The family of =Nematoda= possesses numerous parasitic members. The common thread worm (_Oxyuris Vermicularis_) is one of the most common of these. The female is 1∕3 to 1∕2 inch in length, and inhabits chiefly the lower bowel. The ova, which are from 1∕490 to 1∕1100 inch in diameter, often gain access to drinking water, or are carried by flies, or received on salads, etc. The injection of salt and water into the bowel, and treatment tending to improve the general health, are the proper remedies. The round worm (_Ascaris Lumbricoides_) inhabits chiefly the small intestine; hence medicines for its removal require to be given by the mouth. The female is from 10 to 14 inches long; the ova, of which each female discharges on an average 160,000 daily, are from 1∕340 to 1∕440 inch in diameter. The whip-worm (_Trichocephalus Dispar_) is a smaller nematode, which is rarely met with in this country. The _Dochmius Duodenalis_ is met with chiefly in Italy and Egypt. It sucks the blood in the intestine, causing dangerous anæmia. The =Strongylus Gigas= is chiefly found in the kidneys of the ox, dog, etc., and is very rare in man. It resembles a very large round worm. In the kidney it produces severe disorders. How it gets there is not known. The _Trichina Spiralis_ has been already described (page 23). The _Filaria Dracunculus_ (Guinea Worm) seems to gain access into the stomach along with water, or possibly in some cases, by perforating the skin. It burrows among the tissues, especially of the legs, and attains a length of several feet. It causes large boils and sores, and through these the eggs escape and pass into water. Here the embryo which has escaped from the egg meets with a fresh water crustacean (_cyclops_), enters its body, undergoes larval growth, and is swallowed with its host by a man, in whom it burrows and undergoes its next stage of life. The embryos of three species of =Filaria= infest the blood of man, chiefly in the tropics. One embryonic species is found in the blood of infested patients by day, one by night, and one during both day and night. The length of the embryos varies from 1∕75 to 1∕125 inch, and its width from 1∕3000 to 1∕3500 inch. The night embryo, which is the most common, is produced by the _Filaria Bancrofti_. This adult worm infests the lymphatic system of man, sometimes reaching a length of three to four inches. Its embryos may obstruct lymphatic vessels, causing obstruction of the flow of chyle (hence originates _chyluria_), and _elephantiasis_, in which enormous swelling of the legs and other parts ensues.