It is a phylum of unsegmented triploblastic pseudocoelomic cylindrical or thread-like worms which are covered by a body wall having cuticle and epidermis. Phylum Nemathelminthes includes roundworms and threadworms. It includes more than 15,000 species. This group was established by Grobben in 1910.
Characteristics of Phylum Nemathelminthes:
Most of the animals of nemathelminthes are parasitic. A few are free living. The free-living forms are found in terrestrial, marine and freshwater habitats.
These have organ-system organization and bilateral symmetry.
These are triploblastic animals i.e. with three germ layers- ectoderm, mesoderm and endoderm.
Pseudocoelm cavity is found between the body wall and the digestive tract.
The skin is covered by a tough resistant cuticle. It is followed by snycytial (multinucleated) epidermis and muscle layer.
These are the first animals to have a straight and complete alimentary canal. The pharynx is muscular and tri-radiate. The intestine is non-muscular. So these have a tube-within-a-tube plan.
The respiratory system is absent. Respiration is aerobic in free-living forms. Parasites usually have anaerobic respiration. The exchange of gases occurs through the general body surface.
Excretion occurs from the body cavity through excretory pores.
The blood vascular system is absent. Transporation of materials may take place with the help of hydrolymph or psuedocoelomic fluid.
The animals are commonly unisexual. Sexual dimorphism is also found in most species. Males are smaller with penial spicules for copulation. Separate gonopore is absent in them, i.e., they have a cloaca. A separate genital pore is present in females. Fertilization is internal. The animals are oviparous and ovoviviparous.