Phylum - Aschelminthes (Nematoda) (Old NCERT) - NEET PDF Download

The organisms belonging to the phylum Nematoda are also known as “roundworms”. 

• There are 28000 species of Nematoda identified till date. They are unsegmented vermiform animals. The epidermis has dorsal and ventral nerve cords.
• The Nematodes present in the soil feed on the bacteria, fungi, and other nematodes, and play an important role in nutrient recycling. They also attack the insects and control the pests. However, they cause severe damage to plants. They feed on the plant roots and reduce the nutrient uptake and stress tolerance of the plant.

Nematoda

• A spadeful of soil contains numerous Nematodes. They cause diseases such as Ascariasis, Trichuriasis, Hookworm, Enterobiasis, Filariasis, and Angiostrongyliasis in humans.

Characteristics of Nematoda

• Grobben gave the name = Aschelminthes 
• Gagenbaur named it as as Phylum Nemathelminthes. The phylum includes roundworms.
• Aquatic, terrestrial, free-living or parasites.
• Animals of this phylum are Cylindrical, tapering at both ends without segmentation.
• Bilaterally Symmetrical - Triploblastic, Organ-system level and having tube within tube plan.
• The anterior end does not show a distinct head.
• No appendage.
• Body wall consists of:
  Cuticle - Firm, non-living, resistant to digestive enzymes of the host.
  Moult (changed) many times during the growth period (usually four times).
  Epidermis - Syncytial
• Muscle layer - Longitudinal fibres divided into four quadrants. (each with 150 muscle cells)
• Body cavity is Pseudocoel (developed from blastocoel) and contains Pseudocoelomic fluid.
• High fluid pressure in the pseudocoelom maintains body shape. It is called Hydroskeleton.
• Digestive tract is complete and differentiated into mouth, pharynx, intestine & Anus.
  The mouth is surrounded by 3 - lips having sensory papillae and amphids. The pharynx is muscular. It is used to suck food. The intestine is non-muscular.
• Respiration is through the body surface by diffusion.
• The circulatory system is undeveloped
• Nervous system comprises of circum pharyngeal ring (Brain).
• Sense organs like Papillae (Tangoreceptors), Amphids (chemoreceptor) are present on the lip.
  Paired unicellular Phasmids (chemoreceptor) are found near the hind end of the body.
• Excretory system is H-shaped formed by single cell called Renette cell.
  The excretory substance is ammonia.
• The reproductive system is developed and the sexes are generally separate.
• Sexual dimorphism is present.
• Male is smaller than female and curved from its caudal end.  
• Male has pineal spicule for copulation. The genital tract joins the digestive tract to form the cloaca.
• Female is larger than male and straight. Genital tract opens independently. Female lays numerous eggs with Chitinous shells.
• Fertilization is internal and development is mostly direct.    
• Cleavage is a Holoblsatic spiral and determinate type.
• Number of cells are fixed from larva to adult such development is known as Eutely.

Additional Information

Classification of Nematoda

Class–Aphasmidia

• Members of this class lack phasmid.
• Many types of amphids are found.
• One pair of excretory canal are present.
• Caudal adhesive glands are found.

Ex. - Enoplus, Desmoscolex, Trilobus, Lapillaria

Class–Phasmidia

• Phasmid is present.
• Caudal adhesive glands are not found.

Ex – Ascaris*

• It is a parasite of the small intestine.
• It is also called roundworm.
• Rhabditiform larva exists in the life cycle of Ascaris.
• Ascaris leads to a disease called Ascariasis.
• Infective stage of Ascaris to man is embryonated egg with 2nd stage juvenile.
• Trichinella spiralis (Trichinia worm) occurs in voluntary muscles, causes trichinosis. 

Ex– Trichuris trichiura

• It is also called whipworm.
• It is a parasite of the large intestine of man.
• No larval stage in the life cycle.
• It leads to anaemia and Diarrhoea.               

Ex-Enterobius

• It is also called pinworm or seatworm.
• It is a parasite of the human intestine.
• It causes enterobiasis.
• No intermediate host.

Ex-Ancyclostoma

• Cause - Ancyclostomiasis.
• It is also called hookworm.
• Its larva penetrates through the sole of the person.
• Live in the intestine and suck tissue fluid.             

Ex-Wuchereria

• It is also filaria worm.
• Its life cycle gets completed in two hosts. Man is the primary host, while female culex/Aedes mosquito is the secondary host.
• It leads to a disease called Elephantiasis.
• Adult worm blocks lymph duct which leads to swelling of limbs.
• Females are ovoviviparous. Female lays juvenile microfilaria.
• Infective stage to man is microfilariae.

Ex- Dracunculus

• They are also called Guinea worms and also referred to as “fiery serpent”.
• They are digenetic, man is the primary host while water flea (Cyclops) is the secondary host.
• Completely eradicated from India (eye worm)

Ex- Loa-loa (eye worm)

• Found in subdermal connective tissues. Sometimes they pass across eyeball.
• Tabanas fly act as vector.