Nematoda

Nematoda, commonly known as roundworms, is a phylum of unsegmented, cylindrical worms that are found in almost every environment on Earth—soil, freshwater, marine habitats, and even inside other organisms as parasites.

1.0Introduction

• The body of members is circular in cross section and tapering at both ends, without segmentation. So also called Roundworms. 
• Habit and habitat - Nematodes are found everywhere, they may be free living (aquatic and terrestrial) or parasites in plants and animals. 
• Shape - They have a worm shaped long, cylindrical body with tapering ends. 
• Symmetry - Bilateral. 
• Germ layers – All three primary germ layers are present so animals are triploblastic. 
• Level of organisation - Organ-system level. 

2.0Body Wall

The body wall consists of: 

• Cuticle - Non-living, thick, and resistant to digestive enzymes of the host. 
• Epidermis - Syncytial i.e. a continuous layer of cytoplasm having scattered nuclei. 
• Muscle layer - Only longitudinal muscle fibres present.
• They are pseudocoelomate animals, body cavity is there between body wall and digestive tract which is not lined by mesodermal epithelium i.e. Pseudocoel. 
• The skeleton is absent but fluid pressure in the pseudocoelom maintains body shape. It is called Hydro skeleton.

3.0Digestive System

• The digestive tract is complete and differentiated into mouth, pharynx, intestine and anus. 
• Pharynx is muscular and well developed. It is used to suck the liquid food. Intestine is non-muscular. 

4.0Respiration and Excretion

• Respiration is through the body surface by diffusion. 
• The circulatory system is absent. 
• The excretory system is H-shaped (Renette cell) and consists of excretory canals (Protonephridia) which removes body wastes from the body cavity through excretory pore. 
• They develop from an embryonic "Renette cell". Excretory matter is ammonia.

5.0Nervous System and Reproduction

• The nervous system comprises a nerve ring (Brain) and longitudinal nerve cords.
• Reproductive systems are developed, and sexes are separate (Dioecious) i.e., male and female are distinct. 
• Fertilization is internal and development may be direct or indirect.

6.0Examples

• Ascaris (Roundworm) – Parasite of small intestine. 
• Ancylostoma (Hookworm) – Parasite of small intestine. 
• Wuchereria (Filarial worm) – Viviparous.
• Enterobius (Pinworm or seat worm) – Parasite of large intestine