Widely distributed. Aquatic or Terrestrial. Parasitic or Free living.
Body elongated, cylindrical, unsegmented, worm-like, bilaterally symmetrical.
Triploblastic body. Organ system grade of organisation.
Body wall with thick resistant cuticle, cellular or syncytial epidermis and only longitudinal muscle fibres in 4 bands.
True coelom absent. Persistent blastocoel or pseudocoel present not lined by mesoderm.
No cilia. No circulatory or respiratory system.
Digestive system complete with anus. Muscular pharynx and non-muscular intestine.
Excretory system of glandular organs or canals or both. Flame cells are absent.
Nervous system with anterior and posterior nerves.
Sense organs poorly developed.
Sexual dimorphism. Male smaller than female. No asexual reproduction.
Fertilisation internal.
Representative Organism: Ascaris lumbricoides