When the editors of this memorial volume invited me to write a contribution, I was glad to have an opportunity to express my regard and esteem for Prof. MAX WEBER whom I have the privilege to reckon amongst my teachers. When I was a student and afterwards, when I examined material, collected by the Siboga-Expedition, which he led with so much success and the results of which are so exquisitely published, I had plenty of occasion to appreciate WEBER as man, professor and investigator. My colleague, Prof. J. POELS, director of the Rijksseruminrichting at Rotterdam, provided me with the material for this investigation: 5 worms (2 ♂♂, 3 immature ♀♀), found in nodules of the wall of the intestine of an orang-outan. They proved to belong to Oesophagostomum apiostomum (Willach), a species of which it is not superfluous to give a somewhat detailed description.