A “family” name not definable. They comprise a vast number of small forms, most of which live free in fresh water, in soil, or in macerating substances; amongst them there are some which live parasitically on plants, more rarely on animals. They do not exceed 8 mm. in length. The large majority are only 1 to 2 mm., or even 0·5 mm. The uterus is straight. Eggs in the uterus at one time, one to four. Genera very numerous, but many of them insufficiently defined (Anguillula, Anguillulina, Rhabditis, Heterodera, etc.).
Family. Angiostomidæ, Braun, 1895.
Small Nematodes undefined morphologically, but characterized by heterogony, i.e., there is a free-living “rhabditic” generation and a parasitic “filariform” generation which succeed one another (e.g., Angiostomum, Strongyloides, Probstmayria).
Family. Gnathostomidæ.
Cuticle covered totally or partly with cuticular laminæ fringed posteriorly with multiple points. Head subglobular, covered with simple spines. Two spicules. Vulva behind middle of body, parasitic in vertebrates, especially mammals (e.g., Gnathostoma, Tanqua, Rictularia).
Family. Dracunculidæ, Leiper, 1912.
Males very small in proportion to females. Anus absent. Vulva absent (?). Genera: Dracunculus, Icthyonema (in body cavity of eel and other fish).
Family. Filariidæ, Claus, 1885.
Long thread-like Nematodes. Anus present. Œsophagus without bulb. Vulva usually in anterior half of body. Two ovaries. Generally ovoviviparous. Development often requires an intermediate host. This family is at present ill-defined, but has been already subdivided into several sub-families, Filariinæ, Onchocercinæ, Arduenninæ.
Family. Trichinellidæ, Stiles and Crane, 1910.