The Protozoa are the very lowest forms of animal existence, the beginning and dawn of living things. They first appear as minute shapeless particles of semi-fluid sarcode moving on the surface of the waters. The pseudopodia, or false feet, with which they move, are merely lobes of their own substance which they project and retract. In creatures of a somewhat higher grade the form is definite, the pseudopodia, numerous and filamental, serving for locomotion and catching prey; and from the resemblance they bear to the slender roots of plants are called Rhizopods.^[3] The microscopic organisms possessing these means of locomotion and supply, are of incalculable multitudes, and of innumerable forms. Thus the waters, as of old, still ‘bring forth abundantly the moving creature that hath life;’ in them the lowest types of the two great kingdoms have their origin, yet they are diverse in the manifestation of the living principle, that slender but decided line which separates the vegetable from the animal Amœba.

Class I.—Rhizopoda.

The Amœba, which is the simplest of the group, is merely a mass of semi-fluid jelly, ‘changing itself into a greater variety of forms than the fabled Proteus, laying hold of its food without members, swallowing it without a mouth, digesting it without a stomach, appropriating its nutritious material without absorbent vessels or a circulating system, moving from place to place without muscles, feeling (if it has any power to do so) without nerves, multiplying itself without eggs, and not only this, but in many instances forming shelly coverings of a symmetry and complexity not surpassed by those of any testaceous animal.’

Such is the description given by Dr. Carpenter of the Amœba and its allies. The Amœba princeps, which is the type of the naked group, fig. 86, is merely a shapeless mass of semi-fluid sarcode, coated by a soft, pellucid and highly contractile film, called diaphane by Mr. W. J. Carter, and in many forms of Amœba the whole is inclosed in a transparent covering. It is in the interior semi-fluid sarcode alone, that the coloured and granular particles are diffused, on which the hue and opacity of the body depend, for the ectosarc or external coat is transparent as glass. These creatures, which vary in size from the ¹⁄₂₈₀₀ to the ¹⁄₇₀ of an inch in diameter, are found in the sea, but chiefly in ponds inhabited by fresh-water plants. They move irregularly over the surface of the water, slowly and continually changing their form by stretching out portions of their gelatinous mass in blunt finger-like extensions, and then drawing the rest of it into them; thus causing the whole mass to change its place. Before it protrudes these pseudopodia or false feet, there is a rush of the internal semi-fluid matter to the spot, due to the highly contractile power of the diaphane, which is often so thin and transparent as to be scarcely perceptible.

When the creature in its progress meets with a particle of food, it spreads itself over it, draws it into its mass, within which a temporary hollow or vacuole is made for its reception; there it is digested, the refuse is squeezed out through the external surface; the nutritious liquid that is left in the vacuole seems to be dispersed in the sarcode, for the vacuole disappears. An Amœba often spreads itself over a Diatom, draws it into a vacuole newly made to receive and digest it; the siliceous shells of the diatom are pushed towards the exterior, and are ultimately thrust out; then the vacuole disappears, either immediately or soon after. These improvised stomachs are the earliest form of a digestive system.

Besides the vacuoles of which there may be several at a time, the slow and nearly rhythmical pulsations of a vesicle containing a subtle fluid may be seen, which changes its position in the interior of the sarcode with every motion of the Amœba. It gradually increases in size, then diminishes to a point, and as some of the digestive vacuoles nearest the surface of the animal are observed to undergo distension when the vesicle contracts, and to empty themselves gradually as it fills, Dr. Carpenter thinks it can hardly be doubted that the function of the vesicle is to maintain a continual movement of nutritious matter, among a system of channels and vacuoles excavated in the substance of the body. It is the first obscure rudiment of a circulating system.

In all the Amœbæ the semi-fluid sarcode, with the numerous bodies suspended in it, rotates at a varied rate within the pellucid coat; a motion presumed to be for respiration, that is to exchange carbonic acid gas for oxygen, so indispensable for animal life.^[4]

Although like other animals, the Amœba cannot change inorganic into organic matter, as the vegetable Amœba can do, these two Protozoa are similar in one mode of reproduction; for portions of the animal Amœba or even one of the pseudopodia separate from the gelatinous mass, move to a little distance on the surface of the water, and become independent Amœbæ.

With a high microscopic power, many bodies besides the digesting vacuoles and pulsating vesicles may be seen imbedded in the sarcode of the Amœba princeps; namely, coloured molecules, granules, fat-globules, and nuclei. All these bodies were seen by Mr. Carter, in certain Amœbina he found at Bombay, together with what he believed to be female reproductive cells, and motile particles similar to spermatozoids, or male fertilizing particles.

The Actinophrys, a genus of the order Radiolaria, differs from the Amœba princeps in having a definite nearly spherical form with slender root-like filamental pseudopodia radiating from its surface in all directions as from a centre. They taper from the base to the apex, and sometimes end in knobs like a pin’s head, but vary much in length and number, and can be extended and retracted till they are out of sight. They are externally of a firmer substance than the sarcode of the body, which is merely a viscid fluid inclosed in a pellucid film. The Actinophrys sol, which is the type of the genus, is a sphere of from ¹⁄₁₃₀₀ to ¹⁄₆₅₀ of an inch in diameter, with slender contractile filaments the length of its diameter extending from its surface as rays from the sun. It can draw them in and flatten its body so as to be easily mistaken for an Amœba. This creature, which is common in fresh-water pools where aquatic plants are growing and even in the sea, has little power of moving about like the Amœba; it depends almost entirely on its pseudopodia for food. They have an adhesive property, for when any animalcule or diatom comes in contact with one of them, they adhere to it; the filament then begins to retract, and as it shortens the adjacent filaments apply their points to the captive, enclose it, coalesce round it, the whole is drawn within the surface of the Actinophrys, the captive is imbedded in the sarcode mass, and passes into a vacuole where it is digested, and then the pseudopodia thrust out the undigested matter by a process exactly the reverse of that by which the food was taken in (D fig. 87). The pseudopodia are believed by Professor Rupert Jones to have the power of stunning their prey, for if an animalcule be touched by one of them, it instantly becomes motionless, and does not resume its activity for some time. The pulsations of the contractile vesicle are very regular, and its duty is the same as in the Amœba princeps.

The Actinophryna are propagated like the lowest vegetables by gemmation and conjugation, shown in B fig. 87; moreover Mr. Carter saw the production of germ-cells and motile particles in the Actinophrys exactly after the mode already described in the Amœba.

Mr. Carter mentions an instance in which the Actinophrys sol showed what may possibly be a certain degree of instinct. An individual was in the same vessel with vegetable cells charged with particles of starch; one of the cells had been ruptured and a little of the internal matter was protruded through the crevice. The Actinophrys came, extracted one of the starch-grains, and crept to a distance; it returned, and although there were no more starch-grains in sight, the creature managed to take them out from the interior of the cell one by one, always retiring to a distance and returning again, showing that it knew its way back, and where the starch-grains were to be found. On another occasion Mr. Carter saw an Actinophrys station itself close to the ripe spore cell of a plant, and as the young zoospores came out one after another, the Actinophrys caught every one of them even to the last and then retired to a distance as if instinctively conscious that no more remained. Like Amœbæ these animals select their food, but notwithstanding the superior facility and unfailing energy with which they capture prey larger and more active than themselves, they are invariably overcome even by a very small Amœba which they avoid if possible. When they come into contact the Amœba shows unwonted activity, tries to envelope the Actinophrys with its pseudopodia, but failing to capture the whole animal it tears out portions and conveys them to improvised vacuoles to be digested. Dr. Wallich mentions that he had seen nearly the half of a large Actinophrys transferred piecemeal to the interior of its enemy, where it was quickly digested.

As every part of the body of the Actinophrys is equally capable of performing the part of nutrition, respiration, and circulation; and as in the absence of muscles and nerves they may be presumed to have no consciousness, the marks of apparent intelligence can only be attributed to a kind of instinct, and their motions to the vast inherent contractility of the sarcode and its enclosing film, which is also the case with the Amœbæ.

The Acanthometræ (see fig. 88, Acanthometra bulbosa) are all marine animals; their skeleton consists of a number of long spicules which radiate from a common centre, tapering to their extremities. These spicules are traversed by a canal with a furrow at the base through which groups of pseudopodia enter, emerging at the apex. Besides, there are a vast number of pseudopodia not thus enclosed, resembling those of the Actinophrys in appearance and action. The body is spherical, and occupies the spaces left between the bases of the spicules. The exterior film covering the body seems to be more decidedly membranaceous than that of the Actinophrys, but it is pierced by the pseudopodia which radiate through it. This exterior film itself is enclosed in a layer of a less tenacious substance, resembling that of which the pseudopodia are formed. There is a species of Acanthometra (echinoides) extremely common in some parts of the coast of Norway, which, to the naked eye, resembles merely a crimson point.

The Polycystina are an exceedingly numerous and widely dispersed group of siliceous rhizopods. They are inhabitants of the deep waters, having been brought up from vast depths in the Atlantic and Pacific oceans. Their bodies are inclosed in siliceous shells, which have either the form of a thin hollow sphere perforated by large openings like windows, or of a perforated sphere produced here and there into tubes, spines, and a variety of singular projections: so they have many varied but beautiful microscopic forms. The animal which inhabits these shells is a mouthless mass of sarcode, divided into four lobes with a nucleus in each and covered with a thick gelatinous coat. It is crimson in the Eucyrtidium and Dictyopodium trilobum of Haeckel (figs. 89 and 90): in others, as the Podocyrtis Schomburgi, it is olive brown with yellow globules (fig. 91). These creatures extend themselves in radiating filaments through the perforations of their shells in search of food, like their type the Actinophrys sol, to whose pseudopodia the filaments are perfectly similar in form, isolation, and in the slow movements of granules along their borders. The Polycystine does not always fill its shell, occasionally retreating into the vault or upper part of it, as in the Eucyrtidium (fig. 89, frontispiece to vol. i.). Sometimes the shell is furnished with radiating elongations, as in the Dictyopodium trilobum (fig. 90). In both of these shells the animal consists of four crimson lobes. These beautiful microscopic organisms are found at present in the Mediterranean, in the Arctic and Antarctic seas, and on the bed of the North Atlantic. They had been exceedingly abundant during the later geological periods; multitudes are discovered in the chalk and marls in Sicily, Greece, at Bermuda, at Richmond in Virginia and elsewhere; in all 282 different fossil forms have been described, grouped in 44 genera.

In certain Polycystina, the perforations of the shell are so large and so close together, that the sarcode body of the animal appears to be covered by a siliceous net. This connects them with the Thalassicollæ, minute creatures found passively floating on the surface of the sea. Th. morum, which is one of the most simple of the few forms known, has a spherical body of sarcode covered with a siliceous net, through which the pseudopodia radiate in all directions, as in the Actinophrys, but it is studded at regular distances with groups of apparently radiating siliceous spicules.

The Aulocantha scolymantha (fig. 92), found by M. Haeckel in the Mediterranean, may be taken as an example of the most general form of the Thalassicolla. The siliceous skeleton of some of the Radiolaria resembles the Chinese ivory toy of ball within ball. That of the Actinomma drymodes (fig. 93) consists of three perforated concentric spheres, with six strong spicules attached to the outer surface, perpendicular to one another and prolonged in the interior to the central sphere. Hundreds of finer bristle-like spicules radiate from the surface. The animal is chiefly contained in the central sphere, and from it a perfect forest of fine, long pseudopodia radiate in thick tufts through the apertures of the exterior sphere.

The skeleton of the Haliomma (fig. 94) consists of only two concentric spheres. In many species of Haliomma and Actinomma the animals are of the most vivid vermilion or purple colour. Little or nothing is known of the reproduction of these microscopic organisms.

The Actinomma drymodes and the Haliomma are two of the most beautiful microscopic rhizopods discovered by M. Haeckel.

There is a family of fresh-water testaceous rhizopods of which one group secretes its shell and the other builds it. The horny shell secreted by the group of the Arcella presents various degrees of plano-convexity, the convexity in some cases amounting to a hemisphere. They rarely, if ever, have mineral matter on their surface, which is studded with regular but very minute hexagonal reticulations. The aperture or mouth of the shell is small, and invariably occupies the centre of the plane surface, its margins being more or less inverted. The form of the shell is exceedingly varied, sometimes it even has horns indefinite in number, sometimes symmetrical, sometimes not; when its test or covering becomes too small for its increasing size, it quits it, and secretes a new one. The filamental pseudopodia proceed from the mouth of the shell only, and by means of these it creeps about on its mouth in search of food.

The Difflugia build their own shells, which are usually truncated spheres, ovate, or sometimes elongated into the form of a pitcher or flask. The most minute recognisable of these shells is about the ¹⁄₁₀₀₀ of an inch in diameter, but they are constructed with the most perfect regularity. The Difflugia pyriformis or symmetrica has the form of an egg with an aperture at the small end. It is entirely made up of rectangular hyaline plates, arranged with the greatest regularity in consecutive transverse and longitudinal rows, the smaller ones being at the extremities, while the larger ones occupy the central and widest portion of the structure. The inhabitant of this abode is an Amœba with a sarcode body covered with a thin film, from whence it sends off pseudopodia through the mouth of its shell. The Difflugia is propagated by conjugation, but before that takes place it becomes densely charged with chlorophyll-cells and starch-grains. The former disappear during the subsequent changes, and are replaced by a mass of colourless cells full of granules which are supposed to be the elements of a new generation. The embryo or earliest form is a minute truncated sphere, but the animal builds up its habitation very much according to local circumstances.

The greater number of the Difflugiæ secrete a substance which forms a smooth layer in the interior, which the animal covers with sarcode from its mouth, and then it drags itself with its pseudopodia to the particles which it selects, and they adhere to it. The particles selected are invariably mineral matter. ‘The selective power is carried to such an extent that colourless particles—sometimes quartzose, sometimes felspathic, sometimes micaceous—are always chosen.’ ‘The particles seem to be impacted into the soft matter, laid on the exterior in the same way that a brick is pressed into the yielding mortar, and that too, in so skilful a manner as to leave the smallest possible amount of vacant area; whilst in the specimens of Difflugia in which tabular or micaceous particles are used, they are sometimes disposed with such nicety that there is no overlapping, but the small fragments are placed so as to occupy the space left between the larger ones. These excellent architects seem to know that in the valves of the Diatoms are combined the properties best suited to their wants, that is, transparency and form, capable of being easily arranged.’

Both the Difflugia and Arcella are Amœbæ in the strictest sense of the word; their bodies consist of sarcode, which sends out finger-like lobes from the mouth of the shell at one end, while the other end has an adhesive property, which fixes it to the bottom. The nucleus and contractile vesicles are identical in character with those of the Amœbæ, and exhibit the same tendency to subdivision at certain periods of the creature’s history that is witnessed on a large scale in the Amœba proper; and the reproductive process is the same.^[5]

The Difflugiæ are found in rivulets and pools containing aquatic plants; the condition of the water and the nature of the soil have a great influence on the form of their shell.

The Euglyphæ is the third group of fresh-water rhizopods. They are extremely minute, and there are no mineral particles whatever on their shells, the axes of which do not coincide with the aperture. The interior of the animal is like that of the Arcella and Difflugia, but it differs from them in as much as the pseudopodia and ectosarc, or external coat, are finely granular, and the whole mass of the body possesses a decided degree of adhesive viscidity. The pseudopodia are filiform, tapering, radiating, and readily coalesce; and ‘as if to compensate for the restricted power of locomotion, compared with that of the Amœba proper, the pseudopodia of the Euglyphæ are much more active. The rapidity with which they admit of being projected outwards, and withdrawn into the shell, is unequalled in any other form, presenting the most wonderful example of inherent contractility in an amorphous animal substance, that is to be met with in either of the great organic kingdoms.’^[6]

The order Reticularia, with a very few exceptions, are animals dwelling in calcareous microscopic shells, and differing essentially in constitution from all the preceding Rhizopods. The ectosarc or surface-layer of the sarcode in the Amœba and Actinophrys has so much consistence, that their pseudopodia, which are derived from it, have a decidedly firm outline and never coalesce; whereas in the order Reticularia, the sarcode is merely a semi-fluid protoplasm or colourless viscid fluid, without the smallest surface-layer or film, so that their pseudopodia possess no definiteness either in shape, size or number. Sometimes they are cylindrical, and sometimes form broad flat bands, whilst they are often drawn into threads of such extreme tenuity, as to require a high magnifying power to discern them. They coalesce and fuse into each other so freely and so completely when they meet, that no part of their substance can be regarded as having more than a viscous consistence. Their margins are not defined by continuous lines, but are broken by granules irregularly disposed among them, so that they appear as if torn; and these granules, when the animal is in a state of activity, are in constant motion, passing along the pseudopodia from one end to the other, or passing through the connecting threads of this animated network from one pseudopodium to another, with considerable rapidity, analogous to the movement of the particles in the cells of the hairs of the Tradescantia and other plants.^[7]

The sarcode body of the Gromiæ is inclosed in a yellowish brown horny envelope or test of an oval shape, with a single round orifice of moderate size, through which the pseudopodia extend into the surrounding water, some forms of the animal being marine, others inhabitants of fresh water. When the animal is at rest all is drawn within the test, and when its activity recommences, single fine threads are put out which move about in a groping manner until they find some surface to which they may attach themselves. When fixed, sarcode flows into them so that they rapidly increase in size, and then they put forth finer ramifications, which diverging come in contact with those from other stems, and by mutual fusion form bridges of connection between the different branching systems; for the protoplasm spreads over the exterior of the test, and from it pseudopodia extend and coalesce, wherever they meet, so that the whole forms a living network, extending to a distance of six or eight times the length of the body. Fig. 96 represents the Gromia oviformis with its pseudopodia extended.

In the Gromiæ the granular particles in the semi-fluid protoplasm are in constant motion. In the finer filaments there is but one current, and a particle may be seen to be carried to the extremity, and return again bringing back with it any granules that may be advancing; and should particles of food adhere to the filament they take part in the general movement. In the broader filaments two currents carrying particles pass backwards and forwards in opposite directions at the same time, and the network in which these motions are going on is undergoing continual changes in its arrangements. New filaments are put forth sometimes from the midst of the ramifications, while others are retracted; and occasionally a new centre of radiation is formed at a point where several threads meet. The food consists of diatoms and morsels of vegetable matter; but the Gromiæ have no vent, so that the indigestible matter collects in a heap within them. However, as the form of the test is such that the animal cannot increase its size, it leaves it when it becomes too small for its comfort and forms another, and it is supposed to get rid of the effete matter at the same time. The Gromiæ have no nucleus or contractile vesicle.

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.

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 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.