Extremely small, thin-skinned; lay only a few relatively large eggs, which develop very quickly. With few exceptions either earth eelworms (_i.e._ live in decomposing organic matter or humus soil) or as plant parasites leading to characteristic diseases of wild and cultivated forms. All these parasitic forms have a “mouth spine.” This structure, found in the mouth cavity, is very sharp and pointed in front, and can be worked forwards and backwards so as to penetrate the cell walls of plants. A mouth spine is not only found in the species which infest plant tissues (several species of _Tylenchus_ and _Aphelenchus_, as well as all the known species of _Heterodera_), but also in those forms living free in the earth which bore into the exterior of plant roots (the remaining species of _Tylenchus_ and _Aphelenchus_, _Dorylaimus_, etc.). In all cases an eelworm, devoid of a spine, is not a plant parasite. I will only deal with those parasites by which well-known destructive diseases of cultivated plants are caused. These belong to the genera _Tylenchus_ and _Heterodera_. The first remain eel-shaped throughout life, but the adult females of the latter swell out considerably and become lemon- or pear-shaped. The =Stem Eelworm= (_Tylenchus devastatrix_). Length one-thirtieth to one-fifteenth of an inch long, usually of intermediate size; the two sexes of approximately equal length. Live and reproduce in various cultivated plants (_e.g._, rye, oats, stored onions, hyacinths, buckwheat, potatoes, clover, fuller’s teasel) and wild plants (_e.g._, _Poa annua_, _Anthoxanthum odoratum_, _Dipsacus silvestris_, _Polygonum persicaria_), but not to the same extent in all. It must also be added that eelworms of which the progenitors have developed for a considerable number of generations in the same plant, are not easily transferred to another kind of plant, or at any rate do not multiply vigorously there. Eelworms, of which the ancestors have lived for many years exclusively in rye, or alternately in rye and buckwheat, do not readily pass over to seedling onions, and first only reproduce in them to a small extent. The eelworms live only in stems, branches, and leaves, never in roots. In the places where they have penetrated the plant tissues an abnormal growth in thickness of the parts involved takes place, while the growth in length is either much diminished or even entirely stopped. Also the chlorophyll disappears sooner or later from the attacked spots, and rapid death generally ensues. Since only those parts of an organ which are inhabited by a large number of eelworms swell much, it is obvious that cracks are often developed in the stems and leaves concerned. It is further easily seen that the species and constitution of the infested plants will have an important influence on the progress of the disease caused by the eelworms. But since several generations of this parasite succeed one