The confusion which exists among parasitologists, protozoologists and medical men concerning the validity of species of Nyctotherus reported to be parasitic in man prompted this investigation. It is hoped that a better understanding will be the result. For the past six years, the writer has had a special interest in Nyctotherus from cold-blooded hosts (Wichterman (1934, 1937); Geiman and Wichterman (1937)). For the present study, all the original papers were carefully examined and for the sake of accuracy, illustrations were photographed and are herein represented as faithfully as possible for evaluation.
The genus Nyctotherus was introduced by Leidy (1849) when he discovered and described N. velox from the intestine of the milliped Spirobolus (Julus) marginatus. Dobell (1932) however, believes that Leeuwenhoek was the first to observe and sketch a specimen of Nyctotherus which came from the rectum of a frog. The commonest species is probably Nyctotherus cordiformis (fig. 1) described by Stein (1859–1878) from the frog.