_Tylenchus tritici_ Need. is the cause of "ear-cockles" in corn. These take the form of brown or purple galls, which replace the grains of corn, and which contain hundreds of minute Nematodes. In these galls they are motionless, and are capable of surviving in dryness for at least twenty years; but when moistened,—for instance, by the gall falling on damp earth,—they resume their vitality and make their way to the young wheat plants, and then, wriggling up the leaves and stems, find their way to the ear. Here they pair, and producing a gall-like growth in the flower, lay numerous eggs, from which arise the Nematodes of the ear-cockle. [Illustration: FIG. 77.—A, _a_, Female _Heterodera schachtii_ Schmidt, breaking through the epidermis of a root; the head is still embedded in the parenchyma of the root: B, _a_, larvae boring their way into a root; _b_, larva of the immobile kind surrounded by the old skin, living as an ectoparasite on the outside of the root. (From Strubell.)] _Heterodera schachtii_[193] Schmidt, is the cause of the "beet {156}sickness," and forms galls or swellings on the roots of many plants, in England especially on the roots of tomatoes and cucumbers. The free larvae live in the earth and make their way into the smaller rootlets; here the female larvae shed their skin, lose their characteristic Nematode form, and become citron-shaped (Fig. 78, D). The male larvae undergo a change, and after a period of rest cast their skin and, leaving the rootlet, seek out the females. The female does not undergo this second ecdysis, but its generative organs grow and mature in what is practically a larval stage. The embryos develop within the body of the mother, and, escaping through the uterus, ultimately cause her death. They then make their way into the earth. The cycle of the development takes but four or five weeks, so that, as in the case of _Tylenchus devastatrix_, there are several broods in a year; _T. tritici_, on the other hand, has but one. [Illustration: FIG. 78.—A, Male _Heterodera schachtii_ strongly magnified; _a_, head lappets; _b_, mouth cavity; _c_, spine; _d_, muscle of spine; _e_, gland; _f_, oesophagus; _g_, bulb; _h_, nerve-ring; _i_, excretory pore; _j_, intestine; _k_, testis; _l_, intestine; _m_, muscles moving spicule; _n_, spicule: B, first motile larva: C, second immovable parasitic larva casting its skin: D, a female with one half of the body-wall taken away to show the coiling generative organs; _a_, boring apparatus; _b_, oesophageal bulb; _c_, excretory pore; _d_, alimentary canal; _e_, anus; _f_, ovary: E, a male shortly before casting its larval skin.]