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 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 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.

Class III.—Sponges.

According to the observations of Mr. Carter, sponges begin their lives as solitary Amœbæ which grow by multiplication into masses, and assume endless forms according to the species; turbinate, bell-shaped, like a vase, a crater, a fan, flat, foliaceous and lobed or branching and incrusting the surface of stones. All the Amœbæ are so connected as to form one compound animal. The whole substance of a sponge is permeated by innumerable tubes which begin in small pores on the surface, and continually unite with one another as they proceed in their devious course to form a system of canals increasing in diameter and ending in wide openings called oscula, on the opposite side of the mass. Currents of water enter through the pores on the surface, and bring minute portions of food which are seized upon by a vast multitude of Amœbæ with long cilia which form the walls of the tubes and canals; and after they have extracted the nutritious part, the offal is carried into the sea through the oscula, by the current of water whose flux is maintained by the vibrations of the cilia. In the compressed and many of the tubular sponges the water passes through them in a straight line; in branched and encrusting sponges, the afferent and efferent openings are on the same surface. The water is inhaled continuously and gently like an animal breathing, but it is rapidly and forcibly ejected; and in its passage it no doubt furnishes oxygen to aërate the juices of the compound animal, whose flesh or sarcode is irritable while alive, and which has the power to open and shut the pores and oscula of the canals, for the whole sponge forms one compound creature whose mass is nourished by the myriads of Amœbæ of which it is constituted.

Within the animated sarcode mass of the sponges there is in most cases a complicated skeleton of fibrous network, either horny, calcareous, or siliceous, which supports the soft mass, and determines its form.

Besides the skeleton, the mass of sponges is for the most part strengthened and defended by siliceous, and more rarely by calcareous, spines or spicules, either imbedded among the fibres of the skeleton, or fixed to them by their bases. The fibres of the skeleton network always unite, whether they be horny, calcareous, or siliceous; the spicules never, though they often lie in confused heaps over one another. They are of innumerable forms and arrangements. Some are like long needles lying close together in bundles, pointed or with a head like a pin at one or both ends; a great number are stellate with long or short rays; there may even be several different forms in the same sponge. Many calcareous sponges have cavities full of organic matter; and when the calcareous matter is dissolved by dilute acid, the organic base is left.

The common commercial sponges have a skeleton which consists of a network of tubular, horny, tough, and elastic fibres which cross in every direction. They have no spicules or very few; and when such do project from the horny skeleton, they are generally conical, attached by their bases, and their surface is often beset with little spines arranged at regular intervals, which gives them a jointed appearance. The common sponge which is so abundant in the Mediterranean has many forms; those from the coast of North America are no less varied, but that most used in the United States is turbinate, concave, soft, and tomentose.

In the calcareous sponges a mass of three-rayed spicules surround the interior canals, where they are held together by a cartilaginous substance which is wanting in the horny sponges, but which remains in this order after the destruction of the more delicate matter when the sponge is dried.^[12] The pores are also occasionally defended by the projecting points of half buried spines.

In nearly every species of this order the pores on the surface are protected by spicules; and they are also projected from the surface of the large cloacal cavity, and curved towards its opening, to defend it from Annelids and other enemies.^[13] Some species have a long ciliary fringe at the orifice of the cavity, through which the water may pass out, but no animal can come in.

The spicula and skeleton of most of the marine sponges are siliceous and singularly beautiful; the skeleton of the Dactylocalyx pumiceus of Barbadoes is transparent as spun glass; and a species from Madagascar has numerous simple transparent and articulated spicules implanted in the siliceous fibres of the skeleton. The Cristata, Papillaris, Ovulata, and many more have siliceous skeletons, some garnished with spicules of various forms, and the surface occasionally covered with a layer of siliceous granules.

The variety in the size, structure, and habits of the marine sponges is very great: temperate and tropical seas have their own peculiar genera and species; some inhabit deep water, others live near the surface, while many fix themselves to rocks, sea-weeds, and shells, between high and low water mark. There are very few dead oyster, whelk, scallop, and other shells that escape from the ravages of the Cliona, an extremely minute burrowing sponge of the simplest structure, which has a coat of siliceous spicules supposed to be the tools with which it tunnels a labyrinth through the mid-layer of the shell, in a pattern that varies with the species of the sponge. A communication is formed here and there with the exterior by little round holes, through which the sponge protrudes its yellow papillæ. From the force exhibited by this little sponge, it may perhaps be inferred to possess a rudimentary muscle and nerve.^[14]

Sponges are propagated twice in the year by minute ciliated globules of sarcode, detached from the interior of the aquiferous canals, which swim like zoospores to a distance, come to rest, and lay the foundation of new sponges. The little yellow eggs of Halichondria panicea are lodged in the interstices between the interior canals; when mature, they are oval and covered with cilia, and are carried out by the currents; and after swimming about for some days fix on a solid object, become covered with bristles, spread out into a transparent film, charged with contractile vesicles of different sizes in all degrees of dilatation and contraction, as well as with sponge ovules. Spicules are developed at the same time, and these films ultimately become young sponges, and if two happen to meet they unite and are soldered together.^[15] Besides eggs, larger bodies covered with radiating spicules are produced, containing granular particles of sarcode, each of which when set free by the rupture of the envelope, becomes an Amœba-like creature, and ultimately a sponge.

Fresh-water sponges are sometimes branched, and sometimes spread over stones, wood, and other substances; and one species covers an earthy mass some inches thick formed by its own decayed matter. The skeleton of such species as have one, consists of bundles of siliceous spicules, held together and mixed with groups of needles, the rods of which project through the surface of the sponge and render it spinous. The motions in the gelatinous sarcode mass are the most remarkable feature in the fresh-water sponges, which all belong to the genus Spongilla. Mr. Carter observed that portions of the surface of some individuals of the Spongilla fluviatilis in his aquarium had long cilia by means of which they rapidly changed their places during the spring, but when winter came they emitted processes from such parts of their surfaces as were free from cilia and retracted them again just like Amœbæ. These portions often had cells, and when the Amœba-like motions ceased, a nucleus and nucleolus appeared within them, and at last the whole gelatinous sarcode mass consisted of these cells or globules. Some had no nucleus, but were filled with green or colourless granules.

At certain seasons of the year, whatever the form of the fresh-water sponges may be, a multitude of minute hard yellow bodies are produced in their deeper parts. They consist of a tough coat containing radiating spicules like a pair of spoked wheels united by an axle with a pore in its surface. Within this last there is a mass of motionless granular cells, and when put into water the cells come out at the pore and give rise to new sponges.

Insulated groups of germs covered with cells called swarm-cells seem to form parts of the sponges; they lie completely within the mass of the living sponge. They have the form of a hen’s egg, are visible to the naked eye, and when they come into the water they swim in all directions for a day or two; become fixed; a white spot within is enlarged; and the constituents of young sponges appear.^[16]

The generic forms of fossil sponges augment in number and variety from the Silurian to the Cretaceous beds, where the increase is rapid; but all the sponges which had a stony reticulated form without spicules passed away with the Secondary epoch, so that the family has no representatives in the Tertiary deposits or existing seas. The calcareous sponges which abound in the Oolite and Cretaceous strata, and attain their maximum in the Chalk, are now almost extinct, or are represented by other families with calcareous spicules. Siliceous fossil sponges are particularly plentiful. In England extensive beds of them occur in the Upper Greensand, and in some of the Oolitic and Carboniferous Limestones; and some beds of the Kentish Rag are so full of their siliceous spicules, that they irritate the hands of the men who quarry them. Since every geological formation except the Muschelkalk is found in England, the number and variety of fossil sponges are very considerable. The horny sponges are more abundant now than they were in the former seas. According to M. D’Orbigny the whole number of fossil sponges known and described amount to thirty-six genera and 427 species, which is probably much below the real number.^[17]

Class IV.—Infusoria.

The Infusoria, which form the second group of the Protozoa, are microscopic animals of a higher grade than any of the preceding creatures, although they go through their whole lives as isolated single cells of innumerable forms. They invariably appear in stagnant pools and infusions of animal and vegetable matter when in a state of rapid decomposition. Every drop of the green matter that mantles the surface of pools in summer teems with the most minute and varied forms of animal life. The species called Monas corpusculus by the distinguished Professor Ehrenberg, has been estimated to be ¹⁄₂₀₀₀ part of a line in diameter. ‘Of such infusoria a single drop of water may contain 500,000,000 of individuals, a number equalling that of the whole human species now existing upon the face of the earth. But the varieties in size of these animalcules invisible to the naked eye are not less than that which prevails in almost any other natural class of animals. From the Monad to the Loxades or Amphileptus, which are the fourth and sixth part of a line in diameter, the difference in size is greater than between a mouse and an elephant; within such narrow bounds might our ideas of the range in animal life be limited if the sphere of our observation was not augmented by artificial aid.’^[18]

This singular race of beings has given rise to the erroneous hypothesis of equivocal or spontaneous generation, that is to say, the production of living animalcules by a chemical or even fortuitous combination of the elements of inert matter. That question has been decided by direct experiment, for Professor Schultz kept boiled infusions of animal and vegetable matter for weeks in air which had passed through a red-hot tube, and no animalcules were formed, but they appeared in a few hours when the same infusions were freely exposed to the atmosphere, which shows clearly that the germs of the lowest grade of animal life float in the air, waiting as spores do, till they find a nidus fit for their development.

M. Pasteur, Director of the Normal School in Paris, in a series of lectures published in the ‘Comptes Rendus,’ has not only proved that the atmosphere abounds in the spores of cryptogamic fungi and moulds, but with infusoria of the form of globular monads, the Bacteria, and vibrios, which are like little rods round at their extremities and extremely active. The Bacteria mona and especially the Bacteria terma, are exceedingly numerous. These minute beings are the principal agents in the decomposition of organic matter. They are more numerous in dry than in wet weather, in towns than in the country, on plains than on mountains.

In a memoir read at the Academy of Sciences, Paris, Mr. J. Samuelson mentions that he had received rags from Alexandria, Japan, Melbourne, Tunis, Trieste and Peru. He sifted dust from the rags from each of these localities respectively through fine muslin into vases of distilled water. Life was most abundant in the vases containing dust from Egypt, Japan, Melbourne, and Trieste. The development of the different forms was very rapid, and consisted of protophytes, Rhizopods and true Infusoriæ. In most of the vases monads and vibrios appeared first, and from these Mr. Samuelson traced a change first into one then into another species of infusoria. In the dust from Japan he followed the development of a monad into what appeared to be a minute Paramœcium, then into Lexodes cucullus, and finally into Colpoda cucullus. From these and other experiments it is proved that many infusoria now classed as distinct types are really one and the same animal in different states of development. That appears to be the case also with the Amœbæ. In the dust from Egypt Mr. Samuelson found a new Amœba whose motions were very rapid; as to shape and mode of motion he compared it to soap bubbles blown with a pipe. He traced the gradual changes of the globular form of this Amœba until its pseudopodia were in full action, its increase by conjugation, and other circumstances of its life. In the same dust and in that only, the development of the Protococcus viridis was seen, and that in such abundance that at last the water was tinged green by that plant. In the dust from Egypt a vibrio was changed into a vermiform segmented infusoria of an entirely new type. Its length varied from the ¹⁄₁₅₀ to ¹⁄₁₀₀ of an inch, each ring was ciliated, and the whole series of cilia extending along the body acted in concert; a circlet of them surrounded the anterior segment; a canal seemed to extend throughout the body. It was propagated by bisection; the two parts remained attached to one another; an independent ciliary motion was observed in each which did not interfere with the motion of the whole. It was supposed to be a larval form or series of forms. Mr. Samuelson’s observations show, that the atmosphere in all the great divisions of the globe is charged with representatives of the three kingdoms of nature, animal, vegetable, and mineral: that the animal germs not only include the obscure types of monads, vibrios, and Bacteria, but also the Glaucoma, Cyclides, Vorticella, and other superior Infusoriæ, and occasionally though very rarely germs of the Nematode worms.

It has been already mentioned that many of the microscopic fungi are ferments, aiding greatly in the decomposition of organic matter. They however are by no means the only agents in decomposition. The moment life is extinct in an animal or vegetable, Infusoria of the lowest grade seize upon the inanimate substance, speedily release its atoms from their organic bond, and restore them to the inorganic world whence they came. The ferment which transforms lactic acid into butyric acid is a species of vibrio which abounds in the liquid, isolated or united in chains; they glide, pirouette, undulate, and float in all directions, and multiply by spontaneous division. Vibrios possess the unprecedented property of living and propagating without an atom of free oxygen; they not only live without air, but air kills them. This singular property forms an essential difference between the Vibrios and the Mycoderms: the former cannot live in oxygen; the latter cannot live without it, and as soon as it is exhausted within the infusion, they go to the surface to borrow it from the atmosphere.

There are also two groups of Infusoria which possess these opposite characters, one being unable to live in oxygen, while the other cannot live without it; sometimes they even inhabit the same liquid. When the tartrate of lime is put into water along with some ammoniacal and alkaline phosphates, a Monad, the Bacteria terma, and other Infusoria appear after a time. These little animals bud rapidly in an infusion of animal matter, then a slight motion is produced by the appearance of the Monas corpusculum and the Bacterium terma, which glide in wavy lines in all directions in quest of the oxygen dissolved in the liquid, and as soon as it is exhausted they go to the surface in such numbers as to form a pellicle, where by aid of the oxygen they form the simple binary compounds water, ammonia, and carbonic acid. In the meantime the Vibrios, which are without oxygen, are developed below, and keep up the fermentation, and between the two, the work of decomposition is completed.

It is not the worm that destroys our dead bodies; it is the Infusoria, the least of living beings. The intestinal canal of the higher animals, and of man, is always filled during life not only with the germs of vibrios, but with adult and well-grown vibrios themselves. M. Leewenhoeck had already discovered them in man, a fact which has since been confirmed. They are inoffensive as long as life is an obstacle to their development, but after death their activity soon begins. Deprived of air and bathed in nourishing liquid, they decompose and destroy all the surrounding substances as they advance towards the surface. During this time, the little Infusoria, whose germs from the air had been lodged in the wrinkles and pores of the skin, are developed, and work their way from without inwards, till they meet the vibrios, and after having devoured them, they perish, or are eaten by maggots.

Of all the Infusoria and ferments the Vibrios are the most tenacious of life; their germs resist the destructive effect of a temperature of 100° Cent. The spores of the Mucedines are still more vivacious; they grow after being exposed to a heat of 120° Cent., and are only killed by a temperature of 130° Cent. As neither spores of the fungi nor the germs of the Infusoria are ever exposed to so high a temperature while in the atmosphere, they are ready to germinate as soon as they meet with a substance that suits them.

M. Ehrenberg has estimated that the Monas corpusculum is not more than the ¹⁄_24,000th part of an inch in diameter; whence Dr. M. C. White, assuming that the ova of the Infusoria and the spores of minute fungi are only the ¹⁄₁₀th part in linear dimensions of their parent organisms, concludes that there must be an incalculable amount of germs no larger than the ¹⁄_240,000th or ¹⁄_100,000th part of an inch in diameter; and since according to MM. Sullivant and Wormley, vision with the most powerful microscope is limited to objects of about the ¹⁄_80,000th part of an inch in diameter, we need not be surprised if Infusoria and other organisms appear in putrescible liquids in far greater numbers than the germs in atmospheric dust visible by the aid of microscopes would lead us to expect.

The ferments are the least in size and lowest in organization of all the Infusoria. The higher group which abounds in stagnant pools and ditches are exceedingly numerous, and their forms are varied beyond description. They are globular, ovoid, long and slender, short and thick, many have tails, one species is exactly like a swan with a long bending neck, but whatever the form may be, all have a mouth and gullet. Although the skin of the Infusoria is generally a mere pellicle, that of the red Paramœcium and some others resembles the cellulose covering of a vegetable cell, engraved with a pattern; but in all cases respiration is performed through the skin.

Whatever form the cell which constitutes the body of the Infusoria may have, the highly contractile diaphanous pellicle on its exterior is drawn out into minute slender cilia which are the locomotive organs of these creatures. Vibrating cilia form a circlet round the mouth of some of these animalcules, a group of very long ones are placed like whiskers on each side of it, as in the Paramœcium caudatum, and in some cases there is a bunch of bristles in front. Certain Infusoria have cilia in longitudinal rows, and in many the whole body is either partially or entirely covered with short ones. In some Infusoria their vibrations are constant, in others interrupted, and so rapid that the cilia are invisible. These delicate fibres which vary from the ¹⁄₅₀₀th to the ¹⁄_13,000th part of an inch in length, move simultaneously or consecutively in the same direction and back again, as when a fitful breeze passes over a field of corn. These animalcules seize their prey with their cilia, and swim in the infusions or stagnant pools, in which they abound, in the most varied and fantastic manner; darting like an arrow in a straight line, making curious leaps and gyrations, or fixing themselves to an object by one of their cilia and spinning round it with great velocity, while some only creep. These motions, which bring the animalcules into fresh portions of the liquid, are probably excited by the desire for food and respiration.