Phylum Nemathelminthes – Anatomy, Morphology, Characteristics, Classification

Phylum Nemathelminthes is a group of worm-like animals which was earlier used in classification and also referred to as Aschelminthes. It includes unsegmented, bilaterally symmetrical and triploblastic animals. These organisms are elongated and cylindrical in shape and the body is tapering at both ends. Because of this shape they are commonly called roundworms or threadworms. In modern classification, this phylum is considered obsolete and most of the organisms are now placed under phylum Nematoda.

The body of nemathelminthes possess a pseudocoelom which is a fluid-filled body cavity not fully lined by mesoderm. This pseudocoelom helps in maintaining body shape and also acts as a hydrostatic skeleton helping in locomotion. The body wall is covered by a thick non-cellular cuticle which is protective in nature and is periodically shed during growth. The digestive system is complete and consists of mouth and anus but respiratory and circulatory systems are absent.

The phylum is divided into two main groups based on the presence or absence of phasmids. Phasmida includes forms which possess phasmids and most of them are parasitic in nature. Aphasmida includes those forms which lack phasmids and generally show simpler excretory structures. These organisms are found in soil freshwater and marine habitats. Many species are free living while some are parasitic and cause diseases in plants animals and human beings such as ascariasis hookworm infection and filariasis.

General Characteristics of Nemathelminthes

• They belong to phylum Nemathelminthes which is also known as Aschelminthes or Nematoda in modern classification. These organisms are cosmopolitan in distribution and are found in marine freshwater and terrestrial habitats. They may be free living or parasitic in plants animals and human beings.
• The body is elongated cylindrical and worm like in appearance and tapering at both the ends. The body is unsegmented and shows bilateral symmetry. These animals are triploblastic in nature as they develop from three germ layers namely ectoderm mesoderm and endoderm.
• The body is covered externally by a thick tough non-cellular cuticle which is secreted by the epidermis. This cuticle is protective in nature and is periodically shed during growth. The epidermis may be cellular or syncytial and it contains dorsal and ventral nerve cords.
• Only longitudinal muscles are present beneath the epidermis while circular muscles are absent. Because of this arrangement the movement is jerky and whip like and locomotion occurs by side to side undulations.
• They are pseudocoelomate animals and the body cavity is a pseudocoelom not completely lined by mesoderm. This cavity is filled with fluid and acts as a hydrostatic skeleton helping in maintaining body shape and internal pressure.
• The digestive system is complete and well developed. It consists of mouth pharynx intestine and anus. The pharynx is muscular and helps in sucking or pumping of food while the intestine is simple and non-muscular.
• The excretory system is simple and flame cells are absent. Excretion is carried out by renette cells or a system of excretory canals which open to the exterior through an excretory pore. Nitrogenous waste is mainly excreted in the form of ammonia.
• Respiratory and circulatory systems are absent. Exchange of gases takes place through the general body surface by diffusion and transport of materials occurs through the pseudocoelomic fluid.
• The nervous system consists of a circumpharyngeal nerve ring from which dorsal and ventral nerve cords arise. Sense organs such as papillae and bristles are present. Amphids are present at the anterior end and phasmids may be present at the posterior end in some forms.
• Sexes are separate and this phylum shows clear sexual dimorphism. Males are generally smaller than females and fertilization is internal. Development may be direct or indirect and the life cycle usually includes four larval stages separated by moulting.
• Based on the presence or absence of phasmids the phylum is divided into two groups. Phasmidia possess phasmids and include many parasitic forms. Aphasmidia lack phasmids and are mostly free living in nature.

Morphology of Nemathelminthes

General Body Plan

• Members of phylum Nemathelminthes are bilaterally symmetrical and triploblastic animals.
• The body is unsegmented and vermiform in nature with cylindrical shape tapering at both anterior and posterior ends.
• They show organ system level of organization.
• Size varies greatly ranging from microscopic forms to elongated worms measuring several centimeters or more in length.

Body Wall and Integument

• The body is externally covered by a thick tough and flexible non-cellular cuticle.
• The cuticle is secreted by the underlying epidermis and is periodically shed during growth.
• It may be smooth or provided with ridges rings or bristles in some forms.
• The epidermis (hypodermis) is generally syncytial and forms four longitudinal cords– one dorsal one ventral and two lateral.
• Beneath epidermis a single layer of longitudinal muscles is present.
• Circular muscles are absent in nemathelminthes.
• Due to this muscle arrangement movement is limited to thrashing or sinusoidal motion.

Body Cavity (Pseudocoelom)

• Nemathelminthes are pseudocoelomate animals.
• The pseudocoelom is derived from blastocoel and is not lined by mesoderm.
• It is filled with fluid which maintains high hydrostatic pressure.
• The pseudocoelom acts as hydrostatic skeleton and helps in locomotion.
• Coelomocytes are present in body cavity which helping in transport and storage.

Digestive System

• Digestive system is complete and well developed.
• It consists of straight alimentary canal extending from mouth to anus.
• Mouth is terminal and present at anterior end.
• Mouth may be surrounded by lips bearing sensory papillae or provided with teeth or stylets.
• Mouth opens into muscular pharynx which is triradiate and lined by cuticle.
• Pharynx helps in sucking and pumping food.
• Intestine is long non-muscular tube made up of single layer of epithelial cells.
• The intestine opens into rectum and anus in female or cloaca in male.

Nervous and Sensory System

• Nervous system is simple in structure.
• It consists of circumpharyngeal nerve ring which acts as brain.
• From nerve ring longitudinal nerve cords arise running throughout body.
• Dorsal and ventral nerve cords are well developed.
• Sense organs are poorly developed.
• Papillae and bristles act as tactile receptors.
• Amphids are anterior chemosensory organs present near mouth region.
• Phasmids are posterior sensory organs present near tail region in some forms.

Excretory System

• Flame cells are absent.
• Excretory system may be glandular or tubular type.
• In glandular type one or two renette cells are present opening through excretory pore.
• In tubular type H-shaped canals are present in lateral cords.
• The system mainly helps in osmoregulation and ionic balance.

Reproductive System

• Sexes are separate and sexual dimorphism is present.
• Males are smaller with curved posterior end.
• Male reproductive system usually consists of single testis.
• Copulatory spicules and sometimes bursa are present in male.
• Sperms are amoeboid and lack flagella.
• Females are larger and reproductive system is tubular.
• One or two ovaries are present which opens through vulva.
• Fertilization is internal and development may be direct or indirect.

Respiratory and Circulatory System

• Respiratory and circulatory systems are absent.
• Respiration takes place through general body surface by diffusion.
• Transport of materials occurs through pseudocoelomic fluid.

Anatomy of Nemathelminthes

General Body Plan

• Nemathelminthes are commonly known as roundworms or nematodes. These are unsegmented vermiform animals with elongated cylindrical body tapering at both ends.
• They show bilateral symmetry and are triploblastic animals, where three germ layers is formed during development.
• The body organization may be tissue level or organ system level. It is generally described as tube within tube type body plan.
• Size variation is seen in this group. Some forms are microscopic while parasitic forms like Ascaris lumbricoides female may reach 20–35 cm in length.

Integument (Body Wall)

• The body wall is covered externally by a thick non-cellular cuticle. It is tough flexible and composed mainly of protein with small amount of lipid and carbohydrate.
• The cuticle is secreted by epidermis and it is shed periodically during growth (usually four moults in larval stages).
• Beneath cuticle epidermis or hypodermis is present. It may be cellular or syncytial in nature.
• The epidermis thickens to form four longitudinal cords (one dorsal one ventral and two lateral) which project into body cavity.
• Musculature consists of only longitudinal muscle fibres arranged in four muscle fields. Circular muscles are absent.
• Muscle cells send processes towards nerve cords for innervation which is a characteristic feature.

Skeletal and Support System

• A pseudocoelom is present which is a fluid filled body cavity not lined by mesoderm.
• The pseudocoelomic fluid acts as hydrostatic skeleton and maintains body shape.
• High internal pressure along with cuticle and longitudinal muscles helps in locomotion.
• Special cells called coelomocytes are present in pseudocoelom which are associated with longitudinal cords.

Digestive System

• Digestive system is complete and extends from mouth to anus.
• Mouth is terminal and may be surrounded by lips bearing sensory papillae or teeth.
• Mouth opens into muscular pharynx which helps in sucking and pumping food.
• Pharynx is lined by cuticle and may have bulb or valves in some forms.
• Intestine is a straight non-muscular tube made up of single layer of epithelial cells.
• Rectum is short and cuticle lined which opens outside through anus.

Nervous System and Sense Organs

• The nervous system consists of a circumpharyngeal nerve ring which acts as brain.
• From nerve ring longitudinal nerve cords arise and extend throughout body.
• Dorsal and ventral nerve cords are present within hypodermal cords.
• Sensory organs include papillae and bristles which act as mechanoreceptors.
• Amphids are anterior chemoreceptors present near mouth region.
• Phasmids are posterior glandulo-sensory organs present in some nematodes.

Excretory System

• Flame cells are absent in nemathelminthes.
• Excretory system is either glandular or tubular in nature.
• In Adenophorea it consists of one or two renette cells opening through excretory pore.
• In Secernentea it is H-shaped tubular system with lateral canals.
• The system helps in osmoregulation and removal of metabolic wastes.

Reproductive System

• Nematodes are generally dioecious and sexual dimorphism is present.
• Males are smaller in size and possess curved posterior end.
• Male reproductive system usually has single testis and opens into cloaca.
• Copulatory spicules and sometimes copulatory bursa are present in males.
• Sperm are amoeboid in nature and lack flagella.
• Females generally have two ovaries uterus and vagina opening through vulva on ventral side.
• Fertilization is internal.

Respiratory and Circulatory System

• Respiratory and circulatory systems are absent.
• Respiration occurs through diffusion across body surface.
• Transport of materials is carried out by pseudocoelomic fluid.

Classification of Nemathelminthes

Phylum – Nemathelminthes (Nematoda)

• These are commonly called roundworms.
• The classification is mainly based on presence or absence of caudal sensory organs called phasmids.
• Nature of excretory system is also taken as important basis.

Class I – Phasmidia (Secernentea)

• Phasmids are present.
• Mostly parasitic forms are included.
• Excretory system is tubular type with lateral canals.

Order – Rhabditida

• Free living or parasitic nematodes.
• Cuticle may be smooth or ringed.
• Male possesses copulatory spicules.
• Examples – Rhabditis, Caenorhabditis.

Order – Strongylida

• Endoparasites of vertebrates.
• Lips are absent and buccal capsule is well developed.
• Male has a true copulatory bursa.
• Examples – Strongylus, Ancylostoma.

Order – Oxyurida

• Small to medium sized worms.
• Mouth is surrounded by three or four lips.
• Caudal alae are present.
• Examples – Oxyuris, Enterobius.

Order – Ascaridida

• Large stout nematodes.
• Mouth has three prominent lips.
• Buccal capsule is absent.
• Examples – Ascaris, Toxocara.

Order – Spirurida

• Body is thread like.
• Pharynx does not have a bulb.
• Includes filarial worms.
• Examples – Spirura, Wuchereria, Loa loa.

Order – Camallanida

• Thread like parasites.
• Male lacks copulatory bursa.
• Secondary host is usually a copepod.
• Examples – Camallanus, Dracunculus medinensis.

Order – Tylenchida

• Stylet is present for piercing plant tissues.
• Mostly plant parasitic forms.
• Example – Tylenchus.

Class II – Aphasmidia (Adenophorea)

• Phasmids are absent.
• Mostly free living forms are included.
• Excretory system is glandular or absent.

Order – Enoplida

• Mostly marine forms.
• Cuticle bears bristles.
• Amphids are well developed.
• Example – Anticoma.

Order – Dorylaimida

• Mostly terrestrial forms.
• Cuticle is smooth without bristles.
• Buccal cavity has a protrusible spear.
• Example – Trichodoris.

Order – Mermithida

• Cuticle is smooth.
• Larva is parasitic while adult is free living.
• Amphids are reduced.
• Examples – Mermis, Agamermis.

Order – Chromadorida

• Cuticle may be smooth or ringed.
• Pharynx has posterior bulb.
• Mostly free living forms.
• Example – Paracanthonchus.

Order – Desmoscolecida

• Cuticle is ringed with prominent bristles.
• Four anterior sensory bristles are present.
• Marine forms.
• Example – Desmoscolex.

Order – Monohysterida

• Cuticle smooth or ringed with bristles.
• Amphids are circular.
• Found in marine freshwater or soil.
• Example – Monohystera.

Order – Araeolaimida

• Cuticle is smooth and may bear bristles.
• Amphids are spiral in nature.
• Labial papillae are present.
• Example – Plectus.

Order – Trichinellida (Trichuroida)

• Body is slender and whip like.
• Pharynx is long and thin.
• Mouth lacks lips.
• Examples – Trichinella, Trichuris.

Examples of Nemathelminthes (Aschelminthes)

Parasitic Forms in Human

• Ascaris lumbricoides– It is commonly called giant roundworm of man. The adult worm lives in small intestine and causes ascariasis.
• Enterobius vermicularis– It is known as pinworm or threadworm. It inhabits caecum and colon region and causes irritation and itching around anus.
• Ancylostoma duodenale– It is hookworm parasite of human. It sucks blood from intestinal wall and leads to anemia.
• Necator americanus– Another hookworm of man producing similar symptoms as Ancylostoma.
• Trichuris trichiura– It is called whipworm due to its shape. It lives in large intestine and causes trichuriasis.
• Strongyloides stercoralis– It is a thread like parasite showing autoinfection in human host.

Blood and Tissue Parasites

• Wuchereria bancrofti– It is filarial worm causing elephantiasis. Adult worms live in lymphatic vessels.
• Brugia malayi– It also causes lymphatic filariasis and blocks lymph flow.
• Loa loa– It is called African eye worm. Adult worm migrates in subcutaneous tissue and eye.
• Onchocerca volvulus– It causes river blindness. Microfilariae damage skin and eyes.
• Dracunculus medinensis– It is Guinea worm. Female worm emerges through skin producing painful sores.

Animal Parasites Infecting Human

• Toxocara canis– It is dog roundworm. In human it causes larva migrans.
• Trichinella spiralis– Infection occurs by eating undercooked meat. Larvae encyst in muscles.
• Anisakis species- Marine nematodes transmitted through raw or undercooked fish.

Plant Parasitic Nematodes

• Meloidogyne species- These are root knot nematodes producing galls on roots.
• Heterodera species- These are cyst nematodes affecting crop plants.
• Globodera species- Potato cyst nematodes causing heavy yield loss.
• Bursaphelenchus xylophilus– It is pine wood nematode affecting forest trees.

Free Living Forms

• Caenorhabditis elegans– It is a free living soil nematode widely used in genetic studies.
• Rhabditis– It is free living form found in moist soil and decaying organic matter.
• Panagrellus redivivus– It is found in fermented materials and damp habitats.

Importance of Phylum Nemathelminthes

• It is important in maintaining soil fertility as many free living forms are present in soil and water. These organisms feed on bacteria and fungi and excess nitrogen is released back to soil helping in plant growth.
• It helps in decomposition of organic matter. Dead plants and animals are broken down and nutrients are recycled in terrestrial and aquatic ecosystems.
• It is used as biological indicator of soil health. The presence or absence of certain nematodes indicate nutrient status and disturbance in soil ecosystem.
• It has medical importance as many members are parasitic in human beings. Ascaris lumbricoides Trichuris and hookworms causes malnutrition anemia and weakness in affected persons.
• It causes serious diseases like elephantiasis and river blindness. These diseases is caused by filarial worms such as Wuchereria and Onchocerca which damages lymphatic and eye tissues.
• It has agricultural importance as many species are plant parasites. Root knot nematodes and cyst nematodes damage roots and reduce crop yield.
• It is also beneficial in agriculture. Some nematodes are used as biological control agents to destroy insect pests and reduce use of chemical pesticides.
• It has scientific importance. Caenorhabditis elegans is used as a model organism in genetics development and ageing studies.

FAQ

Q1. What are the phylum Nemathelminthes?
A. Phylum Nemathelminthes is a group of elongated round worm like animals. These animals are unsegmented and cylindrical in shape. The body is tapering at both ends and is covered with a tough cuticle. Many members are parasitic while some are free living forms.

Q2. What is Phylum Nematoda?
A. Phylum Nematoda is the modern scientific name used for Nemathelminthes. It includes roundworms which are bilaterally symmetrical triploblastic and pseudocoelomate animals. It is one of the most abundant animal phylum.

Q3. What are the general characteristics of Phylum Nematoda?
A. The animals are cylindrical and unsegmented. They are triploblastic and bilaterally symmetrical. A pseudocoelom is present. Body is covered with cuticle. Digestive system is complete. Circulatory and respiratory system is absent. Sexes are usually separate.

Q4. What are some examples of Phylum Nematoda?
A. Some common examples are Ascaris lumbricoides Wuchereria bancrofti Enterobius vermicularis Ancylostoma and Trichinella spiralis.

Q5. What is the classification of Nemathelminthes?
A. Phylum Nemathelminthes is commonly divided into two classes–
– Secernentea
– Adenophorea

Q6. Are Nemathelminthes pseudocoelomate?
A. Yes Nemathelminthes are pseudocoelomate animals. The body cavity is not lined completely by mesoderm and is filled with fluid.

Q7. What is the habitat of nematodes?
A. Nematodes are found in soil freshwater marine water plants and animals. Some live freely in moist soil and water while others live as parasites inside host body.

Q8. Are nematodes parasitic or free-living?
A. Nematodes may be parasitic or free living. Many species are parasites of human plants and animals while several species live freely in soil and water.

Q9. What diseases are caused by nematodes?
A. Nematodes cause diseases like ascariasis hookworm disease filariasis elephantiasis enterobiasis and trichinosis.

Q10. Do Nemathelminthes have a complete digestive system?
A. Yes Nemathelminthes have a complete digestive system. Mouth and anus are present and food passes in one direction.

Q11. What is the body plan of Nemathelminthes?
A. The body plan is organ system level. The body is elongated cylindrical unsegmented and bilaterally symmetrical.

Q12. Do nematodes have a circulatory and respiratory system?
A. No circulatory and respiratory system is absent. Exchange of gases and transport of materials occur through body fluid and diffusion.

Q13. What is the difference between Nematoda and Nemathelminthes?
A. Nemathelminthes is the older traditional name. Nematoda is the modern scientific name. Both refer to the same group of roundworms.

Q14. How do nematodes reproduce?
A. Reproduction is sexual. Sexes are separate. Fertilization is internal and development may be direct or indirect.

Q15. What is the significance of the cuticle in nematodes?
A. The cuticle protects the body from digestive enzymes of host and prevents desiccation. It also helps in maintaining body shape and movement.