6,12 or 18 papillae.
- Elongated pharynx without bulb.
- Male with muscular brusa but without rays.
- Some examples of Dioctophymoidea are; Dictyophyme, Hystrichis,
Class 2: Nematomorpha or Gordiacea (Gr., nema=thread+ morphe= shape)
- These are hair worms, which can be found in freshwater. One genus (Nectonema) marine.
- The body of Nematomorpha is very long, thin, slender and cylindrical. The body wall contains a thick cuticle bearing small papillae.
- The Epidermis cellular, single-layered.
- The Pseudocoel is mostly loaded with parenchyma.
- The larva has a complete digestive system while it degenerates in non-feeding adults. Cloaca is present.
- There is no circulatory, respiratory and excretory system in Nematomorpha or Gordiacea.
- The Nervous system of Nematomorpha or Gordiacea contains a circumenteric nerve ring and midventral nerve cord.
- It has a paired Gonad and gonoducts. Oviducts also open within the cloaca.
- Juveniles parasitic in grasshoppers, crickets and other insects.
- Some examples of Nematomorpha or Gordiacea are; Gordius, Paragordius, Nectonema.
Order 1: Cordioidea
- Swimming bristles are wanting.
- The Pseudocoel is loaded with parenchyma.
- It has Paired gonads.
- Some examples of Cordioidea are: Paragordius, Gordius.
Order 2: Nectonematoidea
- The Swimming bristles are arranged in 2 rows.
- It has an Unfilled pseudocoel.
- It contains only one gonad.
- Marine form.
- Some examples of Nectonematoidea are: Nectonema.
Class 3: Rotifera (L., rota= wheel + ferre= to bear)
- Rotifera is microscopic animals which are mainly found in ponds, lakes, and streams. Rarely found in oceans, usually known as wheel bearers.
- The Body wall of Rotifera is thickened into stiff plates or lorica into which the head may.
- The Anterior end of the Rotifera with ciliated corona (wheel organ) helps in feeding and locomotion.
- For attachment, they used the Post- anal foot with toes and adhesive glands.
- The Body musculature of Rotifera comprises longitudinal and transverse muscle bands and strands.
- The digestive system includes a grinding organ, mastax, which is lined internally by a strong cuticle.
- The excretory system of Rotifera contains two protonephridia and two Protonephridial tubes which drain into bladders.
- The nervous system of 3 major ganglia and nerves.
- The Sensory organs of Rotifera are antennae and eyespots.
- As compared to size the Male one is smaller than females. Parthenogenesis common.
- Female oviparous, there are no larval stages.
- Some examples of Rotifera are; Philodina, Asplanchna, Rotaria.
Order 1: Seisonacea
- The Body of Seisonacea is elongated with a long neck.
- Corona of Seisonacea is small in size.
- It has a Paired gonads.
- These are Found as commensals on crustaceans.
- Some examples of Seisonacea are; Seison.
Order 2: Budelloidea
- The Corona of Budelloidea is usually with 2 trochal discs.
- The Pedal glands have more than two.
- Male degenerate; female contain paired germovitellaria.
- Swimming or creeping form.
- Some examples of Budelloidea are; Rotaria or Rotifera, Philodina, Mniobia.
Order 3: Monogonontea
- These are in Swimming or sessile forms.
- The Male is smaller in size or degenerate.
- Male usually contain one testis.
- Female contain single germovitellaria.
- Some examples of Monogonontea are; Mytilina, Limnias.
Class 4: Gastrotricha (L., gaster= stomach + trichos= hair)
- Gastrotricha is microscopic, which are mainly found in marine or freshwater.
- The Body wall of Gastrotricha with cuticle bearing short, curved, dorsal spines.
- Corona is absent, cilia present on the ventral surface which helps in locomotion.
- The Posterior end is forked and with adhesive tubes and glands for attachments.
- The Body musculature of the Gastrotricha comprises 6 pairs of longitudinal muscles.
- Mouth enclosed by bristles.
- The pharynx of Gastrotricha is triradiate and muscular.
- Excretory system contains 2 protonephridia.
- The Nervous system of Gastrotricha contains a saddle-shaped ganglion and 2 lateral nerves.
- Dioecious or Monoecious; parthenogenetic female occurs.
- The Development process is direct. Young and adults are alike.
- Some examples of Gastrotricha are; Chaetonotus, Macrodasys.
Order 1: Macrodasyoidea
- These are Marine worm-like forms.
- Many adhesive tubes.
- The protonephridia is absent.
- Some examples of Macrodasyoidea are; Macrodasys.
Order 2: Chaetonotoidea
- Most of them are freshwater forms which are found on vegetation.
- They have Adhesive tubes on the tail.
- They contain one pair of protonephridia.
- These are reproduced by parthenogenesis.
- Some examples of Chaetonotoidea are; Chaetonotus, Neodesys.
Class 5. Kinorhyncha (Gr., kineo=more + rhynchos= beak)
- Kinorhyncha is Marine, microscopic worm-like.
- Superficial segmentation of body into 13 or 14 overlapping rings (Zonites).
- The Body surface is with a spiny cuticle but cilia are absent.
- Mouth cone or head protrusible and covered with scales.
- The front part of the ventral surface contains a pair of adhesive tubes.
- The Pseudocoel with fluid carrying amoebocytes.
- A nerve ring with a ventral cord with a ganglion in each Zonite. Eyespots in some.
- The digestive system develops with salivary glands.
- Gonads as a pair of tubular sacs.
- Penial spicules in males.
- Fertilization internal. Metamorphosis with several larval stages.
- The development includes a series of juvenile stages.
- Some examples of Kinorhyncha are; Echinoderes, Pycnophyes.
Order 1: Homalorhagida
- The Head and neck portion of Homalorhagida are both protrusible.
- Some examples of Homalorhagida are; Trachydemus.
Order 2: Cyclorhagida
- The only head ring of Cyclorhagida is protrusible.
- Some examples of Cyclorhagida are; Echinoderes.
- 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.