The Ascaris Lumbricoides is the commonest intestinal parasite in man. The female may be 40 cm. long and 5 to 6 mm. thick; the male 25 cm. long and 2 to 4 mm. thick.

The body is brownish and has four longitudinal ridges extending the entire length. The head has three rounded lips, between which is the mouth.

The alimentary canal runs through the entire worm to an opening on the hinder abdominal surface.

The sexual organs occupy the posterior half of the body, the sexual opening being at the junction of its anterior and middle thirds. The uterus is double, thread-like, and twisted; may contain millions of eggs, which are from 0.05 to 0.06 mm. long, are oval, and are covered by a layer of clear albuminous matter which preserves them for a long time after being extruded.

An intermediate host is not needed, the eggs developing in the human intestine. They may occur singly or in numbers. Are found in the small intestine, but may migrate, entering the gall-ducts, the stomach, the esophagus, and even the larynx or nasal cavities.

May obstruct the intestine, or set up inflammation with perforation and abscess formation. Also cause obscure nervous symptoms.