The Filaria Medinensis, or "Guinea-worm," is a very slender round worm about one meter long. The female only is certainly known. It has a circular oral opening with four hook-lets. The tail is sharply pointed.

The greater part of the worm consists of a uterus which contains enormous numbers of embryos. These escape, especially when water is brought in contact with the ulcer from which the worm extrudes. The embryos live in the water, and one view is that a minute crustacean is the intermediate host.

This worm is found in the tropics and occurs in the subcutaneous tissues, particularly of the feet and hands.