Plegepod Protozoa in which the organism during the chief part of its life is provided with one or more vibratile flagella as organs of locomotion and alimentation. A mouth and oesophagus sometimes present; nutrition holozoic, saprophytic, or, when chromatophores are present, holophytic. Reproduction by simple binary fission in the motile condition; or in the encysted state, when it may be simple or continued, giving rise to a few or to numerous small individuals. Conjugation frequent, the conjugating individuals being either similar or dissimilar. Solitary or colonial. Freshwater, marine, in decaying infusions; rarely parasitic.

The number of forms included in the Mastigophora is great. The class may be subdivided into four sub-classes, the Flagellata, Chonano-fiagellata, Dinoflagellata and Cystoflagellata, the mutual relations of which are uncertain.

1 Flagellata. The members of this sub-class are characterised by the possession of one or more vibratile flagella. The body is usually more or less elongated and monaxial, sometimes asymmetrical, sometimes bilaterally symmetrical either in shape or arrangement of parts; contractile in some instances, in others more or less amoeboid. A well-developed cuticle may be present, or a membranous or gelatinous envelope.

The amoeboid condition is most strongly developed in the Monadina Rhizomastigina. The pseudopodia are either simple or branched digitiform processes (Mastigamoeba), or radiant and Heliozooid in aspect (Dimorpha, Ciliophrys, ? Actinomonas), coexisting with one or two flagella, except in the two last-named. They may be partially retracted in Mastigamoeba, wholly in Ciliophrys and Dimorpha. An amoeboid condition, especially of the posterior region of the body, is apt to occur in some other Flagellata, e. g. Cercomonas. The flagella vary in length, thickness, relative size, number, and disposition. When only one or two they are restricted to the anterior pole of the body. And in Heteromastigoda one of them - the pulsellum (gubernaculum of James Clark) - is large and bent backwards beneath the body. An undulatory membrane is found in some parasites, e. g. Trypanosoma, in which it extends along one side of the body. A thick layer of exoplasm is present in Mastigamoeba, and the anterior extremity of the body is in some cases formed of clear protoplasm free from chromatophores, etc. As a rule, however, no distinction into layers is recognisable.

A circulation of the protoplasm has been seldom observed; its vacuolation is rare and perhaps pathological1.

A cuticle is absent in the simpler Monadina, Iso- and Hetero-masti-goda, hence the possibility of an amoeboid condition; but when the outline of the body is fixed it is present. It is best developed in some Euglenoidea where it is hyaline, homogeneous, obliquely or longitudinally striated, and when thick of considerable resistance. Special cuticular structures occur as peduncles or envelopes either gelatinous or membranous. A peduncle secreted by the hinder end of the body, solid, stiff and branched, is seen in Dendromonas and Anthophysa among Monadina, secreted by the anterior end by which the animal fixes itself in Chlo-rangium, and the Euglenid Colacium where it becomes branched1. A gelatinous envelope is formed in many instances under conditions unfavourable to the organism; it occurs normally in Mastigamoeba verrucosa, and as a common investment to the colonies of Uroglena, Spongomonas, Syncrypta and Colacium, or as a branched tubular structure, the individuals inhabiting the ends of the tubes, in Cladomonas and Rhipidodendron. The membranous envelope has two forms.

In the first, it may invest the body loosely as in Haematococcus and Volvox, but more usually closely as in the Euglenid Trachelomonas, the Chlamydomadina and Volvocina. In the two latter it is pierced by holes for the passage of the two flagella, and in Volvocina the colony has a special gelatinoid (?) investment as well. The second form is more or less cup-like, homogeneous, transparent, colourless or brownish, with a large cavity and wide mouth; it is seen in some Monads, e. g. Codonoeca, Bicosoeca, Dinobryon some Isomastigoda, e. g. Epipyxis. Dinobryon is free, Epipyxis fixed by the end of the shell, the others by a peduncle. In Bicosoeca, Dinobryon, and Epipyxis a delicate contractile basal thread connects the animal to the cup. Coccomonas among Chlamydomonads has a shell which readily breaks into two parts, Phacotus a lens-like bivalved shell. The envelope is composed of cellulose in Chlamydomonas and Haematococcus; in others of an unknown substance which dissolves spontaneously in water to set free the products of fission.

1Mastigamoeba aspera has frequently delicate hair-like processes at its posterior end, such as occur in Amoeba, etc. Its surface is covered with very small rod-like bodies of uncertain nature, whether processes of the body or a coat of adherent Bacteria, very similar to what are seen in Deinamoeba.

The colourless Flagellata are for the most part holozoic. Food may be captured by means of pseudopodia, or by a process formed for the purpose containing a vacuole, as in many Monadina and probably some other small forms, or it enters by a distinct aperture near the base of the flagella. A mouth in the same position with an oesophageal tube is present in Euglenoids, some Heteromastigoda, in Chilomonas, and Crypto-monas. In many Euglenoids, however, it serves as an outlet for the contractile vacuole and is not used for purposes of nutrition 2. Where chromatophores are present, the organism is holophytic: some colourless forms related to or derived from coloured are saprophytic3. Food vacuoles occur when the food is ingested by means of a vacuole, rarely in other instances. Indigestible remnants are expelled at the hinder end of the body as a rule, rarely anteriorly as in Bicosoeca, or in amoeboid forms at any spot. A contractile vacuole is seldom absent. It is usually single, rarely double, more rarely multiple.