Once upon a time there were animals and plants: animals moved around and ate things, while plants just stood there and grew. Then came the microscope, and a multitude of new creatures were revealed. Originally the ancient division (fossilised in the question game Animal, Vegetable or Mineral) was adhered to: and the microscopic bugs that moved around and ate things were classed as animals (protozoa), and the rest as plants (algae). In the nineteenth and earlier twentieth century the inadequacy of this was recognised, Fungi were separated into a third kingdom, and various attempts were made at hiving off some of the single-celled bugs into a fourth or fifth kingdom: Monera, Bacteria, Protozoa, Protista, Protoctista.

In the late twentieth century DNA evidence has come to the fore, and the situation has become much more complicated. Most of the diversity is in the microscopic world, and the Plants, Animals, and Fungi together appear to form just one branch among many. The reclassification is not over, and many revisions and disputes will occur over the next twenty years perhaps; but this time we are seeing a classification emerge not by phenotypic similarities (what they do, how they move, what they look like) or even morphological or embryological characters, but the one true classification by evolutionary descent.

So the taxon of Animalia needs to be defined as a clade, an animal-like common ancestor of all animals, together with all its descendants. It may be that among these descendants are some that no longer move, or have returned to photosynthesis. Because of their ancestry they would still be animals.

Instead of three or four kingdoms, there are now several dozen just among the Eucarya, the creatures with cellular nuclei. Of these, five kingdoms produce large multi-celled organisms: Rhodophyta (red seaweeds, first to achieve this body plan), Phaeophyta (brown seaweeds), Animalia, Fungi, and Plantae. Animals and fungi appear to be most closely related to each other (jointly called opisthokonts). Other kingdoms produce multicellular masses in the form of slime, but mostly the cells are independent, not truly specialised. Out of the five true multicellular kingdoms, animals are the most specialised: most animals die if you cut them in half, because the halves no longer have all the machinery required for vital processes, and cannot regenerate it from other kinds of cell. The distinct parts are connected by nerves.

On the edge of the animal kingdom, however, are creatures that can be divided. The sponges, phylum Porifera, are so different from other animals that it may be doubted whether they are animals, that is whether they arose in the same evolutionary event as the rest. They are colonial, relatively unspecialised, and divisible. Genetic and other evidence puts them on a branch of their own. There is another such creature, the tiny and little-known Trichoplax adhaerens, only representative of the phylum Placozoa, another distant branch.

The multi-celled animals, including sponges and Placozoa, are called Metazoa. Of the former taxon of single-celled Protozoa, one lineage seems to be especially close to Animalia: the choanoflagellates are possibly what the single-celled animal ancestor was like. So they're either a sister kingdom to Animalia, or the furthermost branch of them.

Body plans

Apart from the Choanoflagellata, Porifera, and Placozoa, all the rest are the indisputably animalian creatures, and this taxon is called Eumetazoa.

The major division in the Eumetazoa is between the radially symmetric coelenterates and the bilaterally symmetric Bilateria. Coelenterates include coral, jellyfish, and sea anemones. These are also known as Cnidaria, and the old taxon of Coelenterata is now widely regarded as two separate taxa, Cnidaria and the comb jellies Ctenophora. The similarities between them are merely symplesiomorphies, remnants of the common ancestral form of all eumetazoans.

The animal embryo begins by developing as a blastula, a hollow ball of cells one cell thick. This turns in on itself, forming a hollow called the blastopore, and therefore becoming a two-cell-thick cup called the gastrula. The cnidarians and ctenophores are still basically a gastrula in their mature form. They have just an inner layer and an outer layer, endoderm and ectoderm, filled with a jelly called mesogloea, and the blastopore serves as both mouth and anus (eww).

The Bilateria are also known as triploblastic, because they have developed a third layer of cells in the embryonic stage, the mesoderm. These are what form muscles, blood vessels, and other internal parts that give them fast movement and fast response: that make them quintessentially animal, in fact.

Bilateral symmetry comes from the fact that left and right are generally not important; there is no evolutionary pressure to distinguish them. Gravity gives every large life-form an up and down, and another major innovation of triploblasts is the head, a device that brings nerve control together with light detection and a preferred direction. Note that Bilateria is defined by descent: the echinoderms (starfish and sea urchins) have reverted to radial symmetry in their adult form, but are still taxonomically bilaterian.

Within the Bilateria the major division is between Protostomes and Deuterostomes. Recall that coelenterates retain the primitive blastopore as both mouth and anus (eww). Protostomes ("first mouth") use the blastopore for both but stretch it and fuse it in the middle to create two openings. Deuterostomes ("second mouth") create an entirely separate opening for the mouth. Vertebrates and echinoderms are deuterostomes: we're more closely related to starfish than to snails, bees, or earthworms. The major protostome phyla include the molluscs, arthropods, and annelid worms. There is currently considerable dispute about the classification of the 30-odd animal phyla, waiting for enough DNA evidence to preponderate.

The head is one example of a segment, and many animals are segmented, either profuse in number like the millepedes or extravagant in form like the lobsters. One defining common feature of all animals might be the homeobox genes that control segmentation.

Meet the family

Now would be a good place for a tree diagram. In reality all (or almost all) of the branchings must be binary, so showing more than two at equal levels is just a deficiency of the diagram. It's also absurdly anthropocentric, giving much more detail along the line leading to us, like the New Yorker cartoon of New York. If anything it should be centred on beetles (about a fifth of all known animal species). Most minor lineages have been ignored, for reasons of space, and only a few of the most representative members have been named, and a few well-known extinct animal types. A number after a name is an estimate of the number of living species.

Several familiar terms are missing from this diagram, such as reptiles, dinosaurs, and amphibians. The traditional taxon of Reptilia is paraphyletic: that is, it would include the last common ancestor of the Amniota and some but not all of its descendants, since Mammalia and Aves (birds) are traditionally not classed as reptiles. In cladistics the only true taxa are clades, so Amniota belongs on the diagram but there is no place for Reptilia. Likewise, Dinosauria would need to include birds; and Amphibia is ancestral to amniotes.

Source. The explanations and classifications presented here are derived from those given by Colin Tudge in his excellent, accessible book The Variety of Life (Oxford, 2000). He steers between various competing claims, and warns that new DNA studies will make some of the book outdated soon. The same caveats apply to this writeup.