This group is very closely allied to Sporozoa; for the absence of cytogamy, and of sickle-germs,[[96]] and of the complex spores and cysts of the Neosporidia, are the only absolute distinctions.
6. Mycetozoa (Myxomycetes, Myxogastres)
Sarcodina moving and feeding by pseudopodia, with no skeleton, aggregating more or less completely into complex "fructifications" before forming 1-nucleate resting spores; these may in the first instance liberate flagellate zoospores, which afterwards become amoeboid, or may be amoeboid from the first; zoospores capable of forming hypnocysts from which the contents escape in the original form.
| 1. Aggregation taking place without plastogamy, zoospores amoeboid, with a clear ectosarc | Acrasieae. |
| Copromyxa Zopf; Dictyostelium Brefeld. | |
| 2. Aggregation remaining lax, with merely thread-like connexions, except when encystment is to take place; cytoplasm finely granular throughout; complete fusion of the cytoplasm doubtful | Filoplasmodieae |
| Labyrinthula Cienk.; Chlamydomyxa Archer; Leydenia (?) Schaud. | |
| 3. Plasmodium formation complete, eventuating in the formation of a complex fructification often traversed by elastic, hygroscopic threads, which by their contraction scatter the spores; zoospores usually flagellate at first | Myxomycetes. |
| Fuligo Hall.; Chondrioderma Rostaf.; Didymium Schrad. (Fig. 30). | |
I. The Acrasieae are a small group of saprophytes, often in the most literal sense, though in some cases it has been proved that the actual food is the bacteria of putrefaction. In them, since no cell-division takes place in the fructification, it is certain that the multiplication of the species must be due to the fissions of the amoeboid zoospores, which often have the habit of Amoeba limax (Fig. 1, p. [5]).
II. Filoplasmodieae.—Chlamydomyxa[[97]] is a not uncommon inhabitant of the cells of bog-mosses and bog-pools, and its nutrition may be holophytic, as it contains chromoplasts; but it can also feed amoeba-fashion. Labyrinthula is marine, and in its fructification each of the component cells forms four spores. Leydenia has been found in the fluid of ascitic dropsy, associated with malignant tumour.
III. Myxomycetes.—The fructification in this group is not formed by the mere aggregation of the zoospores, but these fuse by their cytoplasm to form a multinucleate body, the "plasmodium," which, after moving and growing (with nuclear division) for some time like a great multinucleate Reticularian, passes into rest, and develops a fructification by the formation of a complex outer wall; within this the contents, after multiplication of the nuclei, resolve themselves into uninucleate spores, each with its own cyst-wall. The fructifications of this group are often conspicuous, and resemble those of the Gasteromycetous fungi (e.g., the Puffballs), whence they were at first called Myxogastres. De Bary first discovered their true nature in 1859, and ever since they have been claimed by botanist and zoologist alike.
The spore on germination liberates its contents as a minute flagellate, with a single anterior lash and a contractile vacuole (Fig. 30, C). It soon loses the lash, becomes amoeboid, and feeds on bacteria, etc. (Fig. 30, D, E). In this state it can pass into hypnocysts, from which, as from the spores, it emerges as a flagellula. After a time the amoeboids, which may multiply by fission, fuse on meeting, so as to form the plasmodium (Fig. 30, F). This contains numerous nuclei, which multiply as it grows, and numerous contractile vacuoles. When it attains full size it becomes negatively hydrotactic, crawls to a dry place, and resolves itself into the fructification. The external wall, and sometimes a basal support to the fruit, are differentiated from the outer layer of protoplasm; while the nuclei within, after undergoing a final bipartition, concentrate each around an independent portion of plasma, which again is surrounded as a spore by a cyst-wall. Often the maturing plasmodium within the wall of the fruit is traversed by a network of anastomosing tubes filled with liquid, the walls of which become differentiated into membrane like the fruit-wall, and are continuous therewith. As the fruit ripens the liquid dries, and the tubes now form a network of hollow threads, the "capillitium," often with external spiral ridges (Fig. 30, A, B). These are very hygroscopic, and by their expansion and contraction determine the rupture of the fruit-wall and the scattering of the spores.
Fig. 30.—Didymium difforme. A, two sporangia (spg 1 and 2) on a fragment of leaf (l); B, section of sporangium, with ruptured outer layer (a), and threads of capillitium (cp); C, a flagellula with contractile vacuole (c.vac) and nucleus (nu); D, the same after loss of flagellum; b, an ingested bacillus; E, an amoebula; F, conjugation of amoebulae to form a small plasmodium; G, a larger plasmodium accompanied by numerous amoebulae; sp, ingested spores. (After Lister.)