On Molecular and Microscopic Science, Volume 2 (of 2)

by Mary Somerville

ROSALINA ORNATA.

Each chamber of the shell is occupied by a reddish-yellow segment of sarcode, from which pseudopodia are seen to protrude; and it is supposed that the sarcode body also fills the vestibule, since without such connecting band it is difficult to understand how the segments which occupy the separate chambers can communicate with each other, or how new segments can be budded off. In the Globigerina the slight cohesion gives reason to believe that the separation of the parts may be a means of reproduction.

The Rosalina ornata, one of the most beautiful specimens of this group, and remarkable for the size of its pores, is represented in [fig. 100] with its pseudopodia extended, and coalescing in some parts.

The shells of the genus Textularia consist of a double series of chambers disposed on each side of an axis, so that they look as if they were mutually interwoven. As the segments for the most part increase gradually in size, the shell is generally triangular, the apex being formed of the first segment, and its base of the two last (H, [fig. 97]).

The aperture is always placed in the inner wall of each chamber, close to its junction with the preceding segment on the opposite side. In the compressed shells it is crescent-shaped, but it is semilunar in the less compressed, and may even be gibbous. The shell is hyaline, with large pores not very closely set, though in some varieties they are minute and near to one another. Sometimes the pores open on the surface in deep hexagonal pits. The older shells are frequently incrusted with large coarse particles of sand, and some specimens from deep water are almost covered with fine sand, but with a good microscope the pores may be seen between them.

The sarcode segments of the animal perfectly correspond in shape and in alternate arrangement with the segments of the shell, and are connected by bands of sarcode passing through the crescent-shaped apertures by which each chamber communicates with that which precedes and follows it.

The Textulariæ are among the most cosmopolitan of Foraminifera; some of their forms are found in the sands and dredgings from all shores, from shallow or moderately deep water. In time they go back to the Palæozoic period.

The Rotalia Beccarii, common on the British coast, affords a good example of the supplemental skeleton, a structure peculiar to some of the higher vitreous Foraminifera. It has a rather compressed turbinoid form with a rounded margin. Its spire is composed of a considerable number of bulging segments gradually increasing in size, disposed with great regularity, and with their opposed surfaces closely fitted to each other. The whole spire is visible on the exterior, with all its convolutions, and on account of the bulging form of the segments, their lines of junction would appear as deep furrows along the whole spire, were they not partly or wholly filled up with a homogeneous semi-crystalline deposit of shell-substance, which is very different in structure and appearance from the porous shell wall of the segments.

The genus Calcarina is distinguished by a highly developed intermediate skeleton with singular outgrowths, which is traversed by a system of canals; through these the animal sends its pseudopodia into the water for food to nourish the whole.

A homogeneous crystalline deposit invests almost the whole of the minute spiral shell of a Calcarina, and sends out many cylindrical, but more generally club-shaped spines in all directions, though they usually affect more or less that of the equator, as in the typical form Calcarina calcar, which is exactly like the rowel of a spur. The spines are for the most part thick and clumsy, and give the shell a very uncouth appearance, especially when their extremities are forked. The turbinoid spire of the shell has a globose centre surrounded by about five whorls progressively increasing in size, and divided by perforated septa into chambers. Each whorl is merely applied to that preceding it, and does not invest it in the least degree. Internally the turns of the spire are separated from each other by the interposition of a solid layer of shell-substance quite distinct from the walls of the chambers. A crystalline deposit begins at the very centre of the spire in a thin layer gradually increasing in thickness as it proceeds, and sending off club-shaped spines from time to time so that the spines are of later and later production, and become thicker and longer. From this it is evident that the intermediate skeleton grows simultaneously with the turns of the spire, but strange as it may seem, their growth is independent, though both are nourished and increased by the sarcode in the interior of the chambers. For the intermediate skeleton is traversed in every part by an elongated network of canals, which begin from irregular lacunæ or openings in the walls of the chambers, and extend to the extremities of the spines. Through these canals threads of the sarcode body of the animal within the chambers have access to the exterior, and provide nourishment for the intermediate skeleton; while pseudopodia, passing into the water through pores in the last partition of the shell, provide for its growth and procure nourishment for the animal. The communication between the adjacent chambers in the whorls, is by means of a series of pores in the septa, or partitions; and it is through the pores of the last septum that the pseudopodia of the animal have access to the water to provide for the growth of the spire, for the punctures on the surface are merely the terminations of some of the branching canals. On approaching the surface the canals become crowded together in some parts, leaving columns of the shelly skeleton unoccupied which either appear as tubercles on the surface, or, if they do not rise so high, form circular spots surrounded by punctations which are the apertures of the canals.