Elongated, thread-shaped or spindle-shaped, unsegmented. A gut, terminating in a ventral anus, is present. Outer investment of the skin smooth, often weakly ringed or striated. The muscle layer underlying the true skin is interrupted here and there, where the skin itself stretches further inwards. In this way are distinguished the _lateral lines_ or _fields_ (Fig. 128, _a_), which run right along the sides of the body, dividing it into a dorsal and a ventral region. There are often present as well two smaller but similar _dorsal_ and _ventral lines_, respectively situated above and below (Fig. 128, _b_). Mouth usually surrounded by lip-like folds, but more or less distinct jaws may also be present. The anus either lies at the apex of the tapering posterior end of the body (_trichina_), or further forward, on the ventral surface. Among the Nematodes are included a number of relatively small species, living free in the earth, and sucking plant parts, while other forms fairly closely related live parasitically in plants, and often cause very serious plant diseases (_e.g._ Beet Eelworm); a larger number of species are parasitic in animals. The free forms and those parasitic in plants lay relatively few, but very large, eggs, although, in several species, increase is furthered by a succession of many generations in the year. The Nematodes parasitic in animals lay very numerous eggs, even several millions. Harmful species are known from the following families: 1. =Palisade Worms= (_Strongylidæ_), 2. =Whip Worms= (_Trichotrachelidæ_), 3. =Slender Thread Worms= (_Filaridæ_), 4. the =Round Worms= (_Ascaridæ_), 5. =Eelworms= (_Anguillulidæ_). I will successively treat of the harmful forms, but since many species belonging to different families infest the digestive organs of the different domesticated animals, causing similar symptoms of disease which may be dealt with in much the same way, the following general sketch is first given:—