The Hymenolepis Nana, or dwarf tapeworm, is from 2 to 4.5 cm. long. Its head is rounded, has four suckers and a rostellum that may be protruded or retracted, on which is a single circle of from 22 to 27 hooklets. Are about 200 segments, all of which are broader than long.

The intermediate hostis not definitely known - is possibly the rat. Embryos can develop in the rat into adult forms without an intermediate host. May do so in man.

The adult form alone is found in man. It attaches itself to the intestine by sinking its head deep into the mucous membrane.