The body of aschelminthes is circular in cross-section, hence they are called roundworms. They may be free-living, aquatic or terrestrial, or parasitic in plants and animals. Roundworms have an organ-system level of body organisation. They are bilaterally symmetrical, triploblastic and pseudocoelomate animals. The alimentary canal is complete, with a well-developed muscular pharynx. An excretory tube removes body wastes from the body cavity through the excretory pore. Sexes are separate, so the animals are dioecious, that is, males and females are distinct, and often the females are longer than the males. Fertilisation is internal, and development may be direct, where the young ones resemble the adult, or indirect. Examples include Ascaris (roundworm), Wuchereria (filaria worm) and Ancylostoma (hookworm). In the broad classification, aschelminthes are the first group in which the pseudocoelom appears, a body cavity not lined by mesoderm because the mesoderm is present only as scattered pouches between ectoderm and endoderm. They are also the first group with a complete digestive system having both mouth and anus. Understanding this contrast with flatworms, which are acoelomate and have an incomplete digestive tract, helps in answering many comparison-based questions in NEET Biology.