The cyst varies in thickness and consistence; fission within it is binary, sometimes continued to 4-8 or even more, and the products of fission may grow, encyst, divide, without assuming a free state; consequently clusters of encysted forms are frequently met with resembling the algal Pleurococcus. Microgonidia are sometimes thus produced.
The products of fission sometimes remain associated, or even organically connected, to form colonies. In Dinobryon and Poteriodendron the young individual attaches itself to the edge of the parental cup.
The Dendromonads are grouped singly or in numbers at the ends of a branching solid stem; the Spongomonads Cladomonas and Rhipidodendron at the ends of branching tubes, whilst the individuals of Uroglena, Synura, and Spongomonas are enveloped in a jelly-like mass, and in the first two may or may not be connected at its centre. In Syncrypta the envelopes of the zooids are united at a common centre; in the Volvocina they are contained by a common investment. The number of individuals in a colony varies from a few, e.g. 4-16 in Gonium, 16 in Pandorina, 32 in Eudorina, to as many as 12,000 in Volvox globator. Each individual may undergo repeated binary fission within its own envelope to form a new colony which is set free; or, as in Volvox, this power is restricted to certain large non-flagellate individuals, the parthenogonidia, always few in number, e. g. eight in V. globator. Every individual in Volvox is connected to each of the six individuals immediately surrounding it by a protoplasmic thread.
The colonies of Eudorina and Volvox are hollow.
Permanent conjugation has been observed in some Monads, in Bodo, in Chlamydomads (except Haematococcus and Coccomonas), and in Volvocina. In small species of Monads and Bodo several individuals may become amoeboid, then non-flagellate, and fuse into a plasmodium, which encysts and undergoes repeated fission. In other instances a partially amoeboid stage may supervene, the individuals fusing in pairs, at first only by their hinder extremities; or if not amoeboid, they may differ in size, points of structure, and life-history. The contents of the cyst may give rise to minute flagellate individuals (zoospores), or to a more or less granular fluid, the granules of which are said to grow into flagellate individuals, e.g. in Dallingeria. The Chlamydomonads have as a rule small individuals or microgonidia, which either fuse one with another in pairs or a microgonidium with an ordinary individual (macrogonidium). The two unite by their anterior ends, their envelopes dissolving at this point. The flagella and stigmata are eventually lost and the zygote or zygospore becomes rounded and encysts, a formation of haematochrome taking place in green-coloured species.
Further development generally takes place only after the zygote has been dried for a time, or, in Polytoma, has been transferred to a fresh infusion. Two or four individuals are then formed by fission. In Carteria, however, a Pleurococcoid condition has been observed (ante, p. 844). As to Volvocina there appears to be a conjugation of microgonidia in both Gonium and Stephanosphaera. In Pandorina each individual gives origin to a colony of eight. The parent colony, now motionless, sinks; its coat and the coats of the original individuals slowly soften and gelatinise. The cells in each colony of eight acquire a transitory common coat and cilia. They are eventually set free, and conjugate in pairs. A difference of size in the conjugating individuals is said to be noticeable. This difference is much more marked in Eudorina and Volvox. One individual, the macrogonidium (female), is large and ovoid, and in Volvox devoid of cilia; the other (male) is elongate, biflagellate, of a yellowish tint, grouped in colonies which are produced by fission from a single large cell, and contain 16, 32, 64 individuals in Eudorina, or even 128 in some species of Volvox. The two sorts of individuals are contained in different colonies in Eudorina, Volvox Carteri, and sometimes in V. minor; or in V. globator in the same colony.
V. minor is said to develope first one, then the other sort. Every individual in a given colony of Eudorina is modified; in Volvox only a certain number which resemble at first the parthenogonidia above mentioned (p. 845), but are as a rule much more numerous. The male colony is set free as a whole in Eudorina and V. minor. After conjugation a double cyst is formed by the zygote, which now developes haematochrome, and is set free by the death and resolution of the original colony. In Volvox ( V. minor) it appears to rest during the winter; in spring its contents undergo fission and are set free as a young colony of about 500 cells.
Hypnocysts are formed by all Flagellata when the conditions of life become unfavourable. They have thick and often multiple walls. In the Chlamydomonads the cyst is formed within the envelope, which is then lost, and in Volvocina (? Volvox itself) every individual of a colony encysts at the same time. Resting green forms turn red. In a few instances encyst-ation of the nucleus, plus a small quantity only of protoplasm, has been observed, the rest of the protoplasm perishing1. Fission of the contents of the hypnocyst occurs in Stephanosphaera and Haematococcus, in the latter sometimes giving rise to Pleurococcoid crusts.
The great majority of known Flagellata inhabit fresh water; a few are marine, and a few parasitic in Vertebrata, Arthropoda, especially In-secta and Myriopoda, in some Mollusca and Nematodes, inhabiting the digestion tract, blood-vascular system, or the epidermis like Bodo necator, so destructive to young trout. They sometimes swarm in great numbers imparting a colour to water, green, e.g. Euglena viridis, Haemaio-coccus, red, e.g. Euglena sanguinea, or even to snow, like the red Haemato-coccus, or the yellowish-green Chlamydomonas flavovirens. The contents of the reproductive cysts of some saprophytic species are capable of withstanding a great heat without being killed, e. g. an Oikomonas s6i°F. in a moist condition, 300°F. in a dry. An artificial adaptation of saprophytic species to life at higher and higher temperatures has been successfully carried out. The green Flagellata evolve oxygen under the influence of sun-light, and how far they can endure darkness and live appears a moot point. They are sensitive to light, being attracted or repelled by differing intensities and perhaps also at different stages of their life.