According to Heller, of the fifty parasites which infest man, twenty-one are found in the intestinal canal. The principal of these are of two kinds, known as tape-worms and round-worms. Three varieties of the former, and five of the latter, together constitute the principal parasites which inhabit the alimentary canal in man. The names of these several varieties are, of tape-worms, toevia solium, toenia saginata or mediocamellata and bothriocephalus latus; of round-worms, ascaris lumbricoides, oxyuris vermicularis or thread worm, tricocephalus dispar or whip worm, anchylostomum duodenale, and trichina spiralis.

The occurrence of parasites in the intestinal canal is much more frequent than is generally supposed, as they often remain for many years undiscovered. It not infrequently happens, on the other hand, that people imagine themselves to be inhabited by worms of various sorts when they are wholly free from parasites of any kind. Less frequently persons become possessed with the idea that they have within their stomachs frogs, lizards, or other reptiles or small animals, a notion which is wholly without foundation, as it would be impossible for one of these creatures to live a day in the stomach or intestines.