Fig. 12. - Shells of Arenaceous Foraminifera. A, Test of Astrorhiza, greatly enlarged ; B, Test of Trochammina ringens, enlarged thirty times; C, Test of Trochammina lituiformis, enlarged eighteen times. (After Carpenter and Brady.)
In a few types of the Foraminifera (e.g., in the Dactyloporidae) the successive chambers of the multilocular test have no direct communication with one another, and simply cohere by their walls. In the majority of the compound shells, the successive chambers are so produced, that the septum between any two of them is formed solely by the anterior wall of the older chamber, which thus constitutes the posterior wall of the newer one (fig. 11, e). In the highest types of the compound Foraminifera, however, each segment is provided with its own proper wall of shell, each segment, as it is produced, forming for itself a posterior wall which applies itself to the anterior wall of the preceding segment, so that each septum ("septal plane") is composed of two lamellae, as seen in fig. 13, A (Carpenter). Moreover, "in the higher types of the hyaline or vitreous series we frequently meet with an ' intermediate ' or ' supplemental' skeleton, formed by a secondary or exogenous deposit upon the outer walls of the chambers, by which they receive a great accession of strength. This deposit not only fills up what would otherwise be superficial hollows at the junctions of the chambers (fig. 13, A, d), or (as in Polystomella) at the umbilical depression, but often forms a layer of considerable thickness over the whole surface, thus separating each whorl from that which encloses it ; and it is sometimes prolonged into outgrowths that give a very peculiar variety to the ordinary contour, as in some varieties of Rotalia and Polystomella, but most characteristically in Calcarina (fig. 13, B). This intermediate or supplemental skeleton, wherever developed to any considerable extent, is traversed by a set of 'canals,' which are usually arranged upon a systematic plan, and are sometimes distributed with considerable minuteness " (Carpenter). The canals of this system are doubtless filled in the living state by prolongations of the sarcode, which serve to keep up the vitality of the intermediate skeleton. This intermediate skeleton, with its canalsystem, is largely developed in many of the highest and largest of the types of the Hyaline Foraminifera (such as Nummulina), and very specially so in the ancient Eozoon, if this be rightly regarded as a Foraminifer.
Fig. 13. - A, Diagram of one of the higher forms of the vitreous Foraminifera, showing the double nature of the septa (b), the stolon-passages between successive chambers (a), and the supplemental skeleton (d) ; B, Test of Calcarina Spengleri, magnified twelve diameters, showing the spines formed by the supplemental skeleton ; C, Part of a section of the test of Calcarina, magnified fifty diameters, showing the tubulated "proper walls " of the chambers (n), and the canal-system of the intermediate skeleton (d); D, Part of the test of Nummulina laeigata, highly magnified, showing the canal-system of the septa (s), and marginal cord (n). (After Carpenter.)
The recognition of a "nucleus" in many Foraminifera - and its probable presence in all - renders it necessary to unite with this group a number of fresh-water Rhizopods, which would otherwise have to be placed with the Amaebea, and to which we may, in a restricted sense, apply Hertwig's name of Thala-mophora.* The test in the forms in question is always one-chambered, and in all except Diaphorophodon it is imperforate. It may be smooth or sculptured, and in composition it may be either membranous or chitinous, in some cases with adventitious siliceous particles in addition. Both a nucleus and contractile vacuoles are present in the protoplasm of the body; and the pseudopodia are long and filamentous or reticulated. Of these simple "Reticularian" Rhizopods, Gromia(fig. 10, b) is both a marine and a fresh-water form, and possesses a delicate membranous test, from a terminal aperture in which the protoplasm gains the exterior. Mic?'ogromia (fig. 14) resembles Gromia in structure, but forms loose colonies by the root-like union of the pseudopodia of a number of individuals. It lives in fresh water, and reproduces itself by giving exit to amoeboid masses of protoplasm, each of which develops two flagella, thus constituting free locomotive "swarm-spores." Euglypha and Diplophrys are other forms allied to Gromia, the former having an inflexible and sculptured test, while the latter has two oppositely-placed apertures in the shell, in place of a single terminal opening.
Fig. 14. - A colony of Microgromia socialis, showing the different members of the colony united by their branching pseudopodia. Greatly enlarged. (After Hertwig.)
* Under the name Thalamophora, Hertwig and Lesser include the true Foraminifera, the monothalamous Rhizopods above alluded to (with "reticularian" pseudopodia), and the Arcellina. The last of these, however, on account of their blunt, lobose pseudopodia, are here placed in the order of the Amaebea.