Phylum Aschelminthes: Definition, Characteristics, Classification and Examples

Phylum Aschelminthes

Phylum Aschelminthes (Gr., askes = cavity ; helmins = worm) is a group of bilaterally symmetrical, unsegmented, and pseudocoelomate animals. They are typically characterized by a cylindrical or thread-like body covered with a tough, flexible cuticle. The phylum includes organisms that are predominantly aquatic or parasitic in nature, such as nematodes (roundworms).

General characteristics of Phylum Aschelminthes

• Bilaterally symmetrical, unsegmented worms.
• Triploblastic and pseudocoelomate animals with organ-system grade of body organization.
• Body usually worm-like cylindrical or flattened.
• Cuticle usually present and cilia absent.
• Alimentary canal straight and complete with mouth and anus.
• Respiratory and circulatory systems absent.
• Excretory system includes a system of canals, protonephridia present in some forms for osmoregulation.
• Nervous system is simple.
• Sense-organs are in the form of pits, papillae, bristles and eye spots.
• Asexual reproduction does not occur.
• Sexes separate, i.e., dioecious and reproductive organs relatively simple.

Classification of Phylum Aschelminthes

* Phylum Aschelminthes is a heterogenous assemblage of different animals. Here only characteristics and classification of Class Nematoda will be described.

CLASS 1. NEMATODA

1. Aquatic or terrestrial, free-living or parasitic, elongated roundworms.
2. Body wall with cuticle, cellular or syncytial epidermis and longitudinal muscles in four bands.
3. No cilia, no circulatory and respiratory systems.
4. Digestive system complete with muscular pharynx and glands.
5. Excretory system of glandular organs or canals or both.
6. Dioecious; male smaller than female.
7. Fertilization internal.

Order 1. Enploidea

1. Cuticle not ringed, often with cuticular bristles.
2. Anterior end with six labial papillae.
3. 10 to 12 bristles in one or two circlets.
4. Oesophagus is divided into two parts, anterior muscular and posterior glandular.
5. A pair of cephalic slits and cyathiform amphids.
6. Examples: Enoplus, Anticoma, Synonchus.

Order 2. Dorylaimoidea

1. Cuticle smooth without bristles.
2. Anterior end with two circlets of papillae of 6 and 10 each.
3. Buccal cavity armed with a protrusible spear.
4. Rear part of pharynx enlarged.
5. Amphids cyathiform.
6. Examples: Dorylaimus, Tylencholaimus, Actinolaimus.

Order 3. Mermithoidea

1. Smooth, filiform nematodes, parasitic in invertebrates (mainly insects) in juvenile stages or free-living as adults in soil or water.
2. Head sense-organs reduced to papillae, usually six.
3. Pharynx long, blind; intestine also blind, altered into a food storing organ.
4. Amphids cyathiform or reduced.
5. Examples: Mermis, Paramermis, Aproctonema.

Order 4. Chromadoroidea

1. Aquatic nematodes with smooth or ringed cuticle.
2. Usually cuticle heavily ornamented with bristles, knobs, punctations, etc.
3. Pharynx with a posterior bulb.
4. Amphids spiral or derivable from a spiral.
5. Examples: Paracytholaimus, Paracanthonchus.

Order 5. Araeolaimoidea

1. Cuticle smooth, sometimes with bristles.
2. Labial papillae present.
3. Anterior end usually with four cephalic bristles.
4. Amphids spiral or loop-like.
5. Examples: Plectus, Wilsonema, Odontophora.

Order 6. Monhysteroidea

1. Cuticle smooth or slightly ringed, often provided with bristles.
2. Anterior end with four, six or eight bristles, or multiples thereof.
3. Amphids circular.
4. Examples: Cylindrolaimus, Siphonolaimus.

Order 7. Desmoscaleoidea

1. Cuticle heavily ringed, with prominent bristles throughout or in restricted areas.
2. Anterior end with 4 bristles.
3. Amphids crescentic.
4. Marine nematodes.
5. Examples: Desmoscolex, Tricoma, Greeffiella.

Order 8. Rhabditoidea or Anguiluloidea

1. Free-living or parasitic nematodes with ringed or smooth cuticle.
2. Pharynx with posterior bulb and also usually with swelling anterior to the nerve ring.
3. Caudal glands absent.
4. Amphids small pockets.
5. Examples: Rhabditis, Diploscapter, Diplogaster.

Order 9. Rhabdiasoidea

1. Smooth nematodes with definite pharyngeal bulb.
2. Hermaphrodite; parthenogenesis also occurs.
3. Parasitic stages in animals.
4. Free living stages may develop into males and females.
5. Examples: Rhabdias, Entomelas.

Order 10. Oxyuroidea

1. Pharynx with a posterior bulb, usually valvulated.
2. Females with a long pointed tail; terminal parts of female system often heavily muscularized.
3. Males with one spicule or two equal spicules.
4. Usually caudal alae forming a cuticular bursa.
5. Example: Oxyuris or Enterobius.

Order 11. Ascaroidea

1. Mouth surrounded by three prominent lips.
2. Pharynx without posterior bulb or if present it is not valvulated.
3. Pharynx or intestine or both often with caeca.
4. Buccal capsule absent.
5. Tail of female blunt; male without caudal alae and possesses two equal or nearly equal spicules.
6. Example: Ascaris.

Order 12. Strongyloidea

1. Mouth without conspicuous lips but often with leaf crowns.
2. Pharynx without bulb.
3. Females usually with ovijectors.
4. Males with copulatory bursa supported by muscular rays; typically 13 in number.
5. Examples: Ancylostoma, Strongylus, Necator.

Order 13. Spiruroidea

1. Mouth usually with two lateral lips; some-times 4 or 6 small ones.
2. Pharynx without bulb, anteriorly muscular and posteriorly glandular.
3. Males without bursa spicules unequal and dissimilar.
4. Examples: Thelazia, Rictularia, Oxyspirura.

Order 14. Dracunculoidea

1. Without definite lips or cuticularized buccal capsule.
2. Pharynx as in Spiruroidea.
3. Vulva near or behind the middle of body; usually not functional.
4. Males with equal filiform spicules; bursa wanting.
5. Examples: Dracunculus, Philometra, Micropleura.

Order 15. Filarioidea

1. Filiform worms without lips.
2. Buccal capsule small or rudimentary.
3. Pharynx as in Spiruroidea, pharyngeal bulb wanting.
4. Vulva anterior in female.
5. Bursa wanting, spicules as in Spiruroidea.
6. Examples: Wuchereria, Loa loa.

Order 16. Trichuroidea or Trichinelloidea

1. Body filiform anteriorly.
2. Mouth without lips.
3. Pharynx slender.
4. Provided with a cirrus, spicule if present one only.
5. Examples: Trichinella, Trichuris.

Order 17. Dioctophymoidea

1. Moderate to very long-sized nematodes.
2. Mouth without lips surrounded by 6, 12 or 18 papillae.
3. Pharynx elongated without bulb.
4. Males with muscular bursa but without rays. Examples:
5. Dictyophyme, Hystrichis, Eustronglides.