Fig. 17.—Shell of Globigerina bulloides, from tow-net, showing investment of spines. (From Wyville Thomson.)
In the Perforate genera, Patellina and Discorbina, plastogamy precedes brood formation, the cytoplasms of the 2-5 pairing individuals contracting a close union; and then the nuclei proceed to break up without fusion, while the cytoplasm aggregates around the young nuclei to form amoebulae, which acquire a shell and separate. In both cases it is the forms with a single nucleus, corresponding to megalospheric forms that so pair, and the brood-formation is, mutatis mutandis, the same as in these forms. Similar individuals may reproduce in the same way, in both genera, without this plastogamic pairing, which is therefore, though probably advantageous, not essential. If pseudopodiospores form their shells while near one another, they may coalesce to form monsters, as often happens in Orbitolites.[[81]]
The direct economic uses of the Foraminifera are perhaps greater than those of any other group of Protozoa. The Chalk is composed largely of Textularia and allied forms, mixed with the skeletons of Coccolithophoridae (pp. [113-114]), known as Coccoliths, etc. The Calcaire Grossier of Paris, used as a building stone, is mainly composed of the shells of Miliolines of Eocene age; the Nummulites of the same age of the Mediterranean basin are the chief constituent of the stone of which the Pyramids of Egypt are built. Our own Oolitic limestones are composed of concretions around a central nucleus, which is often found to be a minute Foraminiferous shell.
The palaeontology of the individual genera is treated of in Chapman's and Lister's recent works. They range from the Lower Cambrian characterised by perforated hyaline genera, such as Lagena, to the present day. Gigantic arenaceous forms, such as Loftusia, are among the Tertiary representatives; but the limestones formed principally of their shells commence at the Carboniferous. The so-called Greensands contain greenish granules of "glauconite," containing a ferrous silicate, deposited as a cast in the chambers of Foraminifera, and often left exposed by the solution of the calcareous shell itself. Such granules occur in deep-sea deposits of the present day.[[82]]
3. Heliozoa
Sarcodina with radiate non-anastomosing pseudopodia of granular protoplasm, each with a stiff axial rod passing into the body plasma; no central capsule, nor clear ectoplasm; skeleton when present siliceous; nucleus single or multiple; contractile vacuole (or vacuoles) in fresh-water species, superficial and prominent at the surface in diastole; reproduction by fission or budding in the active condition, or by brood-formation in a cyst, giving rise to resting spores; conjugation isogamous in the only two species fully studied; habitat floating or among weeds, mostly fresh water.
1. Naked or with an investment only when encysted.
Aphrothoraca.—Actinolophus F.E. Sch.; Myxastrum Haeck.; Gymnosphaera Sassaki; Dimorpha (Fig. 37, 5, p. [112]) Gruber; Actinomonas Kent; Actinophrys Ehrb.; Actinosphaerium St.; Camptonema Schaud; Nuclearia Cienk.
2. Invested with a gelatinous layer, sometimes traversed by a firmer elastic network.