Rhizopod Protozoa (?) with coated spores s. chlamydospores, giving origin to amoebulae, which may or may not become temporarily zoospores or Jlagellulae. The amoebulae grow and multiply by binary fission; they then either collect together into a mass or aggregation-plasmodium, every Amoeba in which is converted into a chlamydospore; or they fuse together and form a multinucleate fusion-plasmodium, which continues to grow, but eventually encysts and is converted into chlamydospores. Inhabiting moist and decaying wood, vegetable dSris or dung.
The spore of a Mycetozoan is a minute spherical or oval body, becoming concavo-convex if dried, inclosed in a membrane or episporium. This membrane may be smooth or ornamented in various ways, generally colourless, violet or violet-brown in the Calcariaceae, yellow or red in Trichiaceae, etc. The contained protoplasm is dense and variably granular; it lodges a single nucleus, or two if the spore is of unusual size. When the spore is suitably moistened and begins to germinate, two contractile vacuoles make their appearance, and the episporium bursts as the protoplasm swells. As soon as the latter is set free, it undergoes in Ceratium (i.e. Exospored) binary fission repeated three times, the products of fission remaining together until the process is complete. But as a rule it commences life as an amoebula, which moves actively about and secretes a pellucid coat. Except in the Sorophora this amoebula developes a cilium and assumes a temporary zoospore condition, and alternate phases, amoeboid and flagellate, may recur. The amoebula or Myxamoeba multiplies by binary fission, but when it does so it becomes spherical and loses its contractile vacuoles.
If food is wanting, or other conditions of life unfavourable, e.g. a lack of moisture, the Myxamoeba contracts into a sphere and developes a more or less firm membrane. This resting phase is known as the microcyst. When the conditions of life again become favourable, the encysted protoplasm perforates the membrane at some one spot and creeps out.
After a period of growth the Myxamoebae begin to gather together, and they either retain their individuality or fuse. The first of these two alternatives obtains in the Sorophora. The collected Myxamoebae constitute an aggregation- or pseudo-plasmodium. The heap they form assumes a determinate shape and is known as a sorus. The Amoebae which make it up either collect by degrees, and are transformed as they collect into chlamydospores, as in Copromyxa, or they collect quickly. Certain of them are then disposed in one or more linear series, enlarge, become vacuolate, acquire a cellulose membrane, and form a stalk or peduncle upon which the remaining cells are grouped. These cells are each converted into a chlamydospore.
Union of the Myxamoebae in a fusion-plasmodium is characteristic of the two other sub-groups of Mycetozoa, the Exo- and Endo-sporea. Two or three unite first of all; the large Myxamoeba thus formed serves as a focus of attraction, and by the repeated addition and fusion of other Myxamoebae, it eventually becomes a fusion-plasmodium, or simply a Plasmodium. The plasmodia vary in size; the majority are either just visible or just invisible to the naked eye; those of the Physareae, however, cover surfaces of iin. to Ift. They have as a rule the shape of a branched tree or network of moving protoplasm. Lycogala epidendron is an exception; it has the shape of cylindrical, varicose, slightly branched threads. Movement takes place by the extension of protoplasmic processes in one direction and their withdrawal in other quarters. If they should happen to be extended simultaneously and energetically in different directions, the result is a rending of the plasmodium into a number of separate portions. These portions, like the portions formed by artificial section of a plasmodium, behave in all ways as so many plasmodia. From a structural point of view, a plasmodium consists of a clear, dense superficial protoplasm, with a granular fluid central portion or medulla.
The granules are of various kinds; some of them in the Calcariaceae, e. g. Euligo, or the (flowers of tan/ consist of Calcium carbonate. Yellow, red, violet, or brown pigment is met with in some instances. Vacuoles are present, and are sometimes contractile. The nuclei of the Myxamoebae persist. The surface is clothed by a soft, sticky, pellucid coat or hypothallus, to which earth and some other foreign bodies adhere, and which may be left behind as a trail in the onward movement of the plasmodium. If the conditions of life become unfavourable before maturity is attained, the plasmodium passes into a resting-phase known as the macrocyst, or thick-walled cyst. It breaks up into portions of unequal size, which become globular and develope in succession two membranes. The encysted protoplasm is only set free after a prolonged soaking of the macrocysts. But if the plasmodium is ripe for sporulation, its resting-phase, the sclerotium, has a different character. All processes are withdrawn by degrees, whilst foreign bodies are extruded and the granules of the protoplasm evenly distributed. The mass becomes rounded, and is resolved into minute globular or polyhedral cells; it becomes in consistence wax-like and finally brittle.
Its cells have one or more vacuoles, and in two instances a cellulose membrane. They are bound together by an outer homogeneous layer covering the mass. If placed in water the sclerotium swells; its cells acquire one or more contractile vacuoles; their membrane, if present, is dissolved; the liberated portions of protoplasm become amoeboid and fuse to reconstitute the plasmodium.
As soon as a plasmodium has attained its definitive growth, it comes to the surface of the decaying wood, etc, whatever it inhabits, and proceeds to form spores. This it does, either on the outer surface of a sporospore s. conidiophore, as in the Exosporea, or as in the Endosporea, within one or more sporocysts or sporangia, which are derived in most instances from a single plasmodium, but in a few, e. g. Fuliigo, from a number of united plasmodia, or, as they are termed, an aethalium.