Animal Systematics

a branch of systematics. The animal kingdom was first categorized in the fourth century B.C. by Aristotle, who described more than 450 forms. Aristotle divided all animals into two large groups—animals with blood and those without—a division similar to the modern division of all animals into vertebrates and invertebrates. He further subdivided animals with blood into groups approximating modern classes. His system was less complete in regard to invertebrates. Of the modern phyla, Aristotle more or less correctly distinguished only Arthropoda (jointed-legged animals).

The next 2,000 years did not contribute anything essentially new to animal systematics. Only in 1693 did the British biologist J. Ray introduce the concept of species, which is basic to systematics. In 1735 the Swedish naturalist C. Linnaeus applied the concept in classifying animals and plants. He improved animal systematics by introducing the interrelated taxonomic categories of species, genus, order, and class. In the tenth edition of his Systema naturae (vols. 1–2, 1758–59), the total number of species exceeded 4,200, including 1,222 species of vertebrates and 1,936 species of insects. However, Linnaeus’ system was incomplete; for example, he combined protozoans, coelenterates, echinoderms, and cephalopods in the artificial group Zoophyta.

The French scientists J. Lamarck and G. Cuvier made significant contributions to animal systematics. In Système des animaux sans vertèbres (1801) and Philosophie zoologique (1809), Lamarck categorized all animals as either invertebrates or vertebrates. In the first group he distinguished the classes Infusoria, Polypi, Radiata (including Coelenterata and Echinodermata), Vermes, Insecta, Arachnoidea, Annelides, Crustacea, Cirripedia, and Mollusca. Cuvier, in Le Règne animal distribué d’après son organisation (vols. 1–4, 1817), established four main branches of animals, and in 1825 the French zoologist H. de Blainville classified the branches as the phyla Radiata, Articulata, Mollusca, and Vertebrata.

The foundations of animal systematics as established by Lamarck and Cuvier were further developed. As early as 1826 the Scottish zoologist R. Grant distinguished the phylum Porifera (sponges), separating it from Cuvier’s phylum Radiata. This completely correct categorization, however, was long disputed, with many zoologists continuing to associate sponges with coelenterates as late as the 20th century. An important reform in animal systematics was made by the German zoologist C. von Siebold, who recategorized Cuvier’s phylum Radiata as the three phyla Protozoa, Zoophyta, and Vermes. Siebold classified the majority of Radiata as Zoophyta and grouped annelids with Vermes, removing the former from Cuvier’s phylum Articulata. The remaining articulates formed the phylum Arthropoda.

In 1851 the German scientist K. Vogt recategorized the phylum Vermes as Platyhelminthes, Nematoda, and Annelida. He classified turbellarians, trematodes, tapeworms, and nemertines as platyhelminths. In 1877 the British comparative anatomist E. Ray Lankester proposed classifying these principal groups of worms as phyla. As early as 1874 the German zoologist C. Claus distinguished the following nine phyla: Protozoa, Coelenterata (together with Porifera), Echinodermata, Vermes, Arthropoda, Mollusca, Molluscoidea, Tunicata, and Vertebrata. This system was used for many years; its proponents included the Russian zoologist V. M. Shimkevich (1923).

With time, ideas about the number and scope of phyla changed radically. Groups that in earlier systems had had no definite position or rank were categorized in the appropriate phyla. Thus, the Tunicata, which even in the late 19th century were considered by many zoologists to be a special group of mollusks, and the Enteropneusta, which were considered to be a special class of worms, were subsequently added to the phylum Vertebrata (later renamed Chordata).

The continuing study of the animal world has revealed new species, genera, and families, as well as new groups of higher taxonomic rank, including orders, classes, and even phyla. Thus, in 1955 the new phylum Pogonophora was established. In recent decades, new orders of coelenterates, turbellarians, and crustaceans have been discovered.

The kingdom Animalia is usually divided into two subkingdoms—Protozoa (or Protozoobionta; unicellular organisms) and Metazoa (or Metazoobionta; multicellular organisms). The latter is subdivided into Parazoa (also called Enantiozoa), which includes Porifera, and Eumetazoa (also called Enterozoa), which include all remaining phyla. In turn, Eumetazoa is divided into Radialia, which includes Cnidaria and Ctenophora, and Bilateria, which includes Protostomia (worms, mollusks, echiuroids, arthropods, sipunculids, tentaculates) and Deuterostomia (hemichordates, echinoderms, chordates).

Some contemporary zoologists have proposed the establishment of a third group, Phagocytellozoa, of equal rank to Parazoa and Eumetazoa. Phagocytellozoa would include Trichoplax adhaerens, the most primitive multicellular organism. At the same time, Tentaculata, Chaetognatha, and Pogonophora would be considered to be independent branches of Coelomata equal to the higher Protostomia (or Trochozoa) and Deuterostomia.

The number of phyla varies from ten to 33, depending on the system used. One of the accepted systems includes the following 16 phyla: Protozoa, Porifera, Archeocyatha (a fossil group of primitive multicellular animals), Coelenterata, Scolecida, Mollusca, Articulata, Prosopygia (or Sipunculida), Kamptozoa (or Entoprocta), Podaxonia, Brachiopoda, Chaetognatha, Pogonophora, Hemichordata, Echinodermata, and Chordata. According to another common system, Coelenterata are reclassified as the independent phyla Cnidaria and Ctenophora, and Scolecida are regarded to be the aggregate of the three phyla Platyhelminthes, Nemathelminthes, and Nemertini. Also considered as independent phyla are Echiurida, Annelida, and Arthropoda; in the first system they are included in the phylum Articulata. Finally, Podaxonia, which includes Bryozoa and Phoronidea, and the anatomically close Brachiopoda, are usually united in the single phylum Tentaculata.

The further study of the animal world brings to light an increasingly larger number of known species. Thus, Aristotle described 454 animals that are known today as species, and Linnaeus described 4,208 species. In the late 18th century the German scientist J. Gmelin listed 18,338 species, and in the early 19th century C. Bonaparte listed 48,266. In the late 19th century the German scientist K. Möbius reported the existence of more than 400,000 species. Modern systematics has described approximately 1.3 million extant animal species (according to some sources, as many as 1.5 million species).