A simple guide to the classification of animals

Introduction

Living things can be classified in many different ways (Can you eat it? Can it fly? etc) but the most useful way and the one which most scientists use is that originally developed by a Swedish scientist called Karl von Linné (1707-1778). He is often known by the Latinised form of his name Carolus Linnaeus, and the classification system he developed is usually called the Linnaean system.

In his original system von Linné used only Latin and Latinised words (for example animalia for animals etc), although later Greek words were incorporated; in this Guide I have not always used Latin or Greek words where there is an equivalent well-known English word that is easier to understand or remember.

Originally animals and other organisms were classified only on the basis of observable characteristics (things which you can actually see or count, for example does it have feathers? or how many legs does it have?), but more recently it has also been possible to classify them according to the amount of DNA they have in common and the way in which they have evolved. All three methods lead to an almost identical classification system.

Classification into groups

We classify living organisms by putting them into groups. At each level each organism in a group possesses one property which it shares with all other organisms in that group but which no organism not in that group possesses. There is no overlapping of groups: an organism cannot be a member of two groups at the same level.

Each group may then be divided into a number of other groups according to the same rules - this takes us through the different levels of classification.

In animal classification all the groups at each level are given names: they are
Kingdom
Phylum
Class
Order
Family
Genus
Species

The Linnaean Classification System

Living organisms are first divided into Kingdoms:

Kingdom Plants
Kingdom Animals
Kingdom Fungi
Kingdom Monera (or Prokaryotes)
Kingdom Protista (or Protoctista)

Animals and plants make up the Higher Kingdoms and fungi, prokaryotae and protoctista the Lower Kingdoms.

Until very recently amoeba and certain other single-celled organisms were classed as animals (the protozoans), but now they are classed not as animals but protoctista.

For more information about these Kingdoms please see the page on Cells, viruses and living things.

Although in this Guide we are concerned only with animals, organisms in the other Kingdoms are classified in a very similar way.

The Animal Kingdom is divided into a number of phyla.

Phylum mollusca (shell-fish, slugs, snails etc)
Phylum porifora (sponges etc)
Phylum annelida (segmented worms etc)
Phylum arthropoda (spiders, insects, crustacea etc)
Phylum chordata (animals with notochords - see below)
Phylum echinodermata (star fishes, sea urchins etc)
Many other phyla

Some phyla are divided into sub-phyla. The phylum chordata is divided into

Sub-phylum vertebrata (animals with backbones)
Sub-phylum tunicata (sea squirts etc)
Sub-phylum cephalochordata (lancelets etc)

A notochord is a hollow tube of nerves running along the back of the animal. In vertebrates it is the spinal cord, which runs down the middle of the backbone. Sea squirts and lancelets are very primitive animals living in the sea: it seems hard to believe that we have more DNA in common with them than we do with any other animal without a backbone!

We often divide animals into vertebrates and invertebrates but this is not a particularly useful way of classifying animals, and of course invertebrates outnumber vertebrates by a factor of several thousand, both in numbers of individuals and number of species - there are three hundred thousand species of beetles alone!

We do this because many vertebrates are about our size whereas most invertebrates are very much smaller than us. If any aliens from another planet visited Earth and they were about one centimetre high they would have no doubt at all that arthropods were the dominant life form on this planet - and we would probably not even notice them!

Each phylum or sub-phylum is divided into a number of classes. The sub-phylum vertebrata is divided into

Class fishes
Class amphibians (Frogs, toads, newts, salamanders etc)
Class reptiles (Turtles, tortoises, lizards, snakes etc)
Class birds (vertebrates with feathers)
Class mammals (vertebrates the female of which produces milk for her young)

Mammals also usually have hair (or fur) on their bodies and this is a more easily observable way of deciding whether a vertebrate is a mammal. Amphibians have smooth bodies, birds have feathers, mammals have hair, and fishes have scales and gills while reptiles have scales and nostrils.

Amphibians (from the Greek for double life) lay their eggs in water and hatch into tadpoles which can live only in water. Although the adults can live on land the eggs must be laid and fertilized in water.

The arthropods (from the Greek for jointed legs) are animals with a hard exoskeleton, like a suit of armour, with joints in it to allow them to move, and they can only grow by shedding this and growing a new one. The phylum arthropoda is divided into

Class crustacea (barnacles, prawns, woodlice etc)
Class arachnida (scorpions, spiders, mites etc)
Class chilopoda (centipedes etc)
Class diplopoda (millipedes etc)
Class insecta (insects)
Many other classes

Some but not all classes are divided into a number of sub-classes. The class mammals is divided into

Sub-class monotremes (egg-laying mammals - the duckbilled platypus and spiny anteater)
Sub-class marsupials (mammals with pouches - kangaroos etc)
Sub-class placentals (mammals which develop using a placenta)

All monotremes are native only to Australasia, and all mammals native to Australasia, except bats, are either monotremes or marsupials. There are a few species of marsupial in South America but the vast majority of mammal species are placentals.

Each class or sub-class is divided into a number of orders. All adult insects have three parts to their body and three pairs of jointed legs, whereas arachnids (spiders etc) have only two parts to their body but four pairs of legs. Insects are divided into

Order odonata (dragonflies etc)
Order orthoptera (grasshoppers etc)
Order coleoptera (beetles etc)
Order hymenoptera (ants, bees, wasps etc)
Order lepidoptera (butterflies, moths etc)
Many other orders

More than a third of all insect species are beetles.

The placental mammals are divided into

Order insectivora (shrews, moles, hedgehogs etc)
Order chiroptera (bats)
Order primates (monkeys, apes, Man etc. - see Note at end)
Order cetacea (whales, dolphins etc)
Order carnivora (bears, lions, wolves, tigers etc - but see also Note at end)
Order pinnipedia (seals, walruses, sea lions etc)
Order proboscidae (elephants)
Order perissodactyla - odd-toed hoofed mammals (horses, rhinoceroses etc)
Order artiodactyla - even-toed hoofed mammals (pigs, cows, giraffes etc)
Order rodentia (rats, mice, hamsters etc)
Several other orders

Of all mammals, half are rodents and a quarter are bats.

The birds are divided into

Order pelecaniformes (pelicans, cormorants, gannets etc)
Order anseriformes (swans, ducks, geese etc)
Order falconiformes (eagles, hawks, vultures etc)
Order galliformes (pheasants, grouse, partridges etc)
Order psittaciformes (parrots, parakeets, cockatoos etc)
Order passeriformes (perching birds)
Order strigiformes (owls etc)
Several other orders

More than half of all species of birds are passeriformes. This order includes larks, swallows, blackbirds, ravens, tits, finches, robins etc.

Each order may be divided into a number of sub-orders.

The order primates is divided into

Sub-order anthropoidea (monkeys, apes and Man etc. - see Note)
Sub-order prosimii (lemurs etc)

Each order or sub-order is divided into a number of families.

The carnivores are divided into

Family felidae (lions, tigers, cats etc)
Family canidae (dogs, wolves, foxes, jackals etc)
Family ursidae (bears etc)
Several other families

Each family is divided into a number of genera

The felidae are divided into

Genus Panthera (lions, tigers etc)
Genus Felis (domestic cats etc)
Several other genera

Each genus is divided into a number of species. The species is the lowest level of classification in general use.

The scientific name of an animal is made up of the generic name (name of the genus) and the specific name (name of the species). The scientific name is always printed in italics (or bold), and when hand-written is always underlined. The generic name is always one word and starts with a capital letter; the specific name is usually one word but can be two words. This is called the binomial system, from the Latin for two names

To allow for the different ways different browsers interpret web sites, in this site scientific names are written thus Ornithorhynchus anatinus (duck billed platypus).

In scientific articles it is usual to abbreviate the scientific name, after its first occurrence in the article, by using only the first (capital) letter of the genus, for example P. leo for Panthera leo.

For animals which reproduce sexually, all animals of the same species are inter-fertile, that is, a male and a female of the same species can mate and produce fertile young. Animals of the same genus but not the same species can mate and produce young, but the offspring are usually infertile and cannot themselves mate and produce young. They are called hybrids. Hybrids are frequently Man-made but do not often occur in Nature. Animals not of the same genus very seldom mate, even if kept together in captivity, and even if they do mate no offspring will result.

A mule is a hybrid, the offspring of a he-ass (Equus asinus) and a mare (E. equus). Mules are very hard-working and docile. A hinney is the offspring of a stallion and a she-ass. Hinneys are much less hard-working and docile than mules and are not usually allowed to happen.

A tigon is the offspring of a tiger and a lioness, and a liger of a lion and a tigress.

The species is the lowest level of classification commonly used. The main difficulty with using further levels of classification, such as skin colour in Man, in any objective way is that it is not easy to find any characteristic which is possessed by all the individuals within the group but not possessed by any individuals outside the group.

Man and his classification

In Greek and Latin, and many other languages, there is one word to mean human being and quite another to mean an adult male human being. A human being would be homo in Latin and anthropos in Greek; the Latin for an adult male human being would be vir. But in English the same word is used for both any human being and an adult male. It is conventional therefore in scientific literature to use ‘Man’ to indicate all human beings, and ‘man’ to indicate a single adult male human being, and to regard ‘Man’ as a singular masculine noun, even though of course Man includes men and women and boys and girls. His classification is

Kingdom He is a member of the animal kingdom because he is a multicellular organism and his cells have a nuclear membrane but no cell walls.

Phylum He is a chordate because he has a notochord.

Sub-phylum He is a vertebrate because he has a backbone.

Class He is a mammal because the female produces milk for her young.

Sub-class He is a placental because the unborn baby is nourished inside his mother’s body by means of a placenta.

Order He is a primate because he has five digits on each hand and foot and grasping hands.

Sub-Order He is in the sub-order anthropoidea because he has eyes on the front of his head giving good stereoscopic vision.

Family He is a hominid (Man-like creature) because he walks erect, on two legs. DNA studies and the fossil record have shown that hominids and apes evolved from a common anthropoidal ancestor about five million years ago. In the past there have been several genera in this family but now there is only one, Homo.

Genus He is in the genus Homo. DNA studies and the fossil record have shown that the first people that we would recognise as human evolved a little less than two million years ago. In the past there have been several other species in this genus (Homo habilis, Homo erectus etc) but now there is only one, Homo sapiens, although Homo neanderthal only became extinct about thirty thousand years ago.

Species We call ourselves Homo sapiens (wise Man) because we think that what makes us different from other animals, but particularly Homo neanderthal, is how clever we are!

DNA and other studies have shown that H. sapiens and H. neanderthal evolved about one hundred thousand years ago from an earlier species. H. sapiens coexisted with H. neanderthal for about seventy thousand years. Neanderthals used very sophisticated flint tools and buried their dead, and had bigger brains than H. sapiens! They became extinct only about thirty thousand years ago.

Neanderthals are so called because their remains were first found in the Neander valley, in Germany, at the end of the nineteenth century. The German for valley has always been pronounced tal but at the time the first Neanderthal remains were discovered it was always spelt thal. Over the past fifty years many German spellings have been simplified and rationalised, so today in Germany it is usually spelt tal, so you may see either neanderthal or neandertal, but it is pronounced nee-and-er-tal whichever spelling you use.

A note on the word carnivore

In English, as in many other languages, the same word may have two, quite different, meanings, and carnivore is one such word.

We can divide animals into herbivores, which eat mainly plants, carnivores, which eat mainly animals, and omnivores, which eat both animals and plants. In this sense hedgehogs, lions, dragonflies, lizards, eagles and perch are all carnivores, and cows, pandas and gorillas are herbivores.

But in the Linnaean system of classification described in this Guide carnivores are members of that order of mammal whose teeth are adapted for the tearing of flesh. In this sense lions are carnivores but so also are pandas because, although wild pandas actually eat mainly bamboo, their ancestors ate meat and their teeth are similar to those of other animals who tear flesh, whereas hedgehogs are insectivores (mammals with long snouts adapted for eating insects) and dragonflies, lizards, eagles and perch are not mammals at all and so of course are not members of any mammalian order.