Phylum Nematoda
Vermiform, tendency towards radial symmetry about longitudinal axis. Pseudocoelomate. Complex, resistant cuticle. Body wall without circular musculature. Longitudinal muscles arranged in 4 regions. Pseudocoel acts as hydrostatic skeleton to oppose contractile force of longitudinal muscles. Muscular gut leading from terminal mouth through muscular esophagus. Nervous system with longitudinal nerve chords. No circulatory system. Excretory system with renette cells (no protonephridia). Cleavage not spiral or radial, but determinate.

Class Adenophorea (Enoplea) (Aphasmida)
Without phasmids. Amphids well developed (except in parasitic taxa). Mostly free living.

Order Trichurida
Anterior end slender. Stichosome esophagus. Both sexes with a single gonad. males with one spicule or none. Eggs with polar plugs. Tissue parasites of birds and mammals.
Trichinella spiralis - larvae in nurse cells

Class Rhabditea
Amphids coiled ventrally. Three esophageal glands.
Subclass Rhabditia (Phasmidea) (Secernentea)
Amphids poorly developed. Phasmids present. Excretory system with one or two lateral canals.

Order Rhabditida
Small worms, generally with 6 lips. Rhabditiform esophagus with anterior corpus, mediam isthmus, and posterior bulb. Spicules equal. Gubernaculum generally present.
Poikilolaimus sp. [1] [2] [3] [4]

Order Strongylida
Strongyliform esophagus swollen posteriorly, but lacking distinct bulb. Male with rayed copulatory bursa.
Necator sp. - hookworm [adult] [copulatory bursa]
Ancylostoma sp. - hookworm

Order Ascaridida
Generally 3 prominent lips. Esophagus variable.
Ascaris sp. - intestinal roundworm [cross section]

Order Oxyurida
Sharply pointed tails. esophagus with prominent, valved posterior bulb. Males with single or no spicule.
Enterobius vermicularis - pinworm [1]

Order Spirurida
Esophagus divided into anterior muscular and posterior glandular portions. No esophageal bulb.
Dirofilaria immitis - dog heartworm [microfilaria]
Folyella sp. - microfilaria in frog blood


Phylum Nematomorpha
Bilateral symmetry. Pseudocoelomate. Complete gut that is degenerate and nonfunctional in adults. Flexible, collagenous cuticle. No circulatory, exretory or gas exchange organs. No circular muscle layer. Adult females free-living, but short-lived. Larvae develop in hemocoel of arthropods.
Gordioidean female


Phylum Rotifera
Pseudocoelomate. Crown of cilia with pre-oral (trochus) and post-oral (cingulum) bands. Complex jaw apparatus (mastax). Body divided into corona, trunk (lorica), and a foot with toes containing adhesive glands.
bdelloid rotifer [1] [2]


Phylum Acanthocephala
No gut. No metamerism. Prominent, eversible, hooked proboscis. Syncitial epidermis with lacunar system. Gonochoristic. All parasitic. Cystacanth larva in arthropod intermediate host.
Macracanthorhynchus hirudinaceus
Pomphorhynchus bulbocollis
Acanthocephalus sp. - male [1] [2] [3]


Phylum Tardigrada
Superficial arthropod affinities. Complete gut with muscular pharynx and stylets for piercing. Body monomeric with 4 pairs of unjointed, claw-bearing legs. Pseudocoelom (?) forming hydrostatic skeleton. Epidermis covered by cuticle containing no chitin. Eutelic. No circulatory or gas exchange system. Gonochoristic (sometimes parthenogenetic).
tardigrade - water bear


Phylum Pentastomida
Complete gut. Terminal anus. Body monomeric with 2 pairs of claw bearing legs. Pseudocoelom (? metameric during development ?) forming hydrostatic skeleton. Chitinous cuticle. Gonochoristic. Development through 3 larval stages. Hematophagous naso-pulmonary parasites of vertebrates. Structural, developmental, and molecular (rRNA) evidence places this taxon within the subclass Branchiura (fish-lice) of the crustacean class Maxillopoda.
pentastome - tongue worm