_Body_, 8 to 10 mm. long, by 4 to 5 mm. broad, by 4 mm. thick; tapers anteriorly to 2·5 mm. Caudal extremity bluntly rounded, venter surrounded by an elevated ridge, surface with transverse ridges best defined ventrally. Genital pore median about one-quarter of body length from anterior end at level of suctorial pouches. Acetabulum 1 mm. in diameter, margin projecting, aperture small. Mouth in a groove with digitate papillæ. Oral sucker very large, one-fifth of length of body, with a pair of irregularly globular pouches. Œsophagus somewhat longer than sucker. Excretory pore at the level of the acetabular aperture. The vesicle extends from the plane of the transverse vitelline ducts to centre of acetabulum. [Illustration: FIG. 136.--_Watsonius watsoni_: ventral view. 4/1. (After Shipley.)] _Male Organs_.--Testes deeply notched adjoining one another. Vesicula seminalis much coiled and dilated, pars musculosa not coiled. Pars prostatica (?) dilated, ejaculatory duct long and narrow, opening on a papilla; genital atrium papillated. _Female Organs._--Ovary dorso-posterior of posterior testis. Shell gland dorsal to ovary. Vitellaria ventral and lateral to gut cæca extending from gut fork to equator of acetabulum. Uterus dorsal to testes, ductus hermaphroditicus absent. Laurer’s canal opens in dorso-median line slightly behind anterior border of sucker. [Illustration: FIG. 137.--_Watsonius watsoni_: ventral projection composed from a series of transverse sections. _o.s._, oral sucker; _s.p._, suctorial pouch; _ga._, genital atrium; _d.e._, ejaculatory duct; _es._, œsophagus; _e.g._, œsophageal ganglion; _p.p._, pars prostatica; _p.m._, pars musculosa; _i._, gut; _ut._, uterus; _v.e._, vas efferens; _v.e.s._, left vas efferens; _v.e.d._, right vas efferens; _v.g._, vitellarium; _t._, testes; _ov._, ovary; _s.g._, shell gland; _t.vd._, transverse vitelline duct. (After Stiles and Goldberger.)] _Eggs._--123 µ to 133 µ long by 75 µ to 80 µ broad. _Habitat._--Jejunum and duodenum of man, German West Africa. The parasite has only been found once in man. The patient, a negro from German West Africa, died at Zola, Northern Nigeria. The symptoms were persistent watery diarrhœa without blood or mucus. The parasites were also passed in the stools. It occurs also in monkeys. Family. *Gastrodisciidæ*. Genus. *Gastrodiscus*, Lkt., 1877. Acetabulum small, caudal and ventral margin raised, aperture relatively large. Genital pore without sucker. Excretory pore post-vesicular, posterior to opening of Laurer’s canal. Œsophagus with muscular thickening; cæca not wavy, long, end post-equatorial and post-testicular. _Male Genitalia._--Testes two, branched pre-ovarial. _Female genitalia._--Ovary and shell gland post-testicular. Vitellaria extracæcal; uterus intercæcal; Laurer’s canal entirely prevesicular. _Type._--_Gastrodiscus ægyptiacus_, Cobbold, 1876. *Gastrodiscus hominis*, Lewis and McConnell, 1876.[267]