Brood-division, either by resolution in the multinucleate species, or preceded by multiple nuclear division in the habitually 1-nucleate, though presumably a necessary incident in the life-history of every species, has only been seen, or at least thoroughly worked out, in a few cases, where it is usually preceded by encystment, and mostly by the extrusion into the cyst of any undigested matter.[[71]]

In Trichosphaerium (Fig. 9) the cycle described by Schaudinn is very complex, and may be divided into two phases, which we may term the A and the B subcycles. The members of the A cycle are distinguished by the gelatinous investment being armed with radial spicules, which are absent from the B form. The close of the A cycle is marked by the large multinucleate body resolving itself into amoeboid zoospores (3), which escape from the gelatinous test, and develop into the large multinucleate adults of the B form. These, like the A form, may reproduce by fission or budding. At the term of growth, however, they retract their pseudopodia, expel the excreta, and multiply their nuclei by mitosis (7). Then the body is resolved into minute 2-flagellate microzoospores (8), which are exogamous gametes, i.e. they will only pair with similar zoospores from another cyst. The zygote (9-11) resulting from this conjugation is a minute amoeboid; its nucleus divides repeatedly, a gelatinous test is formed within which the spicules appear, and so the A form is reconstituted. In many of the test-bearing forms, whether Lobose or Filose, plastogamic unions occur, and the two nuclei may remain distinct, leading to plurinucleate monsters in their offspring by fission, or they may fuse and form a giant nucleus, a process which has here no relation to normal syngamy, as it is not associated with any marked change in the alternation of feeding and fission, etc. In Trichosphaerium also plastogamic unions between small individuals have for their only result the increase of size, enabling the produce to deal with larger prey. Temporary encystment in a "hypnocyst" is not infrequent in both naked and shelled species, and enables them to tide over drought and other unfavourable conditions.

Schaudinn has discovered and worked out true syngamic processes, some bisexual, some exogamous, in several other Rhizopods. In Chlamydophrys stercorea the pairing-cells are equal, and are formed by the aggregation of the chromidia into minute nuclei around which the greater part of the cytoplasm aggregates, while the old nucleus (with a little cytoplasm) is lost. These brood-cells are 2-flagellate pairing-cells, which are exogamous: the zygote is a brown cyst; if this be swallowed by a mammal, the original Chlamydophrys appears in its faeces.[[72]]

Centropyxis aculeata, a species very common in mud or moss, allied to Difflugia, also forms a brood by aggregation around nuclei derived from chromidia. The brood-cells are amoeboid, and secrete hemispherical shells like those of Arcella; some first divide into four smaller ones, before secreting the shell. Pairing takes place between the large and the small forms; and the zygote encysts. Weeks or months afterwards the cyst opens and its contents creep out as a minute Centropyxis. Finally, Amoeba coli produces its zygote in a way recalling that of Actinosphaerium (pp. [73-75], Fig. 21): the cell encysts; its nucleus divides, and each daughter divides again into two, which fuse reciprocally. Thus the cyst contains two zygote nuclei. After a time each of these divides twice, so that the mature cyst contains eight nuclei. Probably when swallowed by another animal they liberate a brood of eight young amoebae. Thus in different members of this group we have exogamy, both equal and bisexual, and endogamy.

Most of the Rhizopoda live among filamentous Algae in pools, ponds, and in shallow seas, etc.; some are "sapropelic" or mud-dwellers (many species of Amoeba, Pelomyxa, Difflugia, etc.), others frequent the roots of mosses. Amoeba coli is often found as a harmless denizen of the large intestine of man. Amoeba histolytica, lately distinguished therefrom by Schaudinn, is the cause of tropical dysentery. It multiplies enormously in the gut, and is found extending into the tissues, and making its way into the abscesses that so frequently supervene in the liver and other organs. Chlamydophrys stercorea is found in the faeces of several mammals. The best monograph of this group is that of Penard.[[73]]

2. Foraminifera[[74]]

Sarcodina with no central capsule or distinction of ectosarc; the pseudopodia fine, branching freely, and fusing where they meet to form protoplasmic networks, or the outermost in the pelagic forms radiating, but without a central or axial filament: sometimes dimorphic, reproducing by fission and by rhizopod or flagellate germs in the few cases thoroughly investigated: all marine (with the exception of some of the Allogromidiaceae), and usually provided with a test of carbonate of lime ("vitreous" calcite, or "porcellanous" aragonite?), or of cemented particles of sand ("arenaceous"); test-wall continuous, or with the walls perforated by minute pores or interstices for the protrusion of pseudopodia.

The classification of Carpenter (into Vitreous or Perforate, Porcellanous or Imperforate, and Arenaceous), according to the structure of the shell, had proved too artificial to be used by Brady in the great Monograph of the Foraminifera collected by the "Challenger" Expedition,[[75]] and has been modified by him and others since then. We reproduce Lister's account of Brady's classification.[[76]] We must, however, warn the tyro that its characterisations are not definitions (a feature of all other recent systems), for rigid definitions are impossible: here as in the case, for instance, of many Natural Orders of Plants, transitional forms making the establishment of absolute boundaries out of the question. In the following classification we do not think it, therefore, necessary to complete the characterisations by noting the extremes of variation within the orders:—

1. Allogromidiaceae: simple forms, often fresh-water and similar to Rhizopoda; test 0, or chitinous, gelatinous, or formed of cemented particles, whether secreted platelets or ingested granules. Biomyxa, Leidy = Gymnophrys, Cienk.; Diaphorodon, Archer; Allogromia, Rhumbl. (= Gromia, auctt.[[77]] nec Duj.) (Fig. 14, 1); Lieberkühnia, Cl. and Lachm. (Fig. 12); Microgromia, R. Hertw. (Fig. 11); Pamphagus, Bailey.

2. Astrorhizidaceae: test arenaceous, often large, never truly chambered, or if so, asymmetrical. Astrorhiza, Sandahl; Haliphysema, Bowerb.; Saccammina, M. Sars (Fig. 13, 1); Loftusia, Brady.