It is usual to classify the Flagellata in four orders: _Euflagellata_, _Dinoflagellata_, _Choanoflagellata_, and _Cystoflagellata_, of which only the _Euflagellata_ are of interest to us. This is a group comprising numerous species, for the further classification of which the number and position of the flagella are utilised. The Euflagellata observed in man belong to the Protomonadina as well as to the Polymastigina. The former possess either only one or two similar flagella, or one principal and one or two accessory flagella. The Polymastigina possess at least three flagella of equal size, or four to eight of unequal size, inserted at different points. An undulating membrane may be present in members of both groups. It must also be pointed out that unicellular organisms with one or several flagella are not always classified with flagellates, for such forms occur in Rhizopods as well as temporarily in the lower plants. In addition, the examination of the flagellates, especially the parasitic species, is very difficult on account of their diminutive size and great activity; thus it happens that certain forms cannot with certainty be included in the group because their description is insufficient. Order. *Polymastigina*, Blochmann. The Polymastigina contains flagellates with three to eight flagella. Some of the Flagellata parasitic in man belong to the Polymastigina, and to two or three genera that are easily distinguishable. Genus. *Trichomonas*, Donné, 1837. The body is generally pyriform, the anterior part usually rounded, the posterior part pointed. There are at the anterior extremity three (? four) equally long flagella that are sometimes matted together. A blepharoplast (kinetic nucleus) and basal granule are present, together with a supporting structure known as an axial filament or axostyle. In addition there is an undulating membrane, bordered by a trailing flagellum, that commences at the anterior extremity and proceeds obliquely backwards. The nucleus, which is vesicular, is situated near the anterior extremity, and behind it are one or more vacuoles, none of which seems to be contractile. These flagellates are parasitic in vertebrate animals, and live chiefly in the intestine. *Trichomonas vaginalis*, Donné. The form of the body is very variable, and is elongate, fusiform or pear-shaped, also amœboid. The length varies between 15 µ and 25 µ, and the breadth between 7 µ and 12 µ. The posterior extremity is drawn out to a point and is about half the length of the remainder of the body. The cuticle is very thin and the body substance finely granular. At the anterior extremity there are three--some say four[36]--flagella of equal length which are frequently united together, at least at the base, and are easily detached.