System of the Organic World

The world of living things numbers approximately 2 million species. The multiplicity of organisms is studied by systematics, whose principal task is the construction of a system of the organic world.

After the acceptance of the evolutionary doctrine in biology, systematics strove to create a phylogenetic system of the organic world, that is, a system that would reflect as completely as possible the evolutionary interrelationships between organisms. Phylogenetic systematics is developed at all taxonomic levels, from species and subspecies to the higher taxons—classes, phyla, and kingdoms. A discussion follows only of the macrosystem of the organic world, which includes the highest taxonomic units—kingdoms and subkingdoms.

Biologists have divided the organic world into plants and animals since Aristotelian times. In C. Linnaeus’ system, plants and animals were respectively given the names Vegetabilia and Animalia. This traditional division has been preserved to this day and is included in almost all biological textbooks. However, the shortcomings of this type of division of the organic world became thoroughly clear in the mid-20th century. Of fundamental significance was the discovery that two phylogenetically related groups—bacteria and blue-green algae (Cyanophyta)—differ sharply from other living things, including fungi, in their lack of a true nucleus. The genetic material deoxyribonucleic acid (DNA) occurs freely in their cells, immersed in nucleoplasm, which has no nuclear membrane to separate it from the cytoplasm. The spindle is absent (with amitotic cell division), as are centrioles, microtubules, mitochondria, and plastids. If present, the flagella are organized simply and have a fundamentally different structure than the flagella in plants and animals. These organisms are called prokaryotes.

In all other unicellular and multicellular organisms the nucleus is surrounded by a nuclear membrane that separates it from the cytoplasm. The genetic material of the nucleus is enclosed in chromosomes. A spindle or its analogue, formed from microtubules, is present. In addition to a clearly differentiated nucleus and cytoplasm, the organisms also have mitochondria. Many also have plastids and complex flagella. These organisms are called eucaryotes. It has gradually become clear that the differences between prokaryotes and eucaryotes are far more profound and fundamental than, for example, those between higher animals and higher plants (both groups being eucaryotes).

Thus, prokaryotes form a unique group that is often assigned the rank of a kingdom or even a superkingdom in the system of the organic world. Therefore, the division of the organic world into prokaryotes and eucaryotes is adequately substantiated and not subject to dispute. Far more complex is the subdivision of eucaryotes, which are usually divided into two kingdoms—the animal kingdom and the plant kingdom. Whereas the taxonomic boundaries of the animal world are relatively clear (with the exception of certain groups of flagellates, including euglenoids, which many zoologists traditionally have classified as protozoans), the boundaries of the plant world have been subjected to radical reexamination. Thus, all prokaryotes, including blue-green algae, should be excluded from the plant kingdom. The position of fungi is more controversial. Traditionally assigned to the plant kingdom, they were grouped as an independent kingdom in the early 19th century by the Swedish mycologist E. Fries. This classification was subsequently accepted by the majority of mycologists.

There are differing opinions on the taxonomic scope, origin, and systematic position of fungi. Fungi represent the most enigmatic group of extant organisms and are classified only with the greatest difficulty. It has long been conjectured that fungi, in the broad sense, do not represent a natural monophyletic systematic group and possibly have various origins. Thus, many scientists exclude slime molds (Myxomyceta) from the group of fungi. Kh. Ia. Gobi (1884) and H. A. de Bary (1887) believed that slime molds originate from flagellate protozoans, whereas others classified them as protozoans. Moreover, many mycologists point to the heterogeneous character of slime molds, various groups of which are descended from different flagellate ancestors.

It has not yet been resolved to which of the two eucaryotic groups—animal or plant kingdom—the fungi are most closely related. As early as 1874 the German scientist J. von Sachs proposed that slime molds and basidium fungi (Basidiomycetes) descended from parasitic red algae (Rhodophyta), and in 1881, de Bary hypothesized that they originated from Phycomycetes, algal fungi. Both hypotheses continue to be supported to this day. Relying chiefly on morphological data, some contemporary mycologists believe that sac fungi (Ascomycetes), basidium fungi, and saprophytes (Zygomycetes) originate from red algae. The majority, however, regard the similarities to red algae to be a result of convergence and theorize that true fungi originate from slime molds, and through them, from protozoans. The closeness of fungi to animals is also confirmed by biochemical data, which reveal similarities in many pathways of nitrogen metabolism and in the primary structure of cytochromes and transfer ribonucleic acids.

Thus, according to the latest system of the organic world, already acknowledged by many scientists, the world of living things consists of four kingdoms. Some contemporary scientists distinguish a fifth kingdom—Protista (E. Haeckel)—which includes some algae, namely, pyrrophyta, Chrysophyta, and Euglenophyta, and all protozoans. According to another system, all algae, protozoans, and primitive lower fungi are grouped in this kingdom. The creation of the extremely heterogeneous kingdom Protista is justifiably objected to by many biologists inasmuch as it only makes classification more difficult and generates new problems. Specifically, it has been challenged that many representatives of this artificial kingdom are much more closely related to representatives of the three other eucaryotic kingdoms than to the remaining Protista.

The differences between the superkingdoms Prokaryota and Eucaryota and their subdivisions (kingdoms and subkingdoms) are summarized below.

Summary of higher taxons

Superkingdom Prokaryota. In prokaryotes, a true nucleus enclosed by a nuclear membrane is absent and genetic material is concentrated in a nucleoid. The DNA usually forms a single ring-shaped strand, which is not bonded to proteins and is not yet a true chromosome (with a much more complex structure). There is no typical sexual process, although the exchange of genetic material is sometimes effected during other (parasexual) processes not accompanied by the mergence of nucleoids.

Prokaryotes lack centrioles, microtubules, spindles (with amitotic cell division), plastids, and mitochondria. The glycoprotein murein (peptidoglycan) serves as the supporting framework of the cell wall. Flagella are absent or relatively simple. Many representatives can fix molecular nitrogen. Prokaryotes may be obligate or facultative anaerobes or aerobes. Nutrients are absorbed through the cell wall, with nutrition being either absorptive (saprotrophic or parasitic) or autotrophic. The superkingdom includes one kingdom.

KINGDOM MYCHOTALIA (OR MYCHOTA SCHIZOPHYTA) (bacteria and blue-green algae). (The term “Mychotalia” is derived from the word mychae, which denotes little balls of chromatin incapable of mitosis.) Many scientists use the unsuitable term “Monera,” which was proposed by Haeckel for the presumably anucleate “genus” Protamoeba; the latter proved to be only an anucleate fragment of an ordinary amoeba.

Subkingdom Bacteriobionta (bacteria). Bacteria are characterized by heterotrophic or autotrophic (chemotropic or, more rarely, phototrophic) nutrition. Chlorophyll, when present, is represented by bacteriochlorophylls, and phycocyanin and phycoerythrin are absent. Molecular oxygen is not produced during photosynthesis. Bacteria often have simple flagella. Besides true bacteria, they include actinomycetes, myxobacteria, spirochetes, mycoplasmas, rickettsiae, chlamydiae, and possibly viruses. The system of the Bacteriobionta has not been sufficiently developed and in the future may be subjected to radical revision. The subkingdom probably includes only the single phylum Bacteriomychota (or Bacteria).

Subkingdom Cyanobionta (blue-green algae). In blue-green algae nutrition is autotrophic (photosynthetic). Chlorophyll is represented by chlorophyll a, and phycocyanin and phycoerythrin are present as supplementary photosynthesizing pigments. Molecular oxygen is generated during photosynthesis. The algae do not have flagella. The subkingdom includes the single phylum Cyanomychota (or Cyanophyta).

Superkingdom Eucaryota. Eucaryotes are organisms with a visibly evident nucleus surrounded by a nuclear membrane. The genetic material of the nucleus is enclosed in chromosomes, in which (except in Pyrrophyta) DNA is bonded to proteins. The typical sexual process alternately consists of the mergence of nuclei and reduction division, which occur during meiosis. The eucaryote sexual process is sometimes characterized by apomixis—reproduction without fertilization but in the presence of sexual organs. An example of apomixis is parthenogenesis. Many eucaryotes have centrioles. Plastids, mitochondria, and a well-developed endoplasmic-membrane system are present, as is a more or less typical spindle or analogue of a spindle, which is formed by microtubules (with mitotic cell division). Flagella or cilia, when present, are usually complex in structure, consisting of nine paired (or triple) tubular fibrils arranged along the periphery of the casing and two singular central fibrils also tubular in shape. Eucaryotes cannot fix atmospheric nitrogen. They may be aerobes or, more rarely, secondary anaerobes. Nutrition is absorptive (with nutrients absorbed through the cell wall), autotrophic, or holozoic (with food ingested by an organism and digested internally). Eucaryotes have food vacuoles. Included in the superkingdom are the kingdoms Animalia, Mycetalia, and Vegetabilia.

KINGDOM ANIMALIA. The Animalia comprise initially heterotrophic organisms. A dense cell wall is usually absent. Nutrition is predominantly holozoic, although it is sometimes absorptive. Carbohydrates are stored in the form of glycogen. Reproduction and distribution occur without the aid of spores, except for certain protozoans of the class Sporozoa (spore-formers). The Animalia are motile organisms that are sometimes sedentary (secondary forms).

Subkingdom Protozoobionta (or Protozoa) (protozoans). Protozoans consist of a single cell or colonies of identical cells. The subkingdom usually includes the single phylum Protozoa, which is sometimes reclassified as two or more independent phyla (corresponding to botanical divisions in taxonomic rank).

Subkingdom Metazoobionta (or Metazoa) (multicellular organisms). The Metazoobionta include organisms consisting of many dissimilar specialized cells. Approximately 16 phyla are distinguished, and sometimes as many as 20–23. The most commonly accepted phyla are Porifera (or Spongia; sponges), Coelenterata (or Cnidaria; hydroids, jellyfishes, corals, and related types), Ctenophora (comb jellies or sea walnuts), Platyhelminthes (flatworms), Nemertinea (ribbon worms), Aschelminthes (or Nemathelminthes; unsegmented worms), Annelida (segmented worms), Arthropoda (jointed-legged animals), Onychophora (velvet worms), Mollusca (mollusks), Lophophorata (or Tentaculata; tentaculates), Echinodermata (spiny-skinned animals, such as starfishes), Pogonophora (beard worms), Chaetognatha (arrow worms), Hemichordata (arrow worms and related types), and Chordata (chordates).

KINGDOM MYCETALIA (OR FUNGI, MYCOTA) (fungi). Fungi are heterotrophic (probably initially heterotrophic) organisms. Cells have a dense cell wall that is chitinous or sometimes cellulosic. More rarely, the cell wall is in the form of a membrane, as in oomycetes. Nutrition is absorptive and rarely holozoic. Carbohydrates are stored mainly in the form of glycogen. Flagellate cells are usually absent. Reproduction is by haploid spores, upon whose germination meiosis occurs. Fungi usually are attached to the substrate. They are subdivided into two taxonomic groups, which differ so fundamentally that they unconditionally merit the taxonomic rank of subkingdoms. The common origin of the subkingdoms has not been proven and has been disputed by many mycologists. The two subkingdoms should be studied within the framework of a single kingdom until their interrelationships and relationships with other subkingdoms of the organic world have been established.

Subkingdom Myxobionta (lower fungi). Lower fungi are characterized by a vegetative phase that consists of a plasmodium (a multinucleate motile mass of protoplasm that lacks cell walls) or a pseudoplasmodium (an aggregate of naked mononucleate amoeboid cells that preserve their individuality). Nutrition may be holozoic or absorptive. When present, flagellate cells usually carry two dissimilar flagella. Lower fungi usually have numerous spores and sporangia (sore receptacles). The subkingdom includes the division (phylum) Myxomycetes, (slime molds).

Subkingdom Mycobionta (higher fungi). Higher fungi are characterized by the absence of plasmodia and pseudoplasmodia. The vegetative phase consists of threads (hyphae) or cells with a definite cell wall. Nutrition is only absorptive. Flagellate cells, when present, have one or two flagella. The subkingdom includes the divisions Mastigomycota, Zygomycota (saprophytes), Ascomycota (sac fungi), and Basidiomycota (basidium fungi), as well as the artificial division Deuteromycota (imperfect fungi).

KINGDOM VEGETABILIA (OR PLANTAE) (plants). Plants are autotrophic (phototropic) organisms that are sometimes secondary heterotrophs (saprophytes or parasites). Their cells have a dense wall consisting usually of cellulose and more rarely of chitin (in some algae). Carbohydrates are stored in the form of starch; more rarely (in red algae) they are stored as rhodamylone, a special starch closely related to glycogen. The kingdom is usually divided into two subkingdoms.

Subkingdom Thallobionta (lower plants). In lower plants, gametangia (sexual organs) and sporangia (spore-producing organs) are unicellular or absent. The zygote usually is not transformed into a typical multicellular embryo. Lower plants are without an epidermis, stomata, or a stele (conducting cylinder). The subkingdom includes all algae except blue-green algae. Depending on the systems, algae are classified into from one (Phycophyta) to nine divisions. The following divisions are most commonly used: Cryptophyta, Euglenophyta (flagellates), Pyrrophyta (dinoflagellates), Chrysophyta (golden-brown algae), Phaeophyta (brown algae), Chlorophyta (green algae), and Rhodophyta (red algae). The taxonomic rank of Rhodophyta is unclear because the red algae differ from all other algae by the absence of flagella and many other morphological and biochemical features. Some scientists place them at the beginning of the system of algae, while others consider them to be a highly specialized group. In many respects, dinoflagellates are undoubtedly very primitive, inasmuch as their chromosomes lack histones and are structurally similar to a prokaryotic nucleoid.

Subkingdom Embryobionta (or Telomobionta) (higher plants). Higher plants have multicellular gametangia and sporangia or reduced gametangia. The zygote is transformed into a typical multicellular embryo. The plants have an epidermis, stomata, and usually a stele. The subkingdom includes the divisions Rhyniophyta, Bryophyta (mosses and liverworts), Lycopodiophyta (club mosses), Psilotophyta, Equisetophyta (horsetails), Polypodiophyta (ferns), Pinophyta (or Gymnospermae; gymnosperms), and Magnoliophyta (or Angiospermae; angiosperms).

The division of the organic world into the four kingdoms Mychotalia (blue-green algae and bacteria), Fungi, Vegetabilia, and Animalia, while on the whole sufficiently substantiated from an evolutionary point of view, does not yet form the basis of the classification system used in handbooks, manuals, and textbooks.