The identification of the species of the genus Trichuris has been much simplified since the appearance of a paper by Chandler in 1930 in which he pointed out that the prepuce-like expansion surrounding the extruded spicule has very little diagnostic value because of its extreme variability. The length and appearance of the spicule is useful only when used in conjunction with other characters for this is a variable structure. However, the six species with which he was concerned could readily be distinguished by certain internal structures which seem to exhibit great constancy and reliability. These structures are the length of the cloacal tube and its diverticulum for the head of the spicule, the length and shape of the ejaculatory duct and the nature of the folding of the vas dcferens and of the testis. Using these characters and other minor ones, he was able to give useful descriptions of the males of Trichuris trichiura, T. vulpis, T. ovis, T. minuta and T. leporis and to describe a new species T. tenuis.