In the sexual condition this lives as a minute worm (male one-seventeenth, female one-eighth to one-sixth of an inch long), the so-called “intestinal trichina,” in the gut of human beings and carnivorous mammals. In this situation it brings forth living young (some fifteen hundred in number), and the multiplication is enhanced by the fact that of the very numerous trichinæ found in the gut there are, on the average, about twelve females to every male. The young trichinæ bore into the walls of the gut, and are carried by the blood-stream into the muscles. At first each little worm extends itself longitudinally in a muscle fibre, but, as it gets larger, curves and becomes spirally twisted, so that the sheath (sarcolemma) of the fibre is bulged more and more outwards. The growth of the young trichina is very rapid—an individual only ¹⁄₂₅₀ of an inch long before leaving the gut may attain the length of one twenty-fifth of an inch in fourteen days. It then grows no more, and the sheath of the muscle fibre gradually thickens to form a capsule or cyst. Later on, in about a year, lime is deposited in the capsule. The “muscle trichinæ” retain their vitality for several years. If now the host of the muscle trichina is devoured by any other mammal, the capsule is digested in the stomach of the latter, and the once more liberated worm becomes a sexually mature “intestinal trichina” in a few days. The pig is the ordinary host of trichinæ, which (without the intervention of any other host) can be permanently parasitic in this domestic animal, since pigs often devour swine’s flesh. They often enough devour even their own young, and, especially in large slaughterhouses, swine are often fed with the offal of their companions. The trichinæ also spread through the body of any living being (men, rats) which eats trichinous pork. The trichinæ thus introduced cause, in the human subject, a dangerous or even fatal disease known as _trichinosis_. The size of this book does not permit me to describe the symptoms of this complaint in man. Pigs suffer much less from the parasite; they may even contain an enormous number of trichinæ in their muscles without being noticeably ill. Symptoms of disease, however, often appear more or less clearly. A short time after eating the trichinous meat the appetite of the pigs is bad; they appear in bad spirits; the tail straightens out, and they often remain standing with bent limbs and arched backs. There may be, in addition, pains in the abdomen, diarrhœa, and fever. Later on, when the trichinæ have settled down in the muscles, the pigs suffer from stiffness in the legs and tenderness in the loins; they often cry out in pain. After this, however, health and appetite may return, and they can be fattened. Trichinous swine therefore appear quite sound, and are slaughtered in due course. The trichinæ in the pig are found most abundantly in the diaphragm, the masticatory, eye-, and other muscles of the head, also in