having either one or two flagella to the order Protomonadina Blochmann. The genus is thus usually considered to belong in that order and to the family Bodonidae Bütschli, which Kudo (1946) defined as embracing those protomonadinids lacking an undulating membrane and having two flagella originating anteriorly, one of these being directed forwards and the other trailing posteriorly.

Unfortunately for this reasoning the closest affinities of Retortamonas he not with the other genera of the family Bodonidae as understood by Kudo but with Chilomastix Alexeieff Flagellates of the latter genus have three anterior flagella and a cytostomal one, and following conventional usage they must be considered as belonging to the order Polymastigina Blochmann. The members of this order never have less than three flagella and seldom more than eight, although in one family 12 or more occur. We are thus confronted with the necessity of assigning two genera of fundamentally similar structure—particularly as regards the cytostomal complex, which is of a unique type—to different orders merely on the basis of flagellar number.

Alexeieff (1912, 1917) and Mackinnon (1915) drew attention to the similarities between (Embadomonas) = Retortamonas and Chilomastix, the former author (1917) suggesting that they should be assigned to a single family for which he proposed the name Embadomonadidae. Wenrich (1932), having demonstrated that Embadomonas is a synonym for Retortamonas, established the family Retortamonadidae to include the two genera. Although remarking that “the tendency to use flagellar number for the separation of larger groups of flagellates has been much overdone in the past,” Wenrich left the ordinal position of his new family unstated.

The inclusion of the family Retortamonadidae, its members having two or four flagella, in either the Protomonadina or the Polymastigina would of course be no more anomalous than the accepted inclusion of the family Callimastigidae—the members of which have 12 or more flagella—in the Polymastigina. At the same time, an increasing number of protozoologists favour the viewpoint expressed by Kirby and Honigberg (1950) who recognized the artificiality of establishing arbitrary barriers between the orders of the Zoomastigina solely on the basis of the number of flagella. One solution would be the establishing of a separate order expressly for Retortamonas and Chilomastix, and Grassé (1952) has recently proposed that the Retortamonadidae and the family Cochlosomidae Tyzzer, the members of which have six flagella, should be considered together as a distinct order. However much of Grassé's drastically revised classification of the Zoomastigina ultimately comes to be accepted, it is felt that the relationships betwen Retortamonas and Chilomastix are best expressed by the retention of both genera in the order Retortamonadia Grassé, 1952.

Having indicated that Eutrichomastix Kofoid and Swezy is a synonym of Monocercomonas Grassi, Travis (1932) designated M. colubroruorum (Hammerschmidt) as the genotype and established a new genus, Monocercomonoides, for certain flagellates previously included with the former ones. Later authors have mostly followed Travis in applying the generic designation of Monocercomonas to those polymastiginid flagellates having three anterior flagella and a trailing one originating at a single basal granule located in front of the anteriorly positioned nucleus, and a more or less well-defined axostyle which usually projects beyond the tapered posterior extremity of the body.

The characteristics of Monocercomonoides, as exemplified by M. melolonthae (Grassi, 1879), the species selected by Travis (1932) as the genotype, are as follows: The body may be ovoidal, pyriform, spherical or subspherical; there are four flagella which originate in pairs at two anteriorly located basal granules, these being connected with one another by a rhizoplast; the proximal part of one of these flagella may adhere to the pellicle, the flagellum in question trailing posteriorly, a slender axostyle, originating at one of the basal granules, curves around the nucleus and terminates at the posterior tip of the body; in some stained examples a halo of

chromatin granules is evident at one side of the periendosomal area, between the endosome and the nuclear membrane.

Butschli (1884) established the genus Polymastix with P. melolonthae (Grassi, 1879) as the genotype, Grassi (1879, 1881) having earlier designated this flagellate Trichomonas melolonthae. Polymastix is referable to the order Polymastigina, its members having four flagella. These originate in pairs at two basal granules as do those of Monocercomonoides, but the axostyle which runs posteriorly from one of the granules differs from that of the last-named genus in being quite thick near its point of origin and thereafter tapering rapidly, ending in a filament which does not reach the posterior extremity (Grassé, 1952). The body is characteristically more elongate than that of Monocercomonoides, and is usually fusiform or flattened and leaf-like. Perhaps the most striking feature of the genus Polymastix is that bacteria of the genus Fusiformis Hoelling, 1910 (Mycobacteriaceae) are always attached longitudinally to the pellicle, which thus appears ridged.

Wenyon (1926) included all three genera, Polymastix, (Eutrichomastix) = Monocercomonas and (Retortamonas) = Monocercomonoides, within his family Trichomonadidae, the diagnosis of which he gave as follows: “The flagellates belonging to this family are characterized by the possession of a variable number of flagella, a definite cytostome, and a rod-like structure, the axostyle, which arises from the blepharoplasts and passes through the body to its posterior end, through which it usually protrudes In some forms, one of the flagella is directed backwards, and its axoneme may be attached to the border of an undulating membrane. In such cases there is usually a stiff basal fibre, which lies along the line of attachment of the undulating membrane to the body.

Kudo (1946), on the other hand, resurrected the polymastiginid family Polymastigidae Bütschli, 1884, for uninucleate Polymastigina having axial organella but lacking an undulating membrane, cresta, rostellum, and adherent flagella He considered Polymastix, (Eutrichomastix] = Monocercomonas and (Monocercomonas) =Monocercomonoides (in part), to be referable to this family, reserving Trichomonadidae Wenyon for uninucleate Polymastrgina having both axial organella and an undulating membrane.

Kirby (1947) refused to recognize a close relationship between Polymastix, Monocercomonoides and Monocercomonas, and transferred the last-named genus to the family Monocercomonadidae Kirby of his new order Trichomonadida. This order was established for zoomastiginids having from three to six flagella, one of which is typically differentiated as a trailing flagellum, an axostyle and a parabasal body of the Janicki type which originates de novo in relation to the basal granule system. Six families were included within the order, one of which, Monocercomonadidae Kirby, included “Trichomonadida with trailing flagellum free from body surface or adherent, but with no cresta and undulating membrane at the base of which there is a costa”; another, Trichomonadidae Wenyon, was to embrace “Trichomonadida with an undulating membrane and costa.”

Grassé (1952) altered the spelling of Kirby's ordinal name to Trichomonadina, defining this order as including small, medium and large zoomastiginids having an axostyle, from three to six flagella—one of which is recurrent and either free or attached to the body or connected with the latter by means of an undulating membrane—and with a parabasal apparatus always well developed. He relegated Monocercomonadidae Kirby to the status of a sub-family of Trichomonadidae Wenyon, emending the definition of this family to cover those zoomastiginids having a parabasal apparatus and from four to six flagella, one of which is always reflected posteriorly, being either free and trailing or adhering to the body with or without the interposition of an undulating membrane. At the same time, Grassé retained Polymastix and Monocercomonoides in the Polymastigidae Bütschli He regarded this family as of uncertain systematic position and did not assign it to an order, defining it