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

by Mary Somerville

Class II.—Foraminifera.

The geological importance of the Foraminifera, their intrinsic beauty, the prodigious variety of their forms, their incredible multitude, and the peculiarity of their structure, have given these microscopic organisms the highest place in the class of Rhizopods. The body of these animals consists of a perfectly homogeneous sarcode or semi-fluid protoplasm, showing no tendency whatever to any film or surface-layer. It is inclosed in a shell; and the only evidence of vitality that the creature gives, is a protrusion and retraction of slender threads of its sarcode, through the mouth or pores of the shell, or through both according to its structure. [Fig. 97] shows some of their forms.

By far the greater number of the Foraminifera are compound or many-chambered shells. When young, the shell has but one chamber, generally of a globular form; but as the animal grows, others are successively added by a kind of budding in a definite but different arrangement for each order and genus of the class. When the creature increases in size, a portion of its semi-fluid sarcode projects like a bud from the mouth of its shell. If it be of the one-chambered kind, the bud separates from its parent before the shelly matter which it secretes from its surface consolidates, and a new individual is thus produced. But if the primary shell be of the many-chambered kind, the shelly secretion consolidates over the sarcode projection which thus remains fixed, and the shell has then two chambers, the aperture in the last being the mouth, from which, by a protrusion of sarcode, a third chamber may be added, the new chamber being always placed upon the mouth of its predecessor, a process which may be continued indefinitely, the mouth of the last segment being the mouth of the whole shell.

Fig. 97. Various forms of Foraminifera:—A, Oolina claxata; B, Nodosaria rugosa; C, Nodosaria spinicosta; D Cristellaria compressa; E, Polystomella crispa; F, Dendritina elegans; G, Globigerina bulloïdes; H, Textularia Mayeriana; I, Quinqueloculina Bronniana.

By this process an ovate shell with a mouth at one extremity may have a succession of ovate chambers added to it, each chamber being in continuity with its predecessor so that the whole shell will be straight and rod-like, the last opening being the mouth. If the original shell be globular, and if all the successive gemmæ given out be equal and globular, the shell covering and uniting them will be like a number of beads strung upon a straight wire. Sometimes the successive gemmæ increase in size so that each chamber is larger than the one which precedes it; in this case the compound shell will have a conical form, the primary shell being the apex, and the base the last formed, the aperture of which is the mouth of the whole shell; a great many Foraminifera have this structure. The spiral form is very common and much varied. A series of chambers increasing in size may coil round a longitudinal axis, like the shell of the snail; but if each of the successive chambers, instead of being developed exactly in the axis of its predecessor, should be directed a little to one side, a curved instead of a straight axis would be the result; there is a regular gradation of forms of Foraminifera between these two types. The convolutions are frequently flat and in one plane, but the character of the spiral depends upon the successive enlargement or not of the consecutive chambers; for when they open very wide and increase in breadth, every whorl is larger than that which it surrounds; but more commonly there is so little difference between the segments after the spiral has made two or three turns, that the breadth of each whorl scarcely exceeds that which precedes it.

However varied the forms may be, the mouth of the last shell is the mouth of the whole, either for the time being or finally. For all the chambers are connected by narrow apertures in the partitions between them. Each chamber is occupied by a segment of the gelatinous sarcode body of the animal, and all the segments are connected by sarcode filaments passing through the minute apertures in the partitions between the chambers, so that the whole constitutes one compound creature.

Although the character and structure assumed by the semi-fluid bodies of the known Foraminifera have been determined in most cases with admirable precision, it is still thought advisable to arrange them according to the substance of the shell: consequently they form three natural orders; namely, the Porcellanous or imperforate, which have calcareous shells often so polished and shining that they resemble porcelain; secondly, the Arenaceous Foraminifera, consisting of animals which secrete a kind of cement from their surfaces, and cover themselves with calcareous or siliceous sand-grains; and lastly, the Vitreous and Perforated order, which is the most numerous and highly organized of the whole class, has siliceous shells transparent as glass, but acquires more or less of an opaque aspect in consequence of minute straight tubes which perforate the substance of the shell perpendicularly to its surface, and consequently interfere with the transmission of light.

Order of Porcellanous Foraminifera.

The Miliolidæ constitute the porcellanous order, which consists of twelve genera and many species, varying from a mere scale to such as have chambered shells of complicated structure.

The genus Miliola has minute white shells resembling millet seeds, often so brilliantly polished that they are perfectly characteristic of the porcelain family to which they belong. No Foraminifera are better suited to give an idea of the intimate connection between the shell and its inhabitant than the Miliola, the fundamental type of this genus. The shell is a spiral (I, [fig. 97]), which is made up of a series of half turns arranged symmetrically on its two sides. Each half turn is longer and of greater area than that on the opposite side, so that each turn of the spire has a tendency to extend itself in some degree over the preceding one, which gives a concave instead of a convex border to the inner wall of the chamber. The sarcode body of the Miliola consists of long segments which fill the chambers, connected by threads of sarcode passing through the tubular constrictions of the shell. As the animal grows, its pseudopodia extend alternately now from one end, and now from the other extremity of the spiral, and by them it fixes itself to seaweeds, zoophytes, and other bodies, for these Foraminifera never float or swim freely in the water. The genus Miliola is more extensively diffused than almost any other group of Foraminifera; they are most abundant between the shore and a depth of 150 fathoms, and are occasionally brought up from great depths. Beds of miliolite limestone show to what an extent the Miliola abounded in the seas of the Eocene period; but the type is traced back to the Lias.

The genus Peneroplis is distinguished by a highly polished opaque white shell; its typical form is an extremely flat spire of two turns and a half opening rapidly and widely in the last half whorl. It is strongly marked by depressed bands which indicate the septa or shelly partitions between the chambers in the interior. The polished surface of the shell is striated between and transversely to the bands by parallel platted-looking folds ¹⁄₁₄₀₀ of an inch apart. But the peculiarity of this shell and its congeners is, that the partitions between the chambers in its interior are perforated by numerous isolated and generally circular pores which in this compressed type are in a single linear row. Their number depends upon the length of the partition between the chambers, which increases with the age of the animal and size of the shell. There is but one pore in each of the consecutive partitions from the globular centre to the fourth chamber. From the fourth to the seventh chamber the communication is by two pores; after this the number is gradually increased to three, four, six, &c., up to forty-eight, so that the last segment may send out forty-eight pseudopodia from the mouth of the shell. In its early youth one pseudopodium appears to have been sufficient to find food for the animal, but as the shell increased in size and the segments in number, a greater supply of food was requisite and a greater number of pseudopodia were necessary to fish for it. Moreover when an addition to the shell is required the pseudopodia coalesce at their base and form a continuous segment upon which the new portion of the shell is moulded.

In varieties of the Peneroplis where the spire is less compressed there are sometimes two rows of pores in the partitions between the chambers. The Dendritine variety deviates most from that described. It is characterised by a single large aperture in each partition which sends out ramifications from its edges. The form of these openings depends upon that of the spire; when compressed the aperture is linear and less branched at its edges; but in shells which have a very turgid spire it is sometimes broader than it is long, and much branched; but these extremes are connected by a variety of forms. The shells of this variety of the Peneroplis are strongly marked by the depressed bands and striæ, as in the Dendritina elegans (F, [fig. 97]). The segments of the animal inhabiting these shells must be more intimately connected than in most of the other Foraminifera; and the pseudopodia sent through these large apertures out of the mouth of the shell must be comparatively quite a mass of sarcode. The Dendritinæ are inhabitants of shallow water and tropical seas, while the other members of the genus Peneroplis abound in the Red Sea and the seas of other warm latitudes, especially in the zone of the great laminarian fuci. They do not appear in a fossil state prior to the beginning of the Tertiary period.

The last whorls of some of the compressed spiral Foraminifera of the Porcellanous order so nearly encompass all their predecessors, that the transition from a flat spiral to the Orbitolite with its flat disk of concentric rings is not so abrupt as might at first appear. The gradual change may be distinctly traced in the species of the genus Orbiculina. The exteriors of the shells of the genus Orbitolites have less of the opaque whiteness than many others of its family. In its simplest form it is a disk about the ¹⁄₅₀₀ of an inch in diameter, consisting of a central nucleus surrounded by from ten to fifteen concentric circular rings. The surface is usually plane, though sometimes it is concave on both surfaces in consequence of the rings increasing in thickness towards the circumference. The rings or zones are distinctly marked by furrows on the exterior of the shell, and each of these zones is divided by transverse furrows into ovate elevations with their greatest diameter transverse to the radius of the disk, so that the surface presents a number of ovate elevations arranged in consecutive circles round the central nucleus. The margin of the disk exhibits a series of convexities with depressions between them; in each of these depressions there is a circular pore surrounded by a ring of shell: these pores are the only means the animal possesses of communicating with the water in which it lives.

Fig. 98. Simple disc of Orbitolites complanatus.

Fig. 99. Animal of Orbitolites complanatus.

[Fig. 98] is a horizontal section of the simple Orbitolite showing the internal structure of the disk. A pear-shaped chamber with a circumambient chamber forms a nucleus which is surrounded by series of concentric rings of ovate cavities. The chambers of the nucleus and all the cavities are filled with segments of homogeneous semi-fluid sarcode, which constitute the body of the animal ([fig. 99]). The segments in the rings are connected circularly by gelatinous bands of sarcode extending through passages which connect the cavities laterally. The segments are also connected radially by similar sarcode bands, which originate in the mass of sarcode filling the nucleus, and extend to the pores in the margin of the disk. The cavities of each zone alternate in position with those of the zones on each side of it. The animal sends out its pseudopodia through the marginal pores in search of food, which consists of Diatoms and Desmidiaceæ; they are drawn in, digested without any stomach, and the nutritious liquid is conducted by the gelatinous bands from segment to segment and from zone to zone, even to the innermost recesses of the shell.

It is supposed that during the growth of the Orbitolite, when the animal becomes too large for its abode, its pseudopodia coalesce and form a gelatinous massive coat over the margin of the exterior zone, which secretes a shelly ring with all its chambers and passages, each ring being a mere vegetative repetition of those preceding it. That vegetative property enables the animal to repair its shell or add a part that is wanting. For, if a small portion of a ring be broken off and separated from the living animal, it will increase so as to form a new disk, the want of the central part or nucleus not appearing to be of the smallest consequence; indeed, the central rings are very often imperfect. The sarcode of these animals is red, and although the shell is of a brownish-yellow by transmitted light, it is so translucent that the red tint is seen through it.

The simple Orbitolite has many varieties. Sometimes it begins its life as a spiral which changes to a circular disk as it advances in age. It varies in thickness, and some of its very large varieties may be said to consist of three disks or stories of concentric chambers and many marginal pores instead of one. The upper and base stories of concentric chambers are alike, the intermediate one very different, but the sarcode segments in all the three are so connected as to form a very complex compound animal.[^[8]] Different as this structure is from that of the simple Orbitolite, they are merely varieties of the same species; for it has been shown by Dr. Carpenter that, although many pass their lives in the simple one-storied state, they may change into the complex form at any stage of their growth; and as an equally extensive range of variation has been proved by Professor Williamson and Mr. Parker to prevail in other groups of Foraminifera, the tendency to specific variation seems to be characteristic of that type of animal life, and consequently the number of distinct species is less than they were supposed to be.

The Orbitolites are found in the dredgings of all the warmer seas, in vast multitudes at the Philippine Islands, but those from Australia are the most gigantic, being sometimes the size and thickness of a shilling.

Order of Arenaceous Foraminifera.

In the numerous family of Lituolidæ the abode of the animal consists of a cement mixed with very fine particles of sand with larger ones imbedded in the surface. The order includes a wide range of forms divided into three genera, the simplest of which consists of a cylindrical tube twisted into a spiral gradually increasing in diameter, and attached to a foreign substance by one of its surfaces. The creature which lives in it is a uniform cord of sarcode, which sends its pseudopodia out through a large aperture at the extremity of its tube in search of food. Although the tube consists of sand imbedded in an ochreous-coloured cement secreted by the animal, its surface is smooth as a plastered wall. The spiral tubes of this genus take various forms, and in some cases are divided into chambers.

The members of the genus Lituola exude from their surfaces a thick coat of cement with a quantity of siliceous particles roughly imbedded in it, but in some instances the particles are so uniform in size and shape, and are so methodically arranged, that the surface resembles a tesselated pavement. The usual form of the Lituola is a mere string of oval convex chambers increasing gradually in size, and fixed to shells and corals by their flat surfaces. In some instances the shells, or rather the substitutes for shells, take a nautiloid form, and become detached from the foreign bodies to which they were attached. In the highest forms of this genus the chambers are divided by secondary partitions.

The typical form of the genus Valvulina is a three-whorled, three-sided pyramidal shell, with three chambers in every turn of the spire. The aperture is large and round, with a valve of smaller size attached by a tooth of shell to its rim. The creature itself has an exceedingly thin perforated vitreous shell, covered by an incrustation of calcareous particles, which so entirely blocks up the perforations that it can only extend its pseudopodia through the mouth of its shell.

Order of Vitreous Foraminifera.

Nearly all the Foraminifera on the British coasts belong to the Vitreous or Perforated order, which consists of three natural families and many genera. Their shells are vitreous, hyaline, and generally colourless, even although the substance of the animal is deeply coloured; in some species both the animal and its shell are of a rich crimson. The glassy transparency of the shells would be perfect were they not perforated by numerous tubes running from the interior of the chambers straight through the shell, and ending in pores on its surface. According to microscopic measurement the tubes in the Rotalia, which are the largest, are on an average the ¹⁄₁₀₀₀ of an inch in diameter, and as they are somewhat more than that apart, the transparency of the shell appears between them and gives the surface a vitreous aspect. The pseudopodia of the animal have been seen to pass through every part of the wall of the chambers occupied by it; the apertures of the tubuli in this case are wide enough to permit particles of food to be drawn into the interior of the shell. But threads of sarcode of extreme tenuity alone could pass through the tubuli of the Operculina, which are not more than the ¹⁄₁₀₀₀₀ of an inch in diameter, and the distance between them not much greater, which gives the shell an opaque appearance. Particles of food can hardly be small enough to pass through such tubes into the interior to be digested. Dr. Carpenter, however, is almost certain, from the manner in which the animal repairs injuries done to its shell, that the semi-fluid sarcode extends itself at certain times, if not constantly, over the exterior of the shell, as in the Gromia; and therefore it is by no means impossible that the digestive process may really be performed in this external layer, so that only the products of digestion may have to pass into the portion of the sarcode occupying the body of the shell.

In such many-chambered shells as are pierced by tubuli wide enough to permit particles of food to be drawn into the interior, each segment of the animal, being fed within its own chamber, has a life of its own, at the same time that it shares with all the others in a common life maintained by food taken in through the mouth of the shell. There are many instances of this individual life combined with a common life among the lowest tribes of animals.

Although the Perforated order contains types widely apart, they are always connected by intermediate forms; but there is no such connection between the two great natural orders, which are not only separated by the tubuli in the shell, but in many instances by the structure of the interior and the corresponding character of the animal.

In the Lagenidæ, which form the first family of the Perforated order, the vitreous shell possesses great hardness, and is pierced by numerous small tubuli. It is very thin, and of glossy transparency. The first four shells in [fig. 97] represent some of its forms.

The genus Nodosaria has a very extensive range of forms, from the elongated structure to the nautiloid spiral, depending upon the relative proportions and arrangement of the segments. The segments are separated by constrictions transverse to the axis of growth, or by bands as in the Nodosaria rugosa, B, [fig. 97]. It frequently happens that parts of the shell are not perforated; and there are generally longitudinal ribs which sometimes have spines projecting from every part of the interior, as in Nodosaria spinicosta, C, [fig. 97].

In the genus Nodosaria, the axis of growth changes from a straight line to that of a spiral, so that the septa or divisions between the segments cross the axis obliquely, and the aperture instead of being exactly central becomes excentric. Between these extremes there is a numerous series of gradations. The Cristallaria is the highest type; the form is a nautiloid spiral, more or less compressed (D, [fig. 97]), of which each whorl has its chambers extended by winged projections so as to reach the centre, and entirely encloses the preceding whorl. The number of chambers in each whorl is much smaller than in most of the nautiloid spirals, not being more than eight or nine. The divisions are always strongly marked externally by septal bands, varying in character according to the species. The margin of the shell runs into a keel, which is sometimes extended into a knife-edge. Nearly all the Lagena family are found in the North Atlantic and Mediterranean, especially in the Adriatic, which is rich in species. In the Nodosaria the cells which compose the shell have so little connection one with another that they may be easily detached; which gives reason to believe that the separation of the parts may be a means of reproduction and dispersion.

The Globigerinidæ are the most numerous family of the perforated series, and the most remarkable in the history of the existing Foraminifera. They are distinguished by the coarseness of the perforations in their shells, and by the crescentic form of the aperture by which the chambers communicate with each other.

The genus Globigerina consists of a spiral aggregation of globose segments, which are nearly disconnected from each other although united by mutual cohesion. The segments are always somewhat flattened against one another in their planes or junctions, and sometimes the flattening extends over a pretty large surface as in G, [fig. 97]. The entire series of segments shows itself on the upper side, but on the lower side only the segments forming the latest convolution are prominent; they are usually four in number, and are arranged symmetrically round a deep depression or vestibule; the bottom of which is formed by the segments of the earlier convolutions. In this vestibule each segment opens by a large crescent-shaped orifice, the several chambers having no direct communication with each other. The entire shell of the ordinary type may attain the diameter of about ¹⁄₃₀ of an inch, but it is usually much smaller; the typical form, however, is subject to very considerable modifications. In newly formed segments of Globigerina, the hyaline shell substance is perforated by tubuli varying from ¹⁄₁₀₀₀₀ to ¹⁄₅₀₀₀ of an inch in diameter, arranged at pretty regular distances; but in deep seas the surface of the shell is raised by an external deposit into tubercles or ridges, the orifices of the pores appearing between them.

Fig. 100, p. 41.

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.

The Rotaline series of the Globigerina family is one of the most numerous and varied of the whole class of Foraminifera; but varied as their forms are, they all bear the characteristic marks which distinguish their order, with this essential difference, that in the genus Globigerina each chamber of the spire has a communication with the central vestibule by a crescent-shaped aperture, while in the Rotalinæ each chamber only communicates by a crescentic aperture with that which precedes and follows it.

In the Rotaline group the internal organization rises successively from the simple porous partition between the chambers, to the double partition with the radiating passages, and from the latter to the double partitions, intermediate skeleton, and complicated system of canals. To these changes the structure of the compound animal necessarily corresponds, for it may be presumed that not only the chambers but all the passages and canals in the interior of the shell are either permanently or occasionally filled with its sarcode body.

However, it is in the Nummuline family that the Foraminifera attain the highest organization of which they are capable. This family surpasses all the Vitreous tribe in the density and toughness of the shell, the fineness of its tubuli, and in the high organization of its canal system. Their forms vary from that resembling a nautilus or ammonite to a flat spiral or cyclical disk, like an Orbitolite, though vastly superior to it in organization both with regard to the animal and to the structure of the shell.

All the species of the genus Nummulite are spiral; in the typical form the last turn of the spire not only completely embraces, but entirely conceals, all that precede it. In general, the form is that of a double convex lens of more or less thickness; some are flat, lenticular, and thinned away to an acute edge, while others may be spheroidal with a round, or obtuse edge. They owe their name to their resemblance to coins, being, in general, nearly circular. Their diameters range from ¹⁄₁₆th of an inch to 4¹⁄₂ inches, so that they are the giants of their race; but the most common species vary from ¹⁄₂ an inch to 1 inch in diameter.

Fig. 101. Section of Faujasina.

[Fig. 101] represents a section of the Nummulite Faujasina near and parallel to the base of the shell. It shows a series of chambers arranged in a flat spiral, and increasing in size from the centre to the last turn of the spire, which embraces and conceals all that precede it. Every segment of the animal is enclosed in a shell of its own, so that they are separated from one another by a double wall and space between; however, they are connected in the spiral direction by narrow passages in the walls.

The segments of the animal in the exterior whorl have direct communication with the water by means of a shelly marginal cord, a, [fig. 101], perforated by multitudes of minute tubes, less than the ¹⁄₁₀₀₀₀ of an inch in diameter, through which threads of sarcode finer than those of a spider’s web can be protruded. These tubuli are so very fine and numerous, that they characterize the Nummuline family.

Fig. 102. Interior of the Operculina.

[Fig. 102] represents the interior of the Operculina, which is an existing representation of the Nummuline type. Every segment of the animal is enclosed in a shell of its own, but all the segments are connected in the spiral direction by narrow passages in the walls as in the Faujasina.

Although each of the interior whorls has its perforated marginal band, the segments can have no direct access to the water; however, they are indirectly brought into contact with it by means of a system of branching shelly canals, radiating from the central chamber, ending in conspicuous pores in the external surface of the shell. During this course the canals send small tubes into the chambers on each side of them; through these the internal segments of the animal can fill the canals with cords of sarcode, and protrude them into the water, whence they are supplied with food.

The genus Polystomella is distinguished by the high development of the intermediate skeleton and the canal system that maintains it. The Polystomella crispa ([fig. 97], E), a beautiful species common on the British coasts and in other temperate seas, has a lenticular form, the ¹⁄₁₆ to the ¹⁄₁₂ of an inch in diameter. It consists of a small number of convolutions winding round the shorter axis of the lens, increasing rather rapidly in breadth, and each one almost entirely enclosing its predecessor, so that the shell is exactly alike on both sides, and only the last convolution is to be seen. At the extremities of the axis there is a mass of solid shell-substance, perforated by orifices which are the apertures of a set of straight, parallel canals. In the figure only the last convolution is visible, upon which the convex septal bands are very conspicuous, dividing the surface into well marked segments, upon the exterior edge of each of which there are strong transverse crenulations. The only communication which the chambers have with the exterior, is by means of a variable number of minute orifices near the inner margin of the sagittate partition-plane, close to its junction with the preceding convolution; a very high microscopic power is required to see them, as well as the minute tubercles with which the surface of the shell is crowded, more especially on the septal bands and in the rows of depressions between the segmental divisions.

The sarcode animal itself corresponds exactly with the form and spiral arrangement of the chambers so strongly marked on the exterior of the shell. The segments form a spiral of crescents, smooth on the convex and crenulated on the concave side; and from the latter threads of sarcode proceed, which pass through pores in the inner margins of the partitions, and unite them into one animal.

The Polystomella lives in tropical seas; P. crispa in temperate latitudes, and P. striato-punctata inhabits the polar waters; the genus is found everywhere.

Although variety of form without specific difference is characteristic of the Foraminifera, it sometimes happens that identity of external form is accompanied by an essential difference in internal structure. Of this the Cycloclypeus is an instance; it is a rare species of nummuline, dredged up from rather deep water off the coast of Borneo. The shell is gigantic, some specimens being two and a half inches in diameter; but its mode of growth is the same with that of the most complicated Orbitolite. It consists of three superposed stages of circular discs, each circle of chambers enclosing all those previously formed. However, each segment of the animal being enclosed in its own shelly envelope, a supplemental skeleton, and a radial, vertical and annular system of canals, prove that the two animals belong to essentially different families of Foraminifera. There are many instances, especially in the Rotaline group, of isomorphism accompanied with generic difference; thus no reliance can be placed on variety of external form, unaccompanied by change of internal structure.

An attempt has been made in the preceding pages to describe a few species most characteristic of some of the genera of this multitudinous class; and of those selected a mere sketch of the most prominent features of the animal and its abode is given, that some idea may be formed of the wonderfully complicated structure of beings, which are mostly microscopic specks. Yet the most minute circumstances in the forms of the animals and their shells, with their varieties and affinities, have been determined with an accuracy that does honour to microscopic science.

They are now arranged in a natural system by William B. Carpenter, M.D. F.R.S. assisted by William K. Parker, Esq., and T. Rupert Jones, Esq., and published in the Transactions of the Ray Society in 1862. To this admirable work, the author is highly indebted.

It was known that different types of Foraminifera abound at different depths on the coasts of the ocean; but it was long believed that no living creature could exist in its dark and profound abyss. By deep-sea sounding, it has been ascertained that the basin of the Atlantic Ocean is a profound and vast hollow or trough, extending from pole to pole; in the far south, it is of unknown depth, and the deepest part in the north is supposed to be between the Bermudas and the Great Banks of Newfoundland. But by a regular series of soundings made by the officers of the navies of Great Britain and the United States, for the purpose of laying a telegraphic cable, that great plain or steppe was discovered, now so well known as the telegraphic plateau, which extends between Cape Race in Newfoundland, and Cape Clear in Ireland. From depths of more than 2,000 fathoms on this plateau, the ooze brought up by the sounding machine consisted of 97 per cent. of Globigerinæ. The high state of preservation of these delicate shells was no doubt owing to the perfect tranquillity which prevails at great depths; for the telegraphic plateau and the bed of the deep ocean everywhere is covered by a stratum of water unruffled by the commotion raised by the hurricane which may be raging on the surface. The greater number of the Globigerinæ were dead empty shells; but although in many the animal matter was quite fresh, Professor Bailly of New York could not believe that such delicate creatures could live on that dark sea bed, under the pressure of a column of water more than 2,000 fathoms high, a weight equal to rather more than that of 340 atmospheres or 5,100 lbs. on every square inch of sea-bed; wherefore he concluded that the tropical ocean and the Gulf Stream, which absolutely swarm with animal life, must have been the birth-place and home of these minute creatures, and that this mighty ‘ocean river,’ which divides at the Great Banks of Newfoundland, and spreads its warm waters like a fan over the north Atlantic, deposits their remains over its bed, which has thus been their grave-yard for unknown periods, and which, in the lapse of geological time, may be raised above the waves as dry land.

Professor Ehrenberg on the contrary concluded that residentiary life exists at the bottom of the ocean, both from the freshness of the animal matter found in the shells, and from the number of unknown forms which are discovered from time to time at various and often great depths along the coasts. This opinion has been confirmed beyond a doubt on several occasions, especially by Dr. Wallich, who accompanied an expedition sent under the command of Sir Leopold M‘Clintock, to sound the North Atlantic for laying a telegraphic line.

In doing that two operations are requisite. The first is to ascertain the depth: when that is known, the nature of the sea-bed must be determined, and on that account a sample of it is then sounded for; but owing to the difficulty of ascertaining the exact time at which the ground is struck, a quantity of rope in excess of the depth is given out, which lies on the bottom of the sea while the machine is being drawn up, which occupies a considerable time when the depth is great. About midway between Greenland and the north of Ireland, when the machine was hauled up from a depth of a mile and a half, several starfish were clinging with their long spiny arms to fifty fathoms of the rope that had been lying on the surface of the sea-bed while the machine was being drawn up, and to that part of the rope alone. They continued to move their limbs energetically for more than a quarter of an hour after they were out of the water. They certainly had not been entangled in the line while swimming, because star-fishes are invariably creeping animals. The deposit on which they had rested at the bottom of the ocean contained ninety-five per cent. of Globigerinæ. Abundance of these minute Foraminifera were found in the stomachs of the starfish; which seemed to prove not only that the starfish were caught on their natural feeding ground, but that their food was living organisms whose normal abode is the surface of the bed of the deep ocean.

Dr. Wallich also discovered in the ooze brought up from a depth of nearly two miles and a quarter a number of small bodies from ¹⁄₁₆ to ¹⁄₄ of an inch in length and about a line in breadth. They consisted of equal globes arranged in a straight line like the Nodosaria, or built up, each lying on part of the one below it, and increasing in size from the uppermost about ¹⁄₁₂₅₀ to the undermost about ¹⁄₄₅₀ of an inch in diameter. Both of these forms, called coccospheres, consisted of sarcode enclosed in a calcareous deposit; and were studded at nearly regular distances by minute round or oval bodies concave below, and with an aperture on their convex surface sometimes single, sometimes double. These coccospheres were also found free in the ooze, and had been seen previously by Capt. Dayman. They have likewise been seen as free organisms living on the surface of the ocean.

The ooze in the bed of the Atlantic ocean, as well as of the Mediterranean and Adriatic contains fifty per cent. of Globigerinæ; they exist in the Red Sea, in the vicinity of the West Indian Islands, on both sides of South America and near the Isle of France, but not in the Coral Sea which is occupied by different genera. Though in utter darkness, at the bottom of a deep ocean, these little creatures can procure food by means of their pseudopodia, whose extreme sensibility makes up for the want of sight; and the very excess of pressure under which they live insures them a supply of oxygen at depths to which free air cannot penetrate, for it is believed that the quantity of dissolved air that water contains is in proportion to the pressure.

Fossil Foraminifera enter so abundantly into the sedimentary strata, that Buffon declared ‘the very dust had been alive.’ 58,000 of these fossil shells have been computed in a cubic inch of the stone of which Paris and Lyons are built. The remains of these Rhizopods are for the most part microscopic. M. D’Orbigny estimated that an ounce of sand from the Antilles contained 1,800,000 shells of Foraminifera. A handful of sand anywhere, dry sea-weeds, the dust shaken from a dry sponge, are full of them.

When the finer portions of chalk amounting to one half or less are washed away, the remaining sediment consists almost entirely of the shells of Foraminifera, some perfect, others in various stages of disintegration. In some of the hard limestones and marbles, the relics of Foraminifera can be detected in polished sections and in thin slices laid on glass. It is now universally admitted that some crystallized limestones which are destitute of fossil remains, had been originally formed by the agency of animal life, and subsequently altered by metamorphic action; the opinion is gradually gaining ground among geologists that such is the history of the oldest limestones.

At certain geological periods circumstances favoured the development of an enormous multitude of individual animals. In the earlier part of the Tertiary period the Nummulites acquired an extraordinary size. They were like very large coins two or more inches in diameter, and were accumulated in such quantities as to constitute the chief part of the nummulitic limestone; a formation in some places 1,500 feet thick, which extends through southern Europe, Libya, Egypt, Asia Minor, and is continued through the Himalayan mountains into various parts of the Indian peninsula, where it is extensively distributed. The Great Pyramid of Egypt is built of this limestone, which gave rise to singular speculations with regard to the Nummulites in very ancient and even in more recent times. Although this is incomparably the greatest, it is by no means the only instance of an accumulation of the fossil shells of individual animals. The ‘Lingula flags,’ a stratum in the upper Cambrian series of North Wales, was so named from the abundance of the Brachiopod Lingula that it contains.

Professor Ehrenberg discovered that the shells of the Foraminifera sometimes undergo an infiltration of silicate of iron, which fills not only the chambers, but also their canal-system even to its minutest ramifications, so that if the shell be destroyed by dilute acid, a perfect cast of the sarcode matter remains. The greensands in the different geological strata from the Silurian formation upwards, are chiefly composed of these casts; and Professor Baily of the United States more recently discovered that a process of infiltration is even now taking place in some parts of the ocean bed, and that beautiful casts of Foraminifera may be obtained by dissolving their shells with dilute acid.

A most extensive comparison of the Foraminiferous group of Rhizopods, recent and fossil, has been made by Messrs. Parker and Rupert Jones from almost every latitude on the globe, from the arctic and tropical seas, from the temperate zones in both hemispheres, and from shallow as well as deep-sea beds. They have also reviewed the fossil Foraminifera in their manifold aspects as presented by the ancient geological faunas throughout the whole series from the Tertiary down to the Carboniferous strata inclusive; and have come to the astonishing conclusion that scarcely any of the species of the Foraminifera met with in the secondary rocks have become extinct. All that they had seen have their counterparts in the recent Mediterranean deposits. Throughout that long series of geological epochs even to the present day, the Foraminifera show no tendency to rise to a higher type; but variety of form in the same species prevailed then as it does now.

Subsequently to this investigation, a gigantic Orbitulite twelve inches in diameter, and the third of an inch thick, has been found in the Silurian strata in Canada. The largest recent species Dr. Carpenter had seen was about the size and thickness of a shilling.

The lowest stratum of the Cambrian formations has been regarded as the most ancient of the Palæozoic rocks; now, however, strata of crystallized limestone near the base of the Laurentian system, which is 50,000 feet thick in Canada, are discovered by Sir W. E. Logan to have been the work of the Eozoön Canadense, a gigantic Foraminifer, at a period so inconceivably remote that it may be regarded as the first appearance of animal life upon the earth. In a paper published by Dr. Carpenter, in May 1865, he expressed his opinion that the Eozoön would be found in the older rocks of central Europe; and in the December following he received specimens from the fundamental quartz rocks of Germany, in which he found undoubted traces of the Eozoön. Here the superincumbent strata are 90,000 feet thick; the transcendent antiquity of the Eozoön is therefore beyond all estimation.

The fossil Eozoön consists of a succession of parallel rows or tiers of chambers, in which the sarcode of the living animal had been replaced by a siliceous infiltration, so that when the calcareous shell was destroyed by dilute acid, the cast was found to be precisely like that of a Nummulite; thin slices of it taken in different directions being examined with a microscope, it was found that the siliceous matter had not only filled that portion of the chambers which had been occupied by the sarcode-body of the animal and the canal-system, but had actually taken the place of the pseudopodial threads, the softest and most transitory of living substances, which were put forth through tubuli in the shell-walls of less than the ¹⁄₁₀₀₀₀ part of an inch in diameter. ‘These are the very threads themselves turned into stone by the substitution which took place, particle by particle, between the sarcode body of the animal and certain constituents of the water of the ocean, before the destruction of the sarcode by ordinary decomposition.’[^[9]] The shell had an intermediate skeleton, but the minute tubes in the walls of the chambers are so characteristic of the Nummulites, that they were sufficient alone to determine the relationship of the Eozoön to its modern representative.

The external shape and limits to the size of the individual Eozoön have not been determined with certainty, on account of its indefinite mode of growth, and the manner in which the fossilized masses are connected with the highly crystalline matrix in which they are imbedded; there is no doubt, however, that they spread over an area of a foot or even more, and attained a thickness of several inches. As they seem to have increased laterally by buds which never fell off, they formed extensive reefs; at the same time they had a vertical growth, for in some of the reefs the older portions appear to have been fossilized before the newer were built up on them as a base, exactly like the coral reefs in the tropical ocean of the present day,[^[10]] with this difference however, that shells and other crustaceans are associated with the corals, while no organic body has been found in the Eozoön reefs; nevertheless the Eozoön must have had food. It may therefore be inferred that parts at least of that primeval ocean swarmed with animal life, whose remains have been obliterated by metamorphic action. Carbon (which in the form of graphite both constitutes distinct beds, and is disseminated through the siliceous and calcareous strata of the Laurentian series, as well in Norway as in Canada), may indicate the existence of vegetation in the Eozoön period.

The Eozoön is by no means confined to Canada and central Europe. The serpentine marble of Tyree which forms part of the Laurentian system on the west of Scotland, and a similar rock in Skye, when subjected to minute examination, are found to present a structure clearly identical with that of the Canadian Eozoön. And the like structure has been discovered by Mr. Sanford in the serpentine marble of Connemara, known as Irish green. The age of that rock however, is doubtful: for when it was discovered to contain Eozoön, Sir Roderick Murchison who had previously studied its relations was at first inclined to believe it belonged to the Laurentian series; now however, he considers the Connemara marble to be of the Silurian age. ‘If this be the case it proves that the Eozoön was not confined to the Laurentian period, but that it had a vast range in time, as well as in geographical distribution; in this respect corresponding to many later forms of Foraminifera which have been shown by Messrs. Parker and Rupert Jones to range from the Trias to the present epoch.’[^[11]]

The Carpenteria found in the Indian seas forms a link between the Foraminifera and Sponges. The shell is a minute cone adhering to the surface of corals and shells, by its wide base which spreads in broad lobes. Double-walled chambers and canals form a spiral within it, and are filled with a spongy sarcode of a more consistent texture than the sarcode of the Foraminifera, which in the larger chambers is supported by siliceous spicules similar to those which form the skeletons in sponges.