Superphylum Aschelminthes was proposed by Grobben in 1910 and was later supported by Hyman in 1951. It includes pseudocoelomate or blastocoelomate phyla, which possess a false coelom or pseudocoelom. Majority of them are free living, freshwater or marine animals. They possess only longitudinal muscles, and digestive system is in a tube plan. They do not have respiratory and circulatory systems. They lay eggs covered by a chitinous shell. The superphylum includes the following phyla which are not closely related to each other and hence do not form a homogenous group.
Phylum NEMATODA (=NEMATHELMINTHES) Parasitic and free living worms.
Phylum NEMATHELMINTHES (=NEMATODA)
The phylum was created by Gegenbaur in 1859 for unsegmented round worms but now phylum Nematoda is commonly used instead. The phylum includes bilaterally symmetrical triploblastic and pseudocoelomate animals with organ system of organization. Excretion involves a giant excretory cell called Renette cell. They have tube-within-tube type of body plan.
It includes round worms that are both free living and parasitic, containing about 28,000 species, of which 16,000 are parasitic. They are slender and unsegmented worms having bilateral symmetry and their skin consists of a syncytium covered by a thick layer of cuticle. They possess only longitudinal muscles. Most of the species are dioecious and lay shelled eggs.
Subclass APHASMIDA (=ADENOPHOREA)
Phasmids generally absent and amphids are of various types and always post-labial. Deirids are not found. Excretory system is simple, non-tubular or sometimes absent. There are three caudal adhesive glands commonly opening through a spinneret near the tail tip. Male generally has two testes and caudal alae are rare. There are sensory papillae in the cephalic region and along the body. They are marine, freshwater or terrestrial worms. Ex. Mermis; Monohystera.
Subclass PHASMIDA (=SECERNENTEA)
Phasmids are present, generally posterior. Amphids with pore-like or slit-like apertures and always labial. Deirids present near the nerve ring. No hypodermal glands. Excretory system tubular. No caudal adhesive glands. Male generally with a single testis. Caudal alae are common. Sensory papillae cephalic only and caudal papillae present in males. Almost exclusively terrestrial worms, rarely freshwater or marine. Ex. Ascaris, Rhabditis, Dracunculus, Wuchereria.