The Sub-kingdom Protozoa.
The consideration of the whole special group of organisms forming the subject matter of this chapter, under the heading of Protozoa, were formerly included among Infusoria, which also embraced every kind of microscopical aquatic body, whether belonging to the vegetable or animal series. A more critical survey of the organisation and affinities of Infusoria and the members which constituted the group led to a re-arrangement, which has been very generally accepted as forming a sub-kingdom, Protozoa. This may be defined as embracing all those forms of life, referable to the lowest grade of the animal kingdom, whose members for the most part are represented by organisms possessing a single cell or aggregation of cells (and also included under the general term of unicellular organisms) the whole of which are engaged in feeding, moving, respiring, and reproducing by segmentation or fission much in the same way as that of the unicellular plants described in a previous chapter. Following out this sub-division of the entire series of Protozoa, the several groups range themselves into four readily distinguishable sections. In the first, the most lowly organised and most abundant have no oral orifice in the literal meaning of the word, food being intercepted at any point of the surface of the body. This most simple elementary type of structure of the Protozoa is represented in the Amœba and Actinophrys, the various representatives of the Foraminifera, and certain Flagellata, as Spumella and Anthrophysa. Next in the ascending scale is a group of Protozoa, in which, though differentiation has not proceeded so far as to arrive at the constitution of a distinct oral aperture, the inception of food substance is limited to a discoidal area occupying the anterior extremity of the body and is associated with the special food-arresting apparatus. To this section of the Protozoa are relegated the minuter flagellate, “collar-bearing” animals, and also the entire group of sponges or Porifera.
Gregarinida, Polycystina, Foraminifera, Rotifera, etc.
Tuffen West, del. Edmund Evans.
Plate III.
In the third section the highest degree of organisation is arrived at. Here is represented a single, simple, often highly-differentiated oral aperture or true mouth. Associated with this section are found the majority of those organisms that collectively constitute the class Infusoria in the proper acceptation of the term, and it embraces the majority of the Ciliata, the Cilio-flagellata, as Euglena, Chilomonas, &c., in which the presence of a distinct and circumscribed oral aperture is clearly seen. With the fourth and remaining section of Protozoa, the oral or inceptive apparatus exhibits a highly characteristic structural modification. This is not restricted to a definite area, nor is it associated with the entire surface of the body, but it consists of a number of flexible, retractile, tentacle-like organs radiating from diverse and definite regions of the periphery, each of which subserves as a tubular sucking-mouth, or for the purpose of grasping food. These may be literally described as many-mouthed, and have been appropriately designated Polystomata. The true zoological position of the Spongida or Porifera is not finally settled, the members of this important section having been formerly regarded as a subordinate group of the Rhizopoda or an independent class of the Protozoa; consequently a tendency has been shown to assign to them a position more nearly approximating to that of the Cœlenterata, or zoophytes and corals, or place them among the more highly organised tissue-constructed animals, the Metazoa, these being characterised by groups of cells set apart to perform certain functions for the whole animal. A division of labour is seen to be marked in these lower animals as the organism becomes more specialised, and the number of functions a cell performs becomes more and more limited as the body becomes more complex.
It has been found convenient to adopt the following definition of the Infusoria as one more generally acceptable. The Protozoa in their adult condition are furnished with prehensile or locomotive organs, that take the form of cilia, flagella, or of adhesive or suctorial tentacula, but not of simple pseudopodia; their zooids are essentially unicellular, free swimming or sedentary; they are either naked, loricate, or inhabit a simple, mucilaginous matrix; single or united in aggregations, in which the individual units are distinctly recognisable; not united and forming a single gelatinous plasmodium, as in Mycetozoa, nor immersed within and lining the interior cavities of a complex protoplasmic and mostly spiculiferous skeleton, as in the Spongida, their food substances being intercepted by a single distinct oral aperture, or by several apertures through a limited terminal region or through the entire area of the general surface of the body. They increase by simple longitudinal or transverse fission, by external or internal gemmation or division, preceded mostly by a quiescent or encysted state, into a greater or less number of sporular bodies. Sexual elements, as represented by true ova or spermatozoa, are entirely absent, but two or more zooids frequently coalesce as an antecedent process to the phenomena of open formation.[63]
The infusorial body in its simplest type of development, as in Amœba, exhibits a structural composition substantially corresponding with that of the lowest organised tissue cell. There is no distinct bounding membrane, or cell-wall, and it is throughout, and apart from the nucleus or endopart, one continuous mass of granular matter, but otherwise homogeneous and undifferentiated protoplasm. Professor Greef, who has made a study of the Amœba, describes motor fibrils in the exoplasm which are active and large in A. terricola. These are readily seen by staining with osmic acid, and, after washing this out with water, immersing in a weak alcoholic solution. In Amœba so prepared and examined with a high power, the whole body will be seen to be surrounded by a distinct double integumentary layer. Highly refractive bodies may also be seen in the interior, connected together by extremely fine filaments. Professor Greef concludes that here we have to do with muscular fibrillæ, which traverse the contractile outer zone in a radial direction and there terminate for the time being. By a similar method, axial filaments can be demonstrated in Heliozoa; these, it is believed, are the true motors of their pseudopodia, and also the axial structures of the Acineta, a marine animal related to ciliate infusoria.
In the Amœba, at one time well known as the Proteus animalcule, [Fig. 325], the marvellous body creeps onward in a flowing manner, occasionally and languidly emitting a single pseudopod first on one side, then on the other. More commonly it puts on a dendroid or palmate form; then again it assumes more or less grotesque shapes in which almost any conceivable image may be imagined. The body, as will be seen in this highly-magnified figure, is full of granules (with the exception of a thin clear outer hyaline zone), and near the centre is a globular or discoid body known as the nucleus, composed of slightly denser material than that which surrounds it. The division of the body into two is preceded by a division of this nucleus. Near the latter is a clear spherical space—the contractile vacuole—which gradually expands, and then rather suddenly collapses and reappears at the same spot, the systole and diastole being slow and continuous. The contractile vacuole contains a clear liquid which is expelled on the collapse of the vacuole. This organ probably serves the double function of respiration and excretion. The Amœba is omnivorous, chiefly a vegetarian, and, therefore, found on the ooze of ponds or on the under surface of the leaves of aquatic plants, especially among Confervæ. It can be readily produced by placing a few fibres of fresh meat in an infusion of hay.
Fig. 325.—Amœba, Proteus animalcule; magnified 600 diameters.—(Warne).
The Gregarinæ consist of a remarkable group of organisms, but these, although unicellular, are, for the most part, confined to the intestinal tract of worms and of the higher animals, and will therefore be described among internal parasites.
Tho fungus-animals, Mycetozoa, have already been referred to in a previous chapter. The best known species, however, is found in tan yards in the form of creeping masses of naked protoplasm, termed Plasmodia. Cakes of protoplasm become segregated from the main mass, and break up into Amœba-like spores, which unite again to form Plasmodia.
Fig. 326.—Rhizopoda lobosa.
A. Difflugia proteiformis; B. Difflugia oblonga; C, D. Arcella acuminata and dentata
The Rhizopoda, or root-footed class of animals, are among the most interesting simple organisms with which the microscope has made us acquainted. In the living state they have the power of protruding pseudopodia from the body, by which they creep about, or cling to plants when in search of food. This group, in fact, includes Amœba, Foraminifera, Sun-animalcules, and Radiolarians. In the first the pseudopodia are simple and lobose; in the second they are slender, confluent and reticulate; while in the two last they are simple, radiating and somewhat stiff, and partake of a calcareous formation.
Of the Lobosa, we may take a well-known representative of the group, the Protomyxa, found at the bottom of fresh-water pools, especially those near bog-moss, where its minute orange-coloured particles of jelly-like substance are seen creeping over stones or shells. If quietly watched the pseudopodia, some of which are broad and others slender, become quiescent spheres, which break up into numerous portions, each of which becomes a new animal.
This group is divided into the shell-less (Nuda) and shell-formed (Testacea). The brown, horny covering is often finely faceted, and is either shaped like a dome, semi-circular, or flat as a box, through which they protrude their few or many pseudopodia (seen in [Fig. 326]).
PLATE XV.
GROMIA.
In the Difflugia the lorica or shell is strengthened by the addition of silicious particles; in Euglypta it is sac-shaped, with a jagged free margin, the surface being covered by overlapping scales; while Arcella are capable of secreting vesicles of air in their interior, whereby they are enabled to rise to the surface. On some parts of our coast, if the sea sand be carefully looked over with a pocket lens, there will often be found minute grains of a porcelain oval kind, belonging to the Miliolina, segmented or strung together not quite in the same plane.
Fig. 327.—Section of Rotalia.
a,a, Radiating interceptal canals; b, Internal bifurcations; c, Transverse branch; d, Tubular wall of chambers.
Fig. 328.—Rosalina varians or Discorbina globularia, with pseudopodia protruding.
The Foraminifera are rhizopods, whose simple protoplasmic bodies send forth, through perforations in the membrane or outer covering of calcium carbonate and silica, branching rays of pseudopodia. The order is divided into two groups, the Imperforata and the Perforata; in the former the shell or harder structure possesses only one or more apertures, whereas in the latter, in addition to the main opening, the shell has its walls perforated throughout, which admits of minute pseudopodia or fine threads being protruded ([Fig. 328]). (See also [Plate III]., Nos. 75-85.) The vast majority of Perforata form their shells, or rather skeletons, of calcium carbonate and silica, which renders them almost indestructible. Consequently the form is preserved through ages, and they present objects of the greatest interest to the microscopist.
A curious and interesting feature of the Foraminifera—often an element of difficulty to the student—is the tendency of modifications of types comprising the larger groups to run into parallel isomorphous series. Thus, if the entire class be roughly divided, as it sometimes has been, into three orders, comprising respectively the forms characterised by porcellaneous, arenaceous, and hyaline “tests,” the same general conformation and arrangement of chambers will be found in each of the three series. The most remarkable example, even among the smaller groups, is the Rotaliidæ, of which three or four genera may be arranged in parallel lines, and in more or less closely isomorphous series. In the report appended to the “Challenger” scheme of classification many examples are enumerated. In Arenacea we have a small family of Foraminifera, the external surfaces of which present a ridge and furrow arrangement, and the incrustations are entirely of a sandy nature held together by a cement secreted by the animal. ([Plate XV]., No. 1, Astrorhiza limicola.)
Gromia.—Among the more remarkable of the Perforata group the Gromia have a foremost place. They are very minute globular or oval-shaped bodies, about one-twenty-fourth of an inch in length, found in fresh, brackish, and salt water. The forms brought up in Dr. Wallich’s deep sea soundings of 1860 were taken attached to pieces of corallines, or found loose among Globigerina ooze. At first there appears to be nothing peculiar about these tiny specks of matter resembling the ova of a zoophyte, but presently, at the smaller end, a very fine thread is protruded, and then another, dividing into finer branches, and, ultimately, a complete network of filaments extends on all sides, and become attached to the side of the glass jar that contains them. Now, on employing magnifying power, every thread exhibits a circulatory motion, an up and down stream or cyclosis of granules suspended in a fluid mass. It is by means of these pseudopodia, as the threads are termed, that the Gromia moves its body along and clings to the glass. We may surmise, then, that these pseudopodia are either gelatinous, glutinous, or terminate in sucker-like processes. Increase in the “test,” integument, is brought about, as in Difflugia, by the secretion of calcareous matter or by cementing fine silicious particles to the outer wall, as the protoplasm is seen to flow over the test, so that when it comes in contact with a diatom it is thereby drawn towards the oral opening and slowly digested.
Some considerable time elapsed between the discovery of Gromia by Mr. W. Archer, F.R.S., and the demonstration of a nucleus and contractile vesicle by Dr. Wallich. It was thought that in the whole of the Monozoa the nucleus was absent, but it is now known that this important body is embedded in the protoplasmic substance, and the reproduction of these curious animals is thereby secured. Among the better known species of Gromia is G. Dujardinii, chiefly distinguishable by the darker colour of the “test,” by the greater quantity of silica that enters into the formation of its pseudopodia, and by the formation of isogamous zoospores, two of which are seen in conjugation in [Plate XV]., No. 2. An excess of protoplasm must also be secreted to admit of so large a protrusion outside the testa.
G. Lieberkühnia (of Claparède and Lachman), No. 5, differs in formation. Its shape is pyriform, and the opening whence the pseudopodia streams out is situated in a lateral depression about midway in the testa, c, o. Hence a trunk branch is seen to issue forth, and from this a ramification of threads, psdp, extends to a considerable distance in all directions.
The Micro-gromia of Hertwig, No. 4, is the minutest form of the genus yet discovered, and differs from those already described in the mode of reproduction. The individual takes the shape of a water bottle with a short neck, whence issue forth a limited number of very slender threads. The test is quite transparent, and it was in this species that the nucleus and contractile vesicle, which lie embedded near the mouth, were first clearly made out.
The zoospores of Micro-gromia have a curious habit of uniting with their neighbours to form a colony, No. 4. Their colonisation is apparently intended to facilitate multiplication. Reproduction is carried on somewhat after the manner of Volvox. The globular bodies formed sink to the bottom of the glass vessel, and there remain for a time in a quiescent state. In the course of a day or two the mass assumes a motive appearance, increases in bulk, becomes more ovoid in shape, and ultimately the nucleus shows the first sign of division. Vertical segmentation takes place, as at A, into two equal parts; each half is seen to possess its fair share of the nucleus and contractile vesicle. It then turns in the horizontal direction, and now there appears to be an upper and a lower division, the uppermost having a neck-like attachment, and this is making its way to the narrow oral opening in the parent testa, as at B. Here it is seen pressing forward, and at C the neck is protruding some distance, and the second half assumes a bottle shape; at D the greater part of the animal is nearly set free, and after a short rest it fully launches forth. It finally pulls itself together, as at E, and either develops a pair of flagella and swims off, or assumes the form of an Actinophrys. In either case, and in a very short space of time, the separated young animal is quite ready to re-unite, as at F, and assist in forming a new colony of the species.
The Polymorphina belong to a low genus of the Foraminifera. They consist of a number of forms and exhibit a rather extensive series of variations, although consisting of a few simple types, and showing transitions between forms which at first seem to be distinct. The majority of species keep to the sea bottom; some few are pelagic, and occur in abundance on the surface of the ocean. Among the latter are the Globigerina: its shell is about one-fortieth of an inch in diameter, and usually composed of seven globular chambers arranged spirally in such a manner that all are visible from above, each chamber opening by a crescentic-shaped orifice into a depression in the middle of the next. Perfect specimens bristle with long slender spines, the pores affording passage to pseudopodia, which stream out along the spines. The more carefully-conducted deep-sea investigations have brought to light the fact that the floor of the ocean, at great depths, and over a vast area, is formed of these white or pinkish coloured bodies, all containing on an average about 60 per cent. of calcium carbonate. It is a question whether the Globigerinidæ which make up the bulk of the ooze actually live at the bottom as well as the surface of the sea. This question has given rise to much discussion. Dr. Murray came to the conclusion that pelagic species do not live near the ocean floor. This opinion is partly based on the fact that the area of the Globigerina ooze coincides with the area of surface of temperature at which these bodies are found to exist. When the surface water is too cold for them, they are not to be found, neither are they found below. Major S. R. J. Owen, while dredging the surface of mid-ocean—the Indian, and the warmer portion of the Atlantic—found attached to his nets a number of these interesting bodies, and which always made their appearance just about sunset. In [Plate III]., Nos. 43-52, a number of these interesting and variously-formed bodies are given, and an attempt is also made to show the richly-tinted colour appearances presented by the sarcode or protoplasm of the Globigerina.
Fig. 329.—Globigerina and other bodies taken in deep sea soundings (Atlantic).
“Many of the forms,” writes Major Owen,[64] “have hitherto been claimed by the geologist, but I have found them enjoying life in this their true home, the silicious shells filled with coloured sarcode, and sometimes this sarcode in a state of distension somewhat similar to that found projecting from the Foraminifera, but not in such slender threads. There are no objects in nature more brilliant in their colouring or more exquisitely delicate in their forms and structure. Some are of but one colour, crimson, yellow, or blue; sometimes two colours are found on the same individual, but always separate, and rarely if ever mixed to form green or purple. In a globular species, whose shell is made up of the most delicate fretwork, the brilliant colours of the sarcode shine through the little perforations very prettily. In specimens of the triangular and square forms ([Plate III]., Nos. 43, 44, 45 and 46), the respective tints of yellow and crimson are vivid and delicately shaded; in one the pink lines are concentric; while another is of a stellate form, the points and uncoloured parts being bright clear crystal, while a beautiful crimson ring surrounds the central portion. A globular form resembles a specimen of the Chinese ball-cutting—one sphere within another; this, however, appears to belong to a distinct species.
Fig. 330.—Globigerina and other bodies taken in deep sea soundings, 1856 (Atlantic).
“The shells of some of the globular forms of these Polycystina, whose conjugation I believe I have witnessed, are composed of a fine fretwork, with one or more large circular holes; and I suspect the junction to take place by the union of two such apertures. That the figures of these shells become elongated, lose their globular form after death, and present a disturbed surface is seen in some of the figures represented in [Plate III]., Nos. 82-85.” Those without internal chambers have been described as Orbulina universa, [Plate III]., [Fig. 78], while Nos. 75 and 76, although members of the same family, have been separated, but all should certainly be united under Globigerina.
“The minute silicious shells of Polycystina present wonderful beauty and variety of form; all are more or less perforated, and often prolonged into spines or other projections, through which the sarcode body extends itself into pseudopodial prolongations resembling those of Actinophrys. When seen disporting themselves in all their living splendour, their brilliancy of colouring renders them objects of unusual attraction. It will appear that they wish to avoid the light, as they are rarely found on the surface of the sea in the daytime; it is after sunset and during the twilight that they make their appearance.”
Many forms of Globigerina and Foraminifera are represented in Figs. 329 and 330. These varied and beautiful forms were dredged up with soundings made in 1856 for the purpose of ascertaining the depth of the Atlantic, prior to the laying down of the electric telegraph wire from England to America, and taken at a depth of 2,070 fathoms.
Heliozoa.—Actinophrys-Sol, “sun-animalcules,” belong to this group; most of them inhabit fresh water ([Plate III]., No. 66). The chief characteristic, and the one to which they owe their name, is the possession of long, slender, somewhat stiff pseudopodia; these radiate from all parts of the body. The living animal usually contains green-coloured particles within a minute translucent spherical globule of about 1⁄250th of an inch in diameter. It is, therefore, variously designated the green sun-animalcule, Acanthocystis, or Actinophrys-Sol. It is commonly found amongst the weeds in clear pools of water, where desmids abound. The pseudopodia appear to be stiff; they are, however, quite flexible, and the body contains more than one clear vesicle with a nucleus; reproduction is secured by the simple division commencing in the nucleus. The little animal can move over a hard surface by the alternate relaxation and stiffening of its pseudopodia; when one of these touches a small organism, it is believed to paralyse it, then envelop, and deliberately digest it. In another species, the lattice-animalcule (Cathrulina), the pseudopodia or silicious threads are arranged tangentially. It grows on a long flexible stalk, attached to an aquatic plant, the total length of which is about 1⁄200th an inch. The globular body is perforated in all directions, through which the fine stiff pseudopodia are thrust out; it is often known to form colonies.
In this order may well be placed the Radiolaria; they are, however, usually separated. But Radiolarians, whether seen alive or in their skeleton form, are surpassingly beautiful. By the favour of Messrs. Warne, I am enabled to append a frontispiece plate to this volume taken from their “Royal Natural History.” These bodies are all marine, and live in zones of several thousand fathoms, and like their congeners, the Globigerina, they avoid a strong light, and only appear after sunset. Their bodies are supposed to emit a phosphorescent glow, but more is known of their silicious skeletons than of their living forms; yet it is not this feature that separates them from other orders of rhizopods, but the possession of a membranous central capsule enclosing the nucleus. The body substance outside this capsule is highly vacuolated in some species, especially in surface forms. A few are without a skeleton, and these consist of oval masses of protoplasm, with slender pseudopodia. In a few species the skeleton is formed of a glassy horny substance, termed acanthin, arranged in the form of radiating spines.
Radiolarians secrete a silicious skeleton, which assumes a variety of forms, as trellis-work, boxes joined by radiating spines, helmets, baskets, bee-hives, discs, rings, and numerous other forms. Haeckel has described upwards of four thousand species, and possibly as many more could be added to this number. Radiolaria are divided into two groups. In the one there is either no skeleton or one of silex; in the other the skeleton is formed of radiating spines of a horny nature. These are again subdivided according to the characters of the central capsule. In those forms with a silicious skeleton the geometrical pattern conforms more or less to the shape of the central capsule, being either spherical or conical. The central capsule is regarded as being homologous with the calcareous shell of Globigerina. Reproduction takes place by simple division into two, or by the body breaking up into spores, each provided with a flagellum, or two spores may fuse together, and the result will be an adult Radiolarian. Certain yellow corpuscles present in the outer part of their body-surface change into unicellular parasitic algals; these can be separated and cultivated independently of their host. The Radiolarians live floating at all depths from 1,000 to 2,500 fathoms, and are distributed over areas in the central Pacific and the south-eastern part of the Indian Ocean, the ooze forming the ocean bed being made up of their skeletons to an extent of 80 per cent. of the deposit; hence it has become known as Radiolarian ooze. The chalky-looking Barbadoes earth, a Tertiary formation, is composed almost entirely of their skeletons. Somewhat similar deposits exist in the Nicobar Islands, in Greece, and in Sicily.
It will have been noticed that by far the greater number of Foraminifera are of marine origin, and these occur in such widespread profusion that the finest calcareous particles which constitute the seashore in some places consist almost wholly of their microscopic remains. At former periods of the earth’s history they appear to have existed even in greater profusion than at the present time. This is evidenced by their remains forming the principal constituent of our largest geological formations.
Moreover, during the Canadian Geological Survey large masses of what appeared to be a fossil organism were discovered in rocks situated near the base of the Laurentian series of North America. Sir William Dawson, of Montreal, referred these remains to an animal of the foraminiferal type; and specimens were sent by Sir W. Logan to the late Dr. Carpenter, requesting him to subject them to a careful examination. As far back as 1858 Sir W. Logan had suspected the existence of organic remains in specimens from the Grand Calumet limestone, on the Ottawa River, but a casual examination of the specimens was insufficient to determine the point. Similar forms being seen by Sir W. Logan in blocks from the Grenville bed of the Laurentian limestone were in their turn tried, and ultimately revealed their true structure to Sir William Dawson and Dr. Sterry Hunt, who named the structure Eozoon Canadense.
The masses of which these fossils consist are composed of layers of serpentine alternating with calc spar. It was found by these observers that the calcareous layers represented the original shell, and the silicious layers the softer parts of the once living Foraminifera. The results were arrived at through comparison of the appearance presented by the Eozoon with the microscopic structure which Dr. Carpenter had previously shown to characterise certain members of the Foraminifera. The Eozoon not only exceeded other known Foraminifera in size to an extent that might have easily led observers astray, but, from its apparently very irregular mode of growth and general external form, no help was derived in its identification, and it was only by microscopical examination of its minute structure that its true character was ascertained. Dr. Carpenter wrote:—“The minute structure of Eozoon may be determined by the microscopic examination either of thin transparent sections, or of portions which have been subjected to the action of dilute acids, so as to remove the calcareous portion, leaving only the internal casts, or models, in silex, of the chambers and other cavities originally occupied by the substance of one animal.” Subsequently he found portions of minute structure so perfect that he was able to say that “delicate pseudopodial threads were originally put forth through openings in the shell wall of less than 1⁄10000th of an inch in diameter” ([Plate III]., Nos. 64, 65). In a paper read at the meeting of the Geological Society he stated that he had since detected Eozoon in a specimen of ophicalcite from Bohemia, in a specimen of gneiss from near Moldau, and in specimens of serpentine limestone sent to Sir C. Lyell by Dr. Gümbel, of Bavaria. These also were found to be parts of the great formation of the “fundamental” gneiss, considered by Sir Roderick Murchison as the equivalent of the Laurentian rocks of Canada.[65]
If the remains of Foraminifera be dissolved in dilute hydrochloric acid, an organic basis is left, after the removal of the calcareous matter, accurately retaining the form of the shell with all its openings and pores. The earthy constituent is mainly calcium carbonate; but there is also a small amount of phosphate of lime in the shells of many of them.
Fig. 331.
1. Separated prisms from outer layer of Pinna shell; 2. Skeletons of Foraminifera from limestone; 3. Recent shell of Polystomella crispa; examined under dark-ground illumination.