Phylum Nemathelminthes or Aschelminthes

Phylum Nemathelminthes:

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.

Examples- Ascaris (roundworm), Rhabditis, Ancylostoma (hookworm), Trichuris, Dracunculus, Enterobius, Wuchereria (filaria worm).

Advancements over Platyhelminthes:

• A true organ system level of organization.
• Tube-within-a-tube body plan.
• A complete digestive tract with mouth and anus.
• Presence of pseudocoelom.
• Unisexual nature.
• Development of sexual dimorphism.
• Absence of asexual reproduction.
• Determinate cleavage.