This section seeks to give a classfication of living animals: invertebrates, fish, amphibians, reptiles, birds, and mammals.

But before we classify the living animals, we might pause to define what we mean. What is "classification"? What are "animals"? And what is "life"?

Classification, Part 1: Linnaeus' hierarchical system.

First arising in the Renaissance, and culminating in the work of the Swedish botanist Karl von Linné (Carolus Linnaeus, 1707-1778), our hierarchical system of biological classification was born. In it, all species are put in a hierarchy of groups within groups, collectively describing the relationships between all organisms on earth. Each hierarchical group is called a "taxon" (plural form, "taxa"). Today there are seven primary taxonomic ranks, each more specific than the last—from kingdom to species, as shown below on the left. These seven ranks are mandatory, even when they are redundant. If more specific information is needed, optional ranks can be added, as shown below in the middle. Prefixes can be added, too, which allow for a tremendous amount of precision, as shown below on the right.

Classification, Part 2: Classification-related terminology for dummies.

To a non-biologist who takes the time to read about biological classification, the subject can seem fraught with confusing terminology.

The first puzzling thing is that the words (1) taxonomy, (2) systematics, and (3) classification all seem to be used interchangeably. That's because they are, basically.

(1) taxonomy: "The description of species and the classification of organisms into groups the reflect evolutionary relationships." —Miller & Harley, Zoology, 3rd ed. (1996)
(2) systematics: "The study of the classification and phylogeny of organisms." —Miller & Harley (1996)
(3) classification: "The systematic grouping of organisms into categories on the basis of evolutionary or structural relationships between them; taxonomy." —American Heritage College Dictionary, 4th ed. (2002)

There's no difference at all that I can see.

(4) Nomenclature is a another taxonomy-related word, one that refers to the scientific naming of species, or to the resulting names themselves. (Thus, binomal nomenclature simply means "two-part scientific naming," and describes the convention of referring to species with two words; for example, Homo sapiens.)

(5) Phylogeny is another classification-related word that may be unfamiliar to the beginner. It refers to the evolution of a species or group of species. Evolution has everything to do with biological classification, just as genealogy is what determines relationships between people. Darwin said, "Our classifications will come to be, as far as they can be so made, genealogies." He was right; his prediction has come true. Phylogeny is essential to classification.

Classification, Part 3: Three approaches to classification.

1. Evolutionary taxonomy. During the century after the theory of evolution was proposed-from about the 1850s to the 1950s-there was only one approach to animal classification. This approach, guided in its essence by the theory of evolution, came to be known as evolutionary taxonomy.

According to this approach, one of the most important distinctions is between animals' analogous traits and their homologous traits. One example of an analogous trait would be the wing of a bird, as compared to that of a bat or insect. These various wings are similar in function, but different in origin. Their similarities are due entirely to convergent evolution, the process by which

2.

3. Cladistic taxonomy.

But what are animals? For centuries, people have understood animals through their contrast with plants. So suppose that we focus in on this crucial distinction. Is there any one essential quality that all animals possess, but all plants lack? The answer is yes.

1. The essential difference between animals and plants: heterotrophic-ness, or the ability to eat. Above all, animals are "heterotrophic." What does that mean? It means that they gain their nourishment by ingesting organic food, rather than simply by manufacturing their own food from sunlight or inorganic chemicals. ######## ######### ######### ############# ######## ########### ############## ################ ################# ########## ########### ########### ############### ####### ############ ############# ##########

2. Another frequent difference between animals and plants: motility. Also, as a general rule, most animals are "motile"; that is, they can usually move by their own power. There are certain exceptions-floating jellyfish, stationary clams, sessile sponges-thus proving that motility is not an essential characteristic of animals; but still it is a very frequent characteristic, and it's also one that (it could be argued) belongs to no other form of life. And it is an important characteristic, for historically it has governed our way of looking at animals, helping us to distinguish them from plants-ever since we as a species had to hunt animals in order to survive.

20th-century changes in biology and classification: non-plant, non-animal classification. This biological dualism between plants and animals reigned all the way to the mid-20th century. Around that time, however, advances in microscopy, embrylogy, and biochemistry revealed that certain organisms, like bacteria and fungi, are even more dissimilar from plants and animals than plants and animals are from each other. Furthermore, it had long been observed that there are a lot of microorganisms that possess both plant and animal characteristics-another confusing blow to the old plant-animal dualism. As a result of all this, in 1969 R. H. Whittaker proposed a new system, organizing earth's life into not two kingdoms, but five. Bacteria, being the most unique of all organisms, were given their own kingdom (Kingdom Monera), as were fungi (Kingdom Fungi). The same distinction was given to all single-celled organisms (Kingdom Protistia), to accurately reflect how impossible it is to class certain such microorganisms as either plants or animals. Finally, plants and animals retained their kingdom status (Kingdoms Plantae and Animalia). This five-kingdom model worked very well; in fact, some scientists accept it even today, though, due to new findings, it is no longer the preferred system. In the late 1970s, scientists discovered a special kind of anaerobic, methane-producing bacteria. By the early 1980s, scientists realized that these so-called bacteria were in fact just as different from typical bacteria as typical bacteria are from all other forms of life. As a result, these new microorganisms, called archaeobacteria, were given not only their own kingdom, but even their own domain. (See the classification list at the bottom of this page.) At any rate, my point here is this: it is now recognized by everyone that animals must be contrasted not only with plants, but also with other living things, such as fungi, protists, and prokaryotes such as eubacteria and archaebacteria. Hence we need to make some more distinctions.

3. The difference between animals and protozoa: multicellularity. All members of Kingdom Animalia are multicellular, while all members of Kingdom Protista are unicellular. It may seem a short leap from the paramecium (a rather advanced protozoan) to Trichoplax adhaerans (an extremely simple invertebrate animal, consisting of just a few cells), but scientists do draw a line between the two, calling the former a protist and the latter an animal.

4. The difference beteeen animals, etc. vs. bacteria. All animals-as well as all plants, fungi, and protists-are "eukaryotic"; that is, they have a true membrane-bound nucleus, an important characteristic of all higher life forms. Bacteria, by contrast, are "prokaryotic," without a true nucleus, and are thus distinguished from all other life on earth.

So—say it softly, as in a sacred mantra: animals are "multicellular eukaryotic heterotrophs." That's it, you've got it!

Unwritten. ####### ####### ######## ########### ######### ######## ######### ########## ########### ########### ########### ############# ########## ########### ########### ####### ####### ######## ########### ######### ######## ######### ########## ########### ########### ########### ############# ########## ########### ########### ####### ####### ######## ########### ######### ######## ######### ########## ########### ########### ########### ############# ########## ########### ########### ####### ####### ######## ########### ######### ######## ######### ########## ########### ########### ########### ############# ########## ########### ########### ####### ####### ######## ########### ######### ######## ######### ########## ########### ########### ########### ############# ########## ########### ###########.

					Species*
	EMPIRE BIOS? Life on earth. In the 18th century, Karl von Linné (a.k.a. Carolus Linnaeus, the founder of biological classification), proposed a huge taxon called Empire Natura, intended to encompass not only all living things, but the entire scope of physical reality itself. In those days, people considered all matter to be divisible into three categories: animal, vegetable, and mineral. As a result, it seemed easier then than it does now to classify the whole realm of physical reality into discrete categories. Now that our world-view has changed, and we no longer consider the universe so easily divisible, Linné's supreme overarching taxon has fallen into disuse. Still, in a way it would be nice to have such an overarching taxon-not necessarily for all physical reality, but, say, for all life on earth. Such a taxon would serve the valuable purpose of unifying the various kingdoms of life, which at present resemble nothing so much as a body without a head. And that is why I, dauntless nonbiologist that I am, would like to propose such a taxon here and now, one that describes all life on earth. I call it Empire Bios. Besides having the advantage of unifying the kingdoms and domains of life, this taxon also has the virtue of emphasizing the vast scope of our current biological knowledge, while at the same time emphasizing its limits. For although we know countless information about countless species of life that span the whole planet-nevertheless, that is all we know. If ever we discover organisms on other worlds, we may find that they possess a different anatomy, a different physiology, even a different chemistry. If that happens, Empire Bios will be demoted to Subempire Terrestria, and our fundamental conceptions of life will be transformed.
		DOMAIN ARCHAEA. Archaebacteria. ##### ####### ###### ###### ##### #### #### #### ##### ###### ####### ########### ######## #######.
		DOMAIN BACTERIA. Bacteria. Description. ##### ####### ###### ###### ##### #### #### #### ##### ###### ####### ########### ######## #######.
		DOMAIN EUKARYOTA. Eukaryotes. ##### ####### ###### ###### ##### #### #### #### ##### ###### ####### ########### ######## #######.
			KINGDOM PROTISTA. Protists. ##### ####### ###### ###### ##### #### #### #### ##### ###### ####### ########### ######## #######.		40,000??*
				Subkingdom Somethingorother. Somethingorothers. I don't know what kind of protists these are, but they sure aren't animal-like. ##### ####### ###### ###### ##### #### #### #### ##### ###### ####### ########### ######## #######.
				Subkingdom Protozoa. See below.
			KINGDOM FUNGI. Fungi. ##### ####### ###### ###### ##### #### #### #### ##### ###### ####### ########### ######## #######.		275,000??*
			KINGDOM PLANTAE. Plants. ##### ####### ###### ###### ##### #### #### #### ##### ###### ####### ########### ######## #######.		100,000??*
			KINGDOM ANIMALIA. Animals. See below.		1,200,000??*

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