Leptodiscus medusoides has the form of a shallow concavo-convex disc .6 - 1.5 mm. in diameter. It swims vigorously by contraction of the disc into a bell-like form, the contraction including the whole disc; a portion only or two opposite portions of its circumference, however, may be thus affected.
The disc thins away towards its periphery, and a circular line of granules divides it on its concave aspect into a central and peripheral portion. There are two tubular depressions on the convex aspect, one wide, the other narrow, both inclined obliquely towards the centre of the concave side where lies a central mass of protoplasm inclosing the nucleus. A bundle of parallel protoplasmic fibres extends to the base of the wide depression. The narrow depression, however, reaches the central mass and gives exit to a fine flagellum. There is said to be a distinct cuticular membrane, thicker on the convex side where it is interrupted by the apertures of the two depressions above-mentioned. Branching and anastomosing cords of protoplasm radiate from the central mass to the periphery on the concave aspect of the disc, and the gaps between the larger branches are filled by a finer network. Branching cords extend from the central mass and its central radial outrunners to the convex aspect of the disc; the corresponding extensions arising towards the periphery are unbranched. The ends of these cords are attached each to a small rod-like body placed vertically to the surface, possibly an internal projection of the cuticular membrane. The intervals between the cords are filled by a clear jelly.
Fine radial fibres, supposed to be contractile, stretch from the circular line of granules (supra) to the centre. Supposed fission-forms have been described by Hertwig, but nothing is known as to the reproduction of the organism1.
The Mastigophora are classified by Butschli as follows: - I. Flagellata: animal provided with one or more vibratile flagella; subdivisible into four orders.
I. Monadina-. small or very small in size, and simple in structure; naked and very frequently more or less amoeboid; sometimes furnished, however, with a theca; for the most part colourless; chromatophores rare; a single large anterior flagellum and sometimes 1-2 small accessory flagella; special oral aperture absent or situate at the base of the flagellum, but never prolonged into a well-developed oesophagus; includes five families, Rhizomastigina (ante, p. 841); Cercomonadina, e.g. Oikomonas; Codonoecina; Bicoeana, e.g. Bicosoeca, Poteriodendron; Hetero-monadina, e.g. Monas, Dendromonas, Anthofihysa, Dinobryon, Uroglena.
2. Euglenoidina: in general large and well-developed; with one, rarely two.
1 Biitschli thinks (Protozoa, pp. 1078-9) that the Gymnodinium pseadonoctiluca, described by Pouchet, Journal de l'Anat. et Physiol. xxi, pp. 71-5, is possibly a spore-form of Leptodiscus.
Flagella; monaxial or a little asymmetrical; sometimes extremely contractile; a cuticle; colourless or green; an oral aperture at or surrounding the base of the flagellum; an oesophagus with a contractile vacuole close to it, often provided with a reservoir; includes seven families - Coelomonadina, e.g. Chronulina; Eugle-nina; Chloropeltina, e.g. Phacus; Menoidina; Paranemina; Petalomonadina; Astasiina.
3. Heteromastigoda: of variable size and colourless; two flagella at least, one smaller and directed forwards, the other large and trailing backwards; an oral spot and in large species a distinct mouth and pharynx; holozoic; includes two families - Bodonina, e. g. Bodo, Dallingeria; Anisonemina.
4. Isomastigoda: of small or medium size; monaxial, rarely bilateral or asymmetrical; anterior end with two, four, rarely five similar flagella, arising as a rule near one another; includes ten families - Amphimonadina; Spongomonadina, - Spongomonas, Clado?nonas, Rhipidodendron; Chrysomonadina, e. g. Synura, Syn-crypta; Chlamydomonadina, e. g. Polytoma, Chlamydomonas, Haematococcus; Volvo-cina - Gonium, Stephanosphaera, Pandorina, Eudorina, Volvox1; Tetramitina; Polymastigina; Trepomonadina; Cryptomonadina, e. g. Chilomonas, Cryptomonas; Lophomonadina.
II. Choanoflagellata: a single flagellum surrounded by a protoplasmic collar: includes two orders.
I. Phalansterina s. Gelatinigera: collar short, narrow, constant in shape; a gelatinous envelope to each individual or to every two individuals; colonial; colony disc-like, or erect and dichotomously branched; Phalansterium.
2. Craspedomanadina: collar large, funnel-shaped, of changeable shape; solitary or colonial; divisible into Codonosiginae, e. g. Monosiga, Codostga, Protero-spongia, and Salpingoecina with a delicate test of variable shape, Sa/pingoeca, Polyoeca.