PROTOZOA.

Think not, reader, that the life of the ocean ends with the innumerable hosts of fishes, molluscs, crustacea, medusÆ, and polyps we have reviewed, and that the waters of the sea or the sands of the shore have now no further marvels for us to admire. The naked eye indeed may have attained the limits of life, but the microscope will soon reveal a new and wonderful world of animated beings.

Take only, for instance, while wandering on the beach, a handful of drift-sand, and examine it through a magnifying glass. You will then not seldom find, among the coarser grains of inorganic silica, a number of the most elegant shells; some formed like ancient amphorÆ, others wound like the nautilus, but all shaped in their minuteness with a perfection which no human artist could hope to equal in the largest size.

The knowledge of these charming little marine productions is of modern date, for they were first observed in the sand of the Adriatic by Beccaria in 1731, and for some time believed to belong exclusively to that gulf. At a later period some species were discovered here and there in England and France, but their universality and importance in the economy of the ocean were first pointed out in 1825, by the distinguished French naturalist Alcide d'Orbigny.

The sand of many sea-coasts is so mixed with Foraminifera, as they have been called from the openings with which their shells are pierced, that they often form no less than half its bulk. Plancus counted 6000 in an ounce of sand from the Adriatic, and d'Orbigny reckoned no less than 3,849,000 in a pound of sand from the Antilles. Along the whole Atlantic coast of the United States, the plummet constantly brings up masses of foraminiferous shells from a depth of ninety fathoms, so that the vast extent of ocean-bottom, which itself forms but a small part of the domains they occupy, is literally covered with their exuviÆ.

Thus their numbers surpass all human conception, nor can any other series of beings be compared to them in this respect; not even the minute crustaceans which colour thousands of square miles on the surface of the sea, and, according to Scoresby, form almost exclusively the food of the huge Greenland whale; nor the infusory animals of the fresh-water, whose shields compose the Bilin slate quarries in Bohemia; for these are limited in their distribution, whereas the Foraminifera occur in all parts of the world.

The resemblance of the Foraminifera to the nautili and ammonites at first led naturalists to suppose that they formed part of the same class, which in a long course of centuries had dwindled down in less congenial seas to almost invisible dimensions; but a closer investigation proved them to belong to a much lower order of beings, nearly related to the AmoebÆ, which likewise occur all over the ocean. Other animals excite our wonder by their complicated structure, but the amoeba raises our astonishment by the excessive simplicity of its organisation. The amoeba is nothing more than a living globule of mucus, a transparent, colourless, contractile substance, or plastic mass, the individual life of which shows itself in manifold changes of form, bearing the character of voluntary motion. When an amoeba approaches another minute animal or plant unable to move out of its reach, it sends out extemporaneous feet, which soon clasp the prey on all sides, and the prisoner lies embedded in the living mucus until all his soluble parts have been absorbed. There is absolutely no trace of particular organs in the amoeba; all its constituent particles may be used for any purpose, all equally move and digest, and each can at any time perform the organic functions pertaining to the whole.

In their internal simplicity the Foraminifera are on a par with the amoebÆ, and differ from them only in respect of their outward form. The amoebÆ are naked, while the Foraminifera are covered with a shell, out of which, through one or numerous openings, the animal protrudes the processes which it requires for creeping or seizing its prey. These processes or filaments of mucus frequently ramify, closing as they spread, and sometimes covering an area of several lines in diameter, in the centre of which the animal inclosed in its shell waits for its prey, like a spider in its net.

The extended filaments appear to have something venomous about them; for Dr. Schultze, to whom we owe an interesting monograph on the Foraminifera, frequently saw small and sprightly parameciÆ, colpodes, and other infusoria drop down paralysed as soon as they touched the net.

The amazing variety of form of the Foraminifera is no less remarkable than the elegance of their delicately chiselled shells, and may well be called immense, as no less than 2,400 living and fossil species have already been distinguished by naturalists, and a far greater number is probably still nameless and unknown. Though generally so minute that the diameter of the pores through which they protrude their filaments usually only ranges from 1/3000 to 1/10000 of an inch yet the diminutive world of the Foraminifera has also its giants, particularly among the fossil species, such as the Nummulites, which occur in such prodigious numbers in the limestone of the Egyptian pyramids, and whose flattened lenticular coin-like forms (d) attain the comparatively gigantic diameter of several inches. Thus the material with which the proud Pharaohs of the Nile constructed their colossal tombs was originally piled up at the bottom of the sea by countless generations of shell-cased Protozoa.

The Foraminifera are among the oldest inhabitants of our globe,[T] and as the present ocean contains them in countless multitudes, thus have they swarmed in the waters of the primeval seas from the first dawn of creation, and piled up the monuments of their existence in vast strata of limestone. A great part of the rocky belt from RÜgen to the Danish isles, the white chalk cliffs which, beginning in England, extend through France as far as Southern Spain, are chiefly composed of the shells of Foraminifera, and the zone of Nummulite limestone, which served to build the huge quadrilateral monument of Cheops, forms a band, often 1,800 miles in breadth, and frequently of enormous thickness, from the Atlantic shores of Europe and Africa through Western Asia up to North India and China; enough to satisfy the most extravagant architectural folly of millions of despots. So important is the part which these beings, individually so minute, have performed and still perform in the geological annals of the globe.

Many of these "minims of nature" consist of only one chamber, and hence are called unilocular or monothalamous; but a vast proportion consist of several chambers, and hence are called multilocular or polythalamous. The latter, however numerous their chambers or seemingly complex their structure, always originate as a single shell. The primitive jelly-sphere, or first sarcode segment, secretes around itself its appropriate calcareous envelope. Having grown too large for its habitation, it protrudes a portion of itself without, and thus forms a second segment. If by a process of spontaneous fission this segment becomes quite detached from its parent, and repeats the life and method of reproduction of the latter, a series of monothalamous shells will be formed. But if by means of a sarcode band the primitive segment maintains its connection with its immediate offspring, and this, repeating the reproductive process, does the same, a compound shell will, of course, be the result.

Among the microscopic denizens of the ocean, the Polycystina rival the Foraminifera both by their number and their wonderful elegance of form and structure. Their body consists of the same viscid homogeneous plastic mass, termed "sarcode" by the naturalists; like them they are capable of protruding it through the foramina with which their shell is pierced, and consequently they are ranked with them among the Rhizopods, or root-footed animalcules, that form the lowest order of the Protozoa, the lowest class of the animal world.

It is a peculiar feature of these beautiful little shells (whose delicate sculpture frequently reminds the observer of the finest specimens of the hollow ivory balls carved by the Chinese) that they are usually surmounted by a number of spine-like projections, very frequently having a radiate disposition. Some have an oblong shape (Podocyrtis), others a discoid form (Haliomma), from the circumference of which the silicious spines project at regular intervals, so as to give them a star-like aspect. They are generally of a smaller size than even the Foraminifera, appear to be almost as widely diffused, and have also largely contributed to the structure of the earth-rind. They were first discovered by Professor Ehrenberg at Cuxhaven, on the North Sea; they were afterwards found by him in collections made in the antarctic seas, and have been brought up by the sounding lead from the bottom of the Atlantic at depths of from 1,000 to 2,000 fathoms.

The term Infusoria, which formerly comprised a most heterogeneous assemblage of minute plants and animals, is now confined to the highest order of the Protozoa, distinguished from the Rhizopods by the possession of a mouth and of ciliary filaments, whose vibrations serve them both for progression through the water and for drawing alimentary particles into the interior of their body. Though most of the Infusoria live in ponds, morasses, pools, wells, or cisterns, yet many are marine, as, for instance, the Carchesium polypinum, which is frequently found attached to corallines, and the Vaginicola valvata, which from its sheath and valve strongly reminds one of a tubicolar annelide.

The wide diffusion both in time and space of the marine Protozoa, and chiefly of the Foraminifera and Polycystina, is a sufficient proof of their vast importance in the household of the seas. Along with the Diatoms and other microscopical forms of vegetation on which their own existence depends, they evidently constitute the basis on which the superstructure of all the higher orders of the animal life of the ocean reposes. Hosts of minute crustaceans, annelides, acalephÆ, and molluscs, feed upon their inexhaustible legions, and serve in their turn to sustain creatures of a larger and still larger size until finally Man is enabled to feast on the abundance of the seas.

The Porifera, or Sponges, were formerly supposed to belong to the vegetable kingdom, but their animal nature is now fully ascertained, for modern researches have proved that the soft glairy substance with which their skeleton is invested during life consists of "sarcode," similar to that which forms the soft parts of the Foraminifera and Polycystina. It is by this animated or organic gelatine, which can generally be pressed out with the finger, and in some species is copious even to nauseousness, that the solid parts of the sponge are deposited, and from it the whole growth of the mass proceeds. The framework or skeleton of the Porifera is usually composed of horny fibres of unequal thickness, which ramify and interlace in every possible direction, anastomosing with each other so as to form innumerable continuous cells and intricate canals, the walls of which in the recent sponge are crusted over with the gelatinous living cortex.

Generally this fibrous mass is interwoven with numerous mineral spicules of a wonderful elegance and variety of forms, for their shapes are not only strictly determinate for each species of sponge but each part of the sponge, it is believed, has spiculÆ of a character peculiar to itself. Sometimes they are pointed at both ends, sometimes at one only, or one or both ends may be furnished with a head like that of a pin, or may carry three or more diverging points, which sometimes curve back so as to form hooks. Sometimes they are triradiate, sometimes stellar; in some cases smooth, in others beset with smaller spinous projections like the lance of the saw-fish. In many species they are embedded in the horny framework; in others, as, for instance, in Tethea Cranium, or in Halichondria, they project from its surface like a tiny forest of spears. They are generally composed of silex or flint, but in the genus Grantia they consist of carbonate of lime. Though the skeleton of most sponges is formed both of horny fibres and of mineral spicules yet the proportions of these two component parts vary considerably in different species. In the common sponge, for instance, the fibrous skeleton is almost entirely destitute of spicules, a circumstance to which it owes the flexibility and softness that render it so useful to man, while they predominate in the HalichondriÆ, and sometimes even, as in the GrantiÆ, completely supersede the horny fabric.

On examining a sponge, the holes with which the substance is everywhere pierced may be seen to be of two kinds; one of larger size than the rest, few in number, and opening into wide channels and tunnels which pierce the sponge through its centre; the other minute, extremely numerous, covering the wide surface, and communicating with the innumerable branching passages which make up the body of the skeleton. Through the smaller openings or pores the circumambient water freely enters the body of the sponge, passes through the smaller canals, and, ultimately reaching the larger set of vessels, is evolved through the larger apertures or oscula. Thus by a still mysterious agency (for the presence of cilia has as yet been detected but in one genus of full-grown marine sponges) a constant circulation is kept up, providing the sponge with nourishing particles and oxygen, and enabling its system of channels to perform the functions both of an alimentary tube and a respiratory apparatus.

Dr. Grant describes in glowing terms his first discovery of this highly interesting phenomenon: "Having put a small branch of sponge with some sea-water into a watch-glass, in order to examine it with the microscope, and bringing one of the apertures on the side of the sponge fully into view, I beheld for the first time the spectacle of this living fountain, vomiting forth from a circular cavity an impetuous torrent of liquid matter, and hurling along in rapid succession opaque masses, which it strewed everywhere around. The beauty and novelty of such a scene in the animal kingdom long arrested my attention, but after twenty-five minutes of constant observation, I was obliged to withdraw my eye from fatigue, without having seen the torrent for one instant change its direction or diminish in the slightest degree the rapidity of its course. I continued to watch the same orifice at short intervals for five hours, sometimes observing it for a quarter of an hour at a time, but still the stream rolled on with a constant and equal velocity."

Subsequent observations have proved that the living sponge has the power of opening and closing at pleasure its oscula, which are capable of acting independently of each other, thus fully establishing the animal nature of these simple organisations, in whom latterly even traces of sensibility have been detected, such as one would hardly expect to meet with in a sponge. For these creatures, as we are entitled to call them, are able to protrude from their oscula the gelatinous membrane which clothes their channels, and on touching these protruded parts with a needle, they were seen by Mr. Gosse to shrink immediately—a proof that the sponge, however low it may rank in the animal world, is yet far from being so totally inert or lifeless as was formerly imagined.

The propagation of the sponges is provided for in a no less wonderful manner than their respiration and nourishment. Minute globular particles of sarcode sprout forth as little protuberances from the interior of the canals. As they increase in size, they are gradually clothed with vibratile cilia, and, finally detaching themselves, are cast out through the oscula into the world of waters. Here their wanderings continue for a short time, until, if they be not devoured on the way, they reach some rock or submarine body on which, tired of their brief erratic existence, they fix themselves for ever, and, bidding adieu to all further rambles, lead henceforth the quiet sedentary life of their parents. In this manner the sponges, which otherwise would have been confined to narrow limits, spread like a living carpet over the bottom of the seas, and in spite of their being utterly defenceless, maintain their existence from age to age. At the same time they serve to feed a vast number of other marine animals, for the waters frequently swarm with their eggs, and these afford many a welcome repast to myriads of sessile molluscs, annelides, polyps, and other creatures small or abstemious enough to be satisfied with feasting on atoms.

Sponges inhabit every sea and shore, and differ very much in habit of growth. For whilst some can only be obtained by dredging at considerable depths, others live near the surface, and others, again, attach themselves to the surfaces of rocks and shells between the tide marks. Like the corals, they revel in every variety of shape and tint, imitate like them every form of vegetation, and adorn like them the submarine grounds with their fantastic shrubberies. The fine collection of West Indian sponges exhibited in the Crystal Palace, but to which fancy must add the additional ornament of colour, may serve to give some idea of their prodigal versatility of growth. More than sixty different species have been discovered in the British waters alone, and as they go on increasing in numbers, size, and beauty, until they attain their highest development along the shores of the tropical ocean, they no doubt hold a conspicuous rank among the living wonders of the sea. The branched sponges, with a compact feltred tissue, are more common than others in the colder maritime domains, where the species of a loose texture, which grow in large massive forms, either do not exist or are very rare. Many sponges are of considerable size, such as the vase-like tropical species known under the name of Neptune's cup; others are almost microscopical; and while by far the greater number grow superficially from a solid base, some penetrate like destructive parasites into the texture of other animals. Thus the Halichondria celata establishes itself in the small holes which some of the smaller annelides drill in the shell of the oyster, eat further and further into the unfortunate mollusc's vitals, causing the softer parts of the shell to rot away, and spread through its whole substance, like the dry-rot fungus through a solid beam of timber, until, sinking under the weight of his misery, the poor victim perishes, and his loosened shell is cast to the mercy of the waves. On the other hand, some marine Acorn-shells nestle habitually in a sponge, the normal construction of the base of the shell being altered to suit the peculiarities of its habitation, so that in this instance, as in many others, there is a foreseen relation between two very dissimilar animals. Amongst the reticulated fibres of its spongy dwelling, the Acorn-shell finds a secure refuge in its infant state, and is soon enclosed by the growing fabric of the sponge-animal, except a small opening, which is kept clear by the vortex occasioned by the constant motion of its feelers or tentacula.

But very few of the manifold species of sponges are of any use to man. The common sponge of commerce (Spongia communis), so serviceable in our households, is most abundant in the Lycian seas, where it is found attached to rocks at various depths between three fathoms and thirty. When alive, it is of a dull bluish black above, and dirty white beneath. There are several qualities, possibly indicating as many distinct species.

"The most valued kinds," says Edward Forbes, "are sought for about the Gulf of Macri, along the Carian coast, and round the opposite islands. The species which live immediately along the shore near the water's edge, though often large, are worthless. These are of many colours; some of the brightest scarlet or clear yellow form a crust over the faces of submarine rocks; others are large and tubular, resembling holothuriÆ in form and of a gamboge colour, which soon turns to dirty brown when taken out of the water; others are again lobed or palmate, studded with prickly points, and perforated at intervals with oscula. These grow to a considerable size, but, like the former, are useless, since their substance is full of needles of flint."

Large quantities of excellent sponge (Spongia usitatissima) are likewise imported from the West Indies.