Ascaris, and especially A. lumbricoides, the common round worm, a convenient type of Nematoda. It has a cylindrical body tapering at both ends; at the anterior is the small head with a triangular mouth. This leads to a muscular oesophagus, continued backwards as a wide tube; this opens at the anus slightly in front of the posterior end of the body. The nervous system consists of a ring round the mouth, and six cords running back through the body. There is neither heart nor vascular system. The full course of development is unknown. The ova are expelled from the body, and after being hatched the embryos gain admittance to the alimentary canal of their future host. They usually remain in the small intestine, but they may enter the stomach and escape through the mouth or perforate the walls of the intestine and even of the abdomen, and cause abscesses. The female is ten to fourteen inches, and the male four to six inches long. The Ascaris lumbricoides is one of the commonest internal parasites in man. Children are more commonly affected than adults, but it is uncertain in what manner the worm is originally introduced into the alimentary canal. The female worm produces a large number of eggs, but these do not develop in the human body, indeed, as a rule, there is no suspicion that anything is wrong with the child that harbours an ascaris, until the worm is expelled. All sorts of symptoms have been ascribed to the presence of ascarides, but as far as the round worm is concerned these are most unreliable. As a rule the ascaris occurs singly, but in some cases a large number may be present and may call for the administration of vermifuge remedies.