Page:Encyclopædia Britannica, Ninth Edition, v. 9.djvu/393

FORAMINIFERA 370 sixteen, do not communicate directly with each other, but each has its own separate mouth, opening into a common " vestibule " which occupies the centre of the under-side of the spire. The younger shells, consisting of from eight to twelve chambers, are thin and smooth; but the older shells are thicker, and are raised into ridges that form a hexagonal areolation round the pores (fig. 15, a) ; and this thickening is shown, by examination of thin sections of the shell (b) to be produced by an exogenous deposit (corresponding with the "in termediate skeleton " of the more complex types) that invests the original shell-wall, sometimes containing little tiask-shapcd cavities filled with sarcode (Wallich). When taken alive by the tow-net, however, floating at or near the surface, the shells of Globigcrince I m. 15.GIobigertna from the Atlantic ooze, showing the thickening of the shell by exogenous deposit : a, entire shell, showing the surface raised into areo- Inted ridges; 6, portion of shell more highly magnified, showing orifices of tubuli, and large cavities filled with sarcot e; r, section of shell, showing exogenous deposit formed around the original chamber wall, which ii raised into ridges with tubuli opening between them, and includes sarcodic cavities (After Walltch.) are found to be beset with multitudes of delicate calcareous spines, which extend themselves radially from the angles at which the ridges meet, to a length equal to four or five times the diameter of the shell (fig. 16). FIG. IS.GIobiyerina, as captured by the tow-net floating near the iurfaee. When the living Globigcrina is examined under favourable cir cumstances, the sarcodic contents of the chambers may be seen to exude gradually through the pores of the shell, and spread out until it forms a kind of flocculent fringe round the shell, filling up the spaces among the roots of the spines, and rising a little way along their length. This external coating of sarcode is rendered very visible by the oil-globules, which are oval, and filled with intensely coloured secondary globules, and are drawn along by the sarcode, and may be seen with a little care following its spreading or con tracting movements. At the same time an infinitely delicate sheath of sarcode, containing minute transparent granules, but no oil- globules, rises on each of the spines to its extremity, and may be seen creeping up one side of the spine and down the other, with the peculiar flowing movement with which we are familiar in the pseudopodia of Gromia. If the cell in which the Olobigcrina is floating receive a sudden shock, or if a drop of some irritating fluid be added to the water, the whole mass of sarcode retreats into the shell with great rapidity, drawing the oil-globules along with it ; and the outline of the surface of the shell and of the hair-like spines is left as sharp as before the exodus of the sarcode." 1 When a sample of the surface-layer of the "globigerina-ooze" is brought up from the sea-bottom by the sounding apparatus or the dredge, the shells are found for the most part perfect, except in the want of spines, but are of a more opaque whiteness than those cap tured near the surface, and hence nave been supposed to be dead. This difference of aspect, however, may be attributed to the increase in the thickness of the shell by exogenous deposit, which is doubt less the cause of the like change in 1 olymorphina ; and as a large; proportion of these bottom-shells have their chambers filled with sarcode exactly resembling that of the types which unquestionably live at the bottom, the fact that in their young state they are found floating in the upper waters does not appear to the writer a suffi cient disproof of the previously accepted belief that they live also at the bottom. And the fact that the thickened shells, when filled with their sarcodic contents, are so much heavier than sea-water as at once to sink to the bottom when placed in a vessel of that liquid seems strongly to indicate that so soon as adult growth (with suc- cessional increase in the number of chambers) has been attained, the carbonate of lime which the animal continues to separate from the sea-water is applied to the thickening of the shell, which will then sink to the bottom of the sea in virtue of its increased specific gravity. And since other Foramim fcra undoubtedly pass their whole lives on the ocean-bottom, drawing their sustenance from sea-water in the manner already explained, there seems no reason why the subsidence of Globigcrince to similar depths should put an end to their vital activity. It is only in the surface-stratum of the globigerina-ooze, however, that the shell-chambers are occupied by sarcodic bodies. Its sub-surface layer consists partly of still re cognizable fragments of the shells, diffused through a mass of amorphous particles, which appears to be the product of a still fur ther disintegration of their substance ; whilst the yet deeper layers of the "ooze" consist entirely of amorphous particles, all trace of shells being lost. It was observed in various parts of the "Chal lenger" voyage that in particular areas the "globigerina-ooze" was altogether wanting (being replaced by a red clay, probably re sulting from the metamorphosis of volcanic products), even though the living animals were found as usual in the upper waters ; and the absence of this calcareous deposit on the sea-bottom seemed attri butable to the pressure resulting from its very great depth, be tween 2500 and 3000 fathoms, which, with the aid of carbonic acid contained in all sea-water, would exert a solvent action, thus restoring to it these portions of the great calcareous deposit of which the materials had been drawn from it by the living Globigcrince. The higher forms of the Globigcrine type constitute two principal series, in one of which the chambers succeed each other along a straight axis, whilst in the other they form a spire. One of the most common types of the first group is the Tcxtularia (1 4 in fig. 1), in which the chambers are arranged bi-serially, those of the two sides alternating with each other, and each communicating with the chamber above and below it on the opposite side ; as is well seen in the "internal cast" (fig. 17, A), which is an exact model ,Fio. 17. Internal casts of I oramluifera : A, 7:rtu!<iria ; I!, RotaltJ. (in glauconite or some other ferruginous silicate) of the sarcodic body that occupied the shell. The type of the second group is the almost universally diffused Rotalia, in which the chambers are dis posed in a turbinoid spire (18 in fig. 1), through which passes a continuous stolon-process (as showTi in the internal cast, fig. 17, B), lying on the lower and inner side. Of this type there are several very interesting modifications. Thus, in Discorbina (15, fig. 1) the chambers are flattened out over the surface to which the shell ad heres; while in Planorbulina (17 in fig. 1) the early spiral plan soon gives place to the cyclical, and the shell thenceforth spreads, by concentric additions, over the surface to which it adheres. In those most-developed forms of this type which occur in warmer seas, the 1 Sir W. Thomson in Voyage of the Challenger, vol. i. p. 213-