Volume II Part 3 (1/2)
Naked cells, with kernels, like those represented in Fig. 10 _B_, which are continuously changing, stretching out and drawing in formless, finger-like processes, and which are on this account called amboid, are found frequently and widely dispersed in fresh water and in the sea; nay, are even found creeping on land. They take their food in the same way as was previously described in the case of the Protamba (vol. i. p.
186). Their propagation by division can sometimes be observed. (Fig. 10 _C_, _D_.) I have described the processes in an earlier chapter (vol. i.
p. 187). Many of these formless Ambae have lately been recognized as the early stages of development of other Protista (especially the Myxomycetae), or as the freed cells of lower animals and plants. The colourless blood-cells of animals, for example, those of human blood, cannot be distinguished from Ambae. They, like the latter, can receive solid corpuscles into their interior, as I was the first to show by feeding them with finely divided colouring matters (Gen. Morph. i. 271).
However, other Ambae (like the one given in Fig. 10) seem to be independent ”good species,” since they propagate themselves unchanged throughout many generations. Besides the real, or _naked_, Ambae (Gymnambae), we also find widely diffused in fresh water _case-bearing_ Ambae (Lepambae), whose naked plasma body is _partially_ protected by a more or less solid sh.e.l.l (Arcella), sometimes even by a case (Difflugia) composed of small stones. Lastly, we frequently find in the body of many lower animals parasitic Ambae (Gregarinae), which, adapting themselves to a parasitic life, have surrounded their plasma-body with a delicate closed membrane.
The simple naked Ambae are, next to the Monera, the most important of all organisms to the whole science of biology, and especially to general genealogy. For it is evident that the Ambae originally arose out of simple Monera (Protambae), by the important process of segregation taking place in their h.o.m.ogeneous viscid body-the differentiation of an inner kernel from the surrounding plasma. By this means the great progress from a simple cytod (without kernel) into a real cell (with kernel) was accomplished (compare Fig. 8 _A_ and Fig. 10 _B_). As some of these cells at an early stage encased themselves by secreting a hardened membrane, they formed the first vegetable cells, while others, remaining naked, developed into the first aggregates of animal cells.
The presence or absence of an encircling hard membrane forms the most important, although by no means the entire, difference of form between animal and vegetable cells. As vegetable cells even at an early stage enclose themselves within their hard, thick, and solid cellular sh.e.l.l, like that of the Ambae in a state of rest (Fig. 10 _A_), they remain more independent and less accessible to the influences of the outer world than are the soft animal cells, which are in most cases naked, or merely covered by a thin pliable membrane. But in consequence of this the vegetable cells cannot combine, as do the animal cells, for the construction of higher and composite fibrous tracts, for example, the nervous and muscular tissues. It is probable that, in the case of the most ancient single-celled organisms, there must have developed at an early stage the very important difference in the animal and vegetable mode of receiving food. The most ancient single-celled animals, being naked cells, could admit solid particles into the interior of their soft bodies, as do the Ambae (Fig. 10 _B_) and the colourless blood-cells; whereas the most ancient single-celled plants encased by their membranes were no longer able to do this, and could admit through it only fluid nutrition (by means of diffusion).
[Ill.u.s.tration: FIG. 11.-A single Whip-swimmer (Euglena striata), greatly magnified. Above a thread-like las.h.i.+ng whip is visible; in the centre the round cellular kernel, with its kernel speck.]
The _Whip-swimmers_ (Flagellata), which we consider as a third cla.s.s of the kingdom Protista, are of no less doubtful nature than the Ambae.
They often show as close and important relations to the vegetable as to the animal kingdom. Some Flagellata at an early stage, when freely moving about, cannot be distinguished from real plants, especially from the spores of many Algae; whereas others are directly allied to real animals, namely, to the fringed Infusoria (Ciliata). The Flagellata are simple cells which live in fresh or salt water, either singly or united in colonies. The characteristic part of their body is a very movable simple or compound whip-like appendage (whip, or flagellum) by means of which they actively swim about in the water. This cla.s.s is divided into two orders. Among the fringed whip-swimmers (Cilioflagellata) there exists, in addition to the long whip, a short fringe of vibrating hairs, which is wanting in the unfringed whip-swimmers (Nudoflagellata). To the former belong the flint-sh.e.l.led yellow Peridinia, which are largely active in causing the phosph.o.r.escence of the sea; to the latter belong the green Euglenae, immense ma.s.ses of which frequently make our ponds in spring quite green.
[Ill.u.s.tration: FIG. 12.-The Norwegian Flimmer-ball (Magosphaera planula) swimming by means of its vibratile fringes, as seen from the surface.]
A very remarkable new form of Protista, which I have named _Flimmer-ball_ (Magosphaera), I discovered only three years ago (in September, 1869), on the Norwegian coast (Fig. 12), and have more accurately described in my Biological Studies(15) (p. 137, Plate V.).
Off the island of Gis-oe, near Bergen, I found swimming about, on the surface of the sea, extremely neat little b.a.l.l.s composed of a number (between thirty and forty) of fringed pear-shaped cells, the pointed ends of which were united in the centre like radii. After a time the ball dissolved. The individual cells swarmed about independently in the water like fringed Infusoria, or Ciliata. These afterwards sank to the bottom, drew their fringes into their bodies, and gradually changed into the form of creeping Ambae (like Fig 10 _B_). These last afterwards encased themselves (as in Fig. 10 _A_), and then divided by repeated halvings into a large number of cells (exactly as in the case of the cleavage of the egg, Fig. 6, vol. i. p. 299). The cells became covered with vibratile hairs, broke through the case enclosing them, and now again swam about in the shape of a fringed ball (Fig. 12). This wonderful organism, which sometimes appears like a simple Amba, sometimes as a single fringed cell, sometimes as a many-celled fringed ball, can evidently be cla.s.sed with none of the other Protista, and must be considered as the representative of a new independent group. As this group stands midway between several Protista, and links them together, it may bear the name of _Mediator_, or _Catallacta_.
[Ill.u.s.tration: FIG. 13.-Labyrinthula macrocystis (much enlarged). Below is a large group of acc.u.mulated cells, one of which, on the left, is separating itself; above are two single cells which are gliding along the threads of the retiform labyrinth which form their ”tramways.”]
The Protista of the fifth cla.s.s, the _Tram-weavers_, or _Labyrinthuleae_, are of a no less puzzling nature; they were lately discovered by Cienkowski on piles in sea water (Fig. 13). They are spindle-shaped cells, mostly of a yellow-ochre colour, which are sometimes united into a dense ma.s.s, sometimes move about in a very peculiar way. They form, in a manner not yet explained, a retiform frame of entangled threads (compared to a labyrinth), and on the dense filamentous ”tramways” of this frame they glide about. From the shape of the cells of the Labyrinthuleae we might consider them as the simplest plants, from their motion as the simplest animals, but in reality they are neither animals nor plants.
[Ill.u.s.tration: FIG. 14.-Navicula hippocampus (greatly magnified). In the middle of the cell the cell-kernel (nucleus) is visible, together with its kernel speck (nucleolus).]
The _Flint-cells_ (Diatomeae), a sixth cla.s.s of Protista, are perhaps the most closely related to the Labyrinthuleae. These primary creatures-which at present are generally considered as plants, although some celebrated naturalists still look upon them as animals-inhabit the sea and fresh waters in immense ma.s.ses, and offer an endless variety of the most elegant forms. They are mostly small microscopic cells, which either live singly (Fig. 14), or united in great numbers, and occur either attached to objects, or glide and creep about in a peculiar manner.
Their soft cell-substance, which is of a characteristic brownish yellow colour, is always enclosed by a solid and hard flinty sh.e.l.l, possessing the neatest and most varied forms. This flinty covering is open to the exterior only by one or two slits, through which the enclosed soft plasma-body communicates with the outer world. The flinty cases are found petrified in ma.s.ses, and many rocks-for example, the Tripoli slate polish, the Swedish mountain meal, etc.,-are in a great measure composed of them.
A seventh cla.s.s of Protista is formed by the remarkable _Slime-moulds_ (Myxomycetes). They were formerly universally considered as plants, as real Fungi, until ten years ago the botanist De Bary, by discovering their ontogeny, proved them to be quite distinct from Fungi, and rather to be akin to the lower animals. The mature body is a roundish bladder, often several inches in size, filled with fine spore-dust and soft flakes (Fig. 15), as in the case of the well-known puff-b.a.l.l.s (Gastromycetes). However, the characteristic cellular threads, or hyphae, of a real fungus do not arise from the germinal corpuscles, or spores, of the Myxomycetes, but merely naked ma.s.ses of plasma, or cells, which at first swim about in the form of Flagellata (Fig. 11), afterwards creep about like the Ambae (Fig. 10 _B_), and finally combine with others of the same kind to form large ma.s.ses of ”slime,” or ”plasmodia.”
Out of these, again, there arises, by-and-by, the bladder-shaped fruit-body. Many of my readers probably know one of these plasmodia, the aethalium septic.u.m, which in summer forms a beautiful yellow ma.s.s of soft mucus, often several feet in breadth, known by the name of ”tan flowers,” and penetrates tan-heaps and tan-beds. At an early stage these slimy, freely-creeping Myxomycetes, which live for the most part in damp forests, upon decaying vegetable substances, bark of trees, etc., are with equal justice or injustice declared by zoologists to be animals, while in the mature, bladder-shaped condition of fructification they are by botanists defined as plants.
[Ill.u.s.tration: FIG. 15.-A stalked fruit-body (spore-bladder, filled with spores) of one of the Myxomycetes (Physarum albipes) not much enlarged.]
The nature of the _Ray-streamers_ (Rhizopoda), the eighth cla.s.s of the kingdom Protista, is equally obscure. These remarkable organisms have peopled the sea from the most ancient times of the organic history of the earth, in an immense variety of forms, sometimes creeping at the bottom of the sea, sometimes swimming on the surface. Only very few live in fresh water (Gromia, Actinosphaerium). Most of them possess solid calcareous or flinty sh.e.l.ls of an extremely beautiful construction, which can be perfectly preserved in a fossil state. They have frequently acc.u.mulated in such huge numbers as to form mountain ma.s.ses, although the single individuals are very small, and often scarcely visible, or completely invisible to the naked eye. A very few attain the diameter of a few lines, or even as much as a couple of inches. The name which the cla.s.s bears is given because thousands of exceedingly fine threads of protoplasm radiate from the entire surface of their naked slimy body; these rays are quasi-feet, or pseudopodia, which branch off like roots (whence the term Rhizopoda, signifying root-footed), unite like nets, and are observed continually to change form, as in the case of the simpler plasmic feet of the Amboidea, or Protoplasts. These ever-changing little pseudo-feet serve both for locomotion and for taking food.
The cla.s.s of the Rhizopoda is divided into three different legions, viz.
the chamber-sh.e.l.ls, or Acyttaria, the sun-animalcules, or Heliozoa, and the basket-sh.e.l.ls, or Radiolaria. The _Chamber-sh.e.l.ls_ (Acyttaria) const.i.tute the first and lowest of these three legions; for the whole of their soft body consists merely of simple mucous or slimy cell-matter, or protoplasm, which has not differentiated into cells. However, in spite of this most primitive nature of body, most of the Acyttaria secrete a solid sh.e.l.l composed of calcareous earth, which presents a great variety of exquisite forms. In the more ancient and more simple Acyttaria this sh.e.l.l is a simple chamber, bell-shaped, tubular, or like the sh.e.l.l of a snail, from the mouth of which a bundle of plasmic threads issues. In contrast to these _single-chambered forms_ (Monothalamia), the _many-chambered forms_ (Polythalamia)-to which the great majority of the Acyttaria belong-possess a house, which is composed in an artistic manner of numerous chambers. These chambers sometimes lie in a row one behind the other, sometimes in concentric circles or spirals, in the form of a ring round a central point, and then frequently one above another in many tiers, like the boxes of an amphitheatre. This formation, for example, is found in the nummulites, whose calcareous sh.e.l.ls, of the size of a lentil, have acc.u.mulated to the number of millions, and form whole mountains on the sh.o.r.es of the Mediterranean. The stones of which some of the Egyptian pyramids are built consist of such nummulitic limestone. In most cases the chambers of the sh.e.l.ls of the Polythalamia are wound round one another in a spiral line. The chambers are connected with one another by pa.s.sages and doors, like rooms of a large palace, and are generally open towards the outside by numerous little windows, out of which the plasmic body can stream or strain forth its little pseudo-feet, or rays of slime, which are always changing form. But in spite of the exceedingly complicated and elegant structure of this calcareous labyrinth, in spite of the endless variety in the structure and the decoration of its numerous chambers, and in spite of the regularity and elegance of their execution, the whole of this artistic palace is found to be the secreted product of a perfectly formless, slimy ma.s.s, devoid of any component parts! Verily, if the whole of the recent anatomy of animal and vegetable textures did not support our theory of plastids, if all its important results did not unanimously corroborate the fact that the whole miracle of vital phenomena and vital forms is traceable to the active agency of the formless alb.u.minous combinations of protoplasm, the Polythalamia alone would secure the triumph of that theory. For we may here at any moment, by means of the microscope, point out the wonderful fact, first established by Dujardin and Max Schulze, that the formless mucus of the soft plasma-body, this true ”matter of life,” is able to secrete the neatest, most regular, and most complicated structures. This secretive skill is simply a result of _inherited adaptation_, and by it we learn to understand how this same ”primaeval slime”-this same protoplasm-can produce in the bodies of animals and plants the most different and most complicated cellular forms.
It is, moreover, a matter of special interest that the most ancient organism, the remains of which are found in a petrified condition, belongs to the Polythalamia. This organism is the ”Canadian Life's-dawn”
(_Eozoon canadense_), which has already been mentioned, and which was found a few years ago in the Ottawa formation (in the deepest strata of the Laurentian system), on the Ottawa river in Canada. If we expected to find organic remains at all in these most ancient deposits of the primordial period, we should certainly look for such of the most simple Protista as are covered with a solid sh.e.l.l, and in the organization of which the difference between animal and plant is as yet not indicated.
We know of but few species of the _Sun-animalcules_ (Heliozoa), the second cla.s.s of the Rhizopoda. One species is very frequently found in our fresh waters. It was observed even in the last century by a clergyman in Dantzig, Eichhorn by name, and it has been called after him, Actinosphaerium Eichhornii. To the naked eye it appears as a gelatinous grey globule of mucus, about the size of a pin's head.
Looking at it through the microscope, we see hundreds or thousands of fine mucous threads radiating from the central plasma body, and perceive that the inner layer of its cell-substance is different from the outer layer, which forms a bladder-like membrane. In consequence of its structure, this, the little sun-animalcule, although wanting a sh.e.l.l, really rises above the structureless Acyttaria, and forms the transition from these to the Radiolaria. The genus Cystophrys is of a nature akin to it.
The _Basket-sh.e.l.ls_ (Radiolaria) form the third and last cla.s.s of the Rhizopoda. Their lower forms are closely allied to the Heliozoa and Acyttaria, whereas their higher forms rise far above them. They are essentially distinguished from both by the fact that the central part of their body is composed of many cells, and surrounded by a solid membrane. This closed ”central capsule,” generally of a globular shape, is covered by a mucous layer of plasma, out of which there radiate on all sides thousands of exceedingly fine threads, the branching and confluent so-called pseudopodia. Between these are scattered numerous yellow cells of unknown function, containing grains of starch. Most Radiolaria are characterized by a highly developed skeleton, which consists of flint, and displays a wonderful richness of the neatest and most curious forms. Sometimes this flinty skeleton forms a simple trellice-work ball (Fig. 16 _s_), sometimes a marvellous system of several concentric trelliced b.a.l.l.s, encased in one another, and connected by radial staves. In most cases delicate spikes, which are frequently branched like a tree, radiate from the surface of the b.a.l.l.s.