FIG. 134.—The Beet Eelworm (_Heterodera Schachtii_): 1, A beet root with adult females attached (natural size); 2, a lateral rootlet of beet which Eelworm larvæ (_a_) are penetrating (enlarged); 3, a lateral rootlet of beet with swelling (_a_) of the outer skin caused by the excessive development of larvæ which have previously entered it, and which have now become nearly mature females (enlarged); 4, a just-hatched larva; 5, a larva which has penetrated a root and swollen up into a club-shaped form; 6–8, development of the male; 9, a nearly adult and, 10, an adult female. The following letters have reference to Figs. 4–10: _a_, mouth spine,—_b_, sucking stomach (pharynx),—_c_, _d_, intestine,—_e_, _f_, rectum,—_f_, anus,—_g_, excretory organ; _h_ (in Figs. 4–7), rudiments of the sexual organs in the undeveloped larvæ; _h_ (in Fig. 8), testis; _i_ (in Figs. 7 and 8), copulatory spicules of the male; _k_ (Figs. 9 and 10), ovary; _l_ (Figs. 9 and 10), female sexual opening; _m_, nerve ring; _w__{1} (Figs. 6–8), larval skin; _w__{2} (Figs. 6–8), new skin which the developing male forms within the larval skin. Figs. 4 and 5 are magnified more than Figs. 6–10. The =Beet Eelworm= (_Heterodera Schachtii_)[4] is the cause of the “beet sickness” of the soil. The course of the disease is as follows. At the end of July light-coloured patches are found here and there among the normally developed beet. The leaves are weak and limp, and the outer ones especially get yellow, spotted, and die off. Later on the inner leaves die as well, after which the top of the beet becomes black and the whole root gradually decays. In less severe cases the beet may recover towards autumn and develop new heart leaves, but the roots remain small and the crop is poor, often being only one-third of its normal amount. Kühn has proved by infection experiments that the sole cause of beet sickness is a nematode, of which the life history is as follows. The female is found fixed to the branches of the root; it is citron-shaped, about one twenty-fifth of an inch long (Fig. 134, 1 and 10), and contains on an average three hundred and fifty eggs. Some few of these, together with a jelly-like substance making up an “egg-sac,” may pass out to the exterior, but the large majority develop in the body of the female, which ultimately becomes a mere sac enclosing the eel-like larvæ. The female is killed by the process. The liberated larva (Fig. 134, 4) seeks out a root (about one twenty-fifth of inch thick), and bores into it. Here it lives as a parasite, causing the disease of the attacked beet plant. The larva quickly sheds its old skin, assumes a thicker form (Fig. 134, 5), ceases to move, and gradually causes the outer skin of the root to bulge out externally (Fig. 134, 3, _a_). The distinction between the sexes now rapidly makes its appearance. A thick motionless larva, destined to