RHIZOPODA.
The organisms of this class are small and for the most part of microscopic minuteness, of a simple gelatinous structure, commonly protected by a shell. The most simple rhizopods, called Amoeba, present a globular form when contracted, but can extend portions of their substance (" sar code") like roots, and use them to draw along the rest of the mass, like the feet or tentacles of polyps, whence the name of the class. These root-like processes can also attach themselves to foreign particles, and draw them into the " sarcode," or substance of the body, where the soluble organic part, so " intus-suscepted," may be assimilated, the insoluble part being extruded. A solid hyaline corpuscle or nucleus is commonly discernible in the interior of the Ameba, sometimes accompanied by one or more clear contractile vesicles. When the productions of the sarcode are numerous, filiform, and seemingly constant, radiating from all parts of the body, the rhizopod presents the characters of Actinophrys. When the tentacles are produced from only one extremity of the body we have the genus Pamphagus. When such a rhizopod is enclosed in a membranous sac it is a Dillugia ; if the sac be discoid with a slit on the flat surface for the protru sion of the tentacles, it is an Arcella. In other rhizopods the sac is calcified, or becomes a "shell," which is sometimes simple, but usually consists of an aggregate of chambers, inter-commu nicating by minute apertures, whence the name Foraminifera given to the testaceous rhizopods. These chambers grow by successive gemmation from a primordial segment, sometimes in a straight line, more commonly in a spiral curve ; and each segment so developed has its own shelly envelope. As, how ever, they are organically connected, the whole seems to form a "chambered" or " polythalamous" shell. The last-formed segment is usually distinguished by the very long, slender, pellucid, colourless, contractile filaments which have suggested the name " Rhizopods" for the class. But, in the Foraminifera, both the outer wall and the septa of the compound shell are perforated by minute apertures, through which either connect ing or projecting filaments of the soft organic tissue can pass.
The several segments or jelly-filled chambers are essentially repetitions of each other ; and there is no proof that the inner and earlier segments derive their nourishment from the outer and last-formed one. A Foraminifer may therefore be regarded either as a series of individuals, organically united, or as a single aggregate being, compounded according to the law of vegetative repetition.
The minute chambered shells of Foraminifera enter largely into the composition of all the sedimentary strata, and are so abundant in many common and familiar materials, like the chalk, as to justify the expression of Bacon, that the very dust had been alive. The deep-sea soundings of the Atlantic Telegraph Company have shown that the bed of that great ocean, at a depth approaching, or even exceeding, two miles, is composed of little else than the calcareous shells of a Globigerirai and a few other Rhizopods, with the silicious shields of the allied Polycystinem The composition of the chalk is extremely similar : when the finer portion, amounting to half or even less, has been washed away, the remaining sediment consists almost entirely of foraminated shells, some perfect, others in various stages of disintegration. They have also been found in other marine formations, which are soft enough to be washed, down to the Lower Silurian ; and in the hard limestones and marbles they can be detected in polished sections, and in thin slices laid on glass. The greater part of these shells are microscopic, but some of the large extinct Foraminifera, called, from resembling a piece of money, " Nummulites," are two inches in diameter.
The generic divisions in use for these shells were mostly invented by M. d'Orhigny, but on artificial grounds, viz., exclusively upon the plan of growth, or mode of increase in the number of chambers. The structure and anamorphoses of these complex atoms have been recently investigated by Messrs. Williamson and Carpenter, and especially by Mr.