Small as they are, Flagellata are not exempt from the attacks of parasites, e. g. the colonies of Volvox afford shelter and food to two species of the Rotiferan Notommata and to the Proteomyxan Pseudospora; and many of them are infested by species of the fungus Chytridium, which live and multiply within them; hence erroneous views as to their reproduction.

1 See Fisch, Z. W. Z. xlii. 1885, p. 72 (Chromulina), p. 110 (Arhabdomonas), p. 114 (Monas guttula).

(ii.) Choanoflagellata (Craspedornonadina, s. Cyclomastiges). This sub-class, found in both fresh and salt water attached to various objects, especially the stalks of Vorticellids, is distinguished by two special features: (I) the possession of a single fine flagellum, implanted at one extremity of the body, within an area surrounded by (2) a clear funnel-shaped collar of protoplasm.

The body is ovate, contractile, and sometimes very liable to assume an amoeboid condition (Proterospongid); it is minute, and never exceeds 1/1000 in. size, of a bluish tint, and consists of a colourless protoplasm charged with more or fewer granules. The collar is expansile and retractile, either short, narrow, and smaller at its aperture than at its base (pha/ansterina), or as is more general, the reverse (Craspedomonadind). It is extremely sensitive to movements in the water. It appears functionally to be connected with nutrition (?in Phalansterina). The motion of the flagellum brings floating particles from behind forwards, and when they impinge against the collar they adhere to its outer surface. They are then either carried by a flow of protoplasm upward to its edge, over it, and downwards to the central area where they are engulfed (Kent); or the direction of the flow is the reverse, and the particles when they reach the base of the collar are either taken up by a vacuole formed temporarily (Butschli) or carried into the body by a separation of a portion of the protoplasm of the collar (Entz). The food may be lodged within food vacuoles, and consists principally of Bacteria. Faecal residues are expelled within the area surrounded by the collar.

The nucleus is small, round, vesicular, placed near the base of the flagellum. Contractile vacuoles are always present at the base of the body, usually two in number, but as many as five have been seen.

The animal may be sessile or stalked, and in the latter case the stalk may be simple and support a single individual, or carry a number of individuals attached to its apex by short contractile threads (Codosiga) or it may be branched (Codonocladiurn). The simple forms are generally social. Hirmidium (=Desmarella) occurs in floating colonies in which the individuals are united side by side in band-like rows, or irregularly; Astro-siga is similarly free, but the members of the colony are united by their posterior extremities into stellate clusters. Some are furnished with a gelatinous investment, either clear (Proterospongia) or somewhat granular, with the outlines of the constituent tubes visible, and either incrusting or erect and branching (Phalansterium). In the Salpingoecina each individual is lodged in a sessile or stalked theca; it may be free, or attached to the bottom of the theca by a delicate filament of protoplasm. The theca itself is rarely thick and soft, usually thin, firm and colourless, ovate, vasiform, horn-like, or, if it incloses the collar as well as the body, expanded terminally. In Polyoeca the peduncle of one theca is attached to the margin of another.

The theca may be detached, and then the animal swims about with it, the flagellum, as is always the case in a free Choano-flagellate, pointing backwards in the line of motion.

Reproduction is by binary fission, transverse in Phalansterium, and most commonly in Sa/pingoeca, longitudinal in others, so far as is known. In the former case the collar and flagellum are retracted, and the detached portion may be amoebiform, but eventually becomes a free-swimming monad with a single flagellum until it settles down and forms a collar, etc. In the latter case, as observed in Codosiga botrytis, the flagellum is withdrawn, the body divides as does the collar, after however the protrusion of two flagella, one for each new individual. Fissiparous individuals of large size have been noticed in Codosiga cymosa. The occurrence of conjugation is uncertain. Retraction of the flagellum and collar, assumption of a spherical shape and formation of a cyst membrane has been observed, in many instances accompanied by division of the contents of the cyst into two, or as a rule into a number of spherical bodies which are set free as uniflagellate monads. After swimming about the latter become fixed and develope a collar, etc.

(iii.) Dinoflagellata (Ciliojlagellata, Peridineay Arthrodele Flagellata in part). This sub-class is characterised by a more or less pronounced bilateral aspect of body, coupled with a certain degree of asymmetry; by the presence, save in a few cases, of an envelope or cuticle of a substance akin to cellulose, though not identical with it1; by having two flagella implanted close to one another, one directed parallel to the long axis of the body, the other usually transverse to it. The majority of genera are marine and widely distributed; a few are marine and freshwater, whilst one genus, the Peridinidan Hemidinium, is exclusively freshwater. Marine forms are said to occur in Alpine lakes, but some doubt attaches to the identification of the species. The sub-class is divisible into the Adinida and Dinifera.

In the Adinida the body is elongated, compressed laterally, and covered by a porous bivalved cuticle; the two flagella are implanted at one of its extremities, hence the anterior. One of them is extended forwards, whilst the other is wound transversely round its base. The Dinifera vary much in shape. The body is as a rule marked by two furrows, a transverse or equatorial furrow, sometimes spiral to a greater or less degree, and a ventral longitudinal furrow which unites the ventral ends of the transverse furrow and extends backwards behind them, and often to a variable degree in front of them. The two flagella are situated at the spot where the furrows meet. The longitudinal flagellum extends backwards along the corresponding furrow; the transverse is lodged in the transverse furrow, and encircles the body from left to right round the dorsal aspect. It appears to be the principal agent in locomotion, and the short wave-like undulations which pass along it from base to apex were formerly interpreted as indicating a ciliation of the furrow; hence the name Cilioflagellata. The longitudinal flagellum is in some instances very contractile; the animal probably glides upon it as do some Flagellata. There are three families of Dinifera.

1 According to Klebs the membrane in Hemidinium and Glenodinium pulvisculus is formed of a substance not akin to cellulose.