Animal Diversity

Animals are multicellular eukaryotes whose cells are bound together by collagen. Unlike plants and fungi, their cells have no stiff walls. Most animals have muscles and can move, and that mobility shaped the rest of their tissues and organs (see Britannica).

Movement matters so much to the idea of an animal that sponges, which have no muscle tissue, were long thought to be plants. Only after their small movements were noticed in 1765 were they slowly accepted as animals (see Britannica).

The first animals appeared in the Ediacaran Period, about 635 to 541 million years ago, as soft-bodied forms (see Britannica). The History of Life lesson tells how they spread in the Cambrian explosion, and Classification shows where animals sit among the domains of life.

Symmetry. Most sponges have no symmetry. Cnidarians and echinoderms have radial symmetry, with body parts arranged around a central axis. The vast majority of animals, from insects to mammals, have bilateral symmetry: one plane splits them into matching left and right halves (see Britannica).

Germ layers. Early in development an embryo forms three germ layers: the ectoderm outside, which makes skin and nerves; the endoderm inside, which lines the gut; and the mesoderm between them, which makes muscle, bone and blood. The mesoderm sets bilateral animals apart from radial ones (see Britannica).

Body cavity. Many animals have a coelom, a body cavity between the gut and the body wall (see Britannica). Flatworms have no body cavity besides the gut (see UCMP). Roundworms have a different cavity, the pseudocoelom, but the animals that have one do not form a natural group (see UCMP).

Protostomes and deuterostomes. In the embryo the gut starts as an opening called the blastopore. In deuterostomes, which include echinoderms and chordates, the blastopore becomes the anus. Textbooks say that in protostomes it becomes the mouth, but its fate actually varies among protostome groups, and in one priapulid worm it becomes the anus (see Martín-Durán and others). So treat this split as a rule of thumb with exceptions.

• Sponges (Porifera) have no mouth. Water is drawn in through pores and driven through the body in one direction by beating flagella. Their cells are more independent than in other animals (see UCMP).
• Cnidarians (Cnidaria), such as jellyfish, corals and sea anemones, all carry stinging cells called nematocysts. Most live in the sea (see UCMP).
• Flatworms (Platyhelminthes) are the simplest animals with bilateral symmetry and three cell layers. Their gut has a single opening, and they are flat because they breathe by diffusion. Planarians are free-living; flukes are parasites (see UCMP).
• Molluscs (Mollusca), such as snails, clams and octopuses, have a soft body with a head, a foot and a visceral mass, covered by a mantle that usually makes the shell (see UCMP).
• Annelids (Annelida) are the segmented worms: earthworms, leeches and polychaetes (see UCMP).
• Nematodes (Nematoda) are the roundworms. A single handful of garden soil can hold many thousands of them (see UCMP).
• Arthropods (Arthropoda), such as insects and crustaceans, have a hard outer skeleton of chitin, segmented bodies and jointed legs (see UCMP).
• Echinoderms (Echinodermata), such as starfish and sea urchins, live only in the sea. Most living ones have fivefold symmetry, and water-filled canals work their tube feet (see UCMP).
• Chordates (Chordata) have pharyngeal slits, a dorsal nerve cord, a notochord and a tail behind the anus at some stage of life. Not all chordates are vertebrates (see UCMP).

DNA can regroup the phyla. In 1997 a study of ribosomal DNA joined nematodes and arthropods with other moulting animals in a group called Ecdysozoa, and found no support for grouping annelids with arthropods because both are segmented (see Aguinaldo and others). Classification explains how to read the trees that show these groups.

The Classification lesson used a dichotomous key for six animals. This one goes a level up and keys an animal to its phylum, using the traits from section 3.

In 2011, one count found about 1.55 million described animal species in 40 phyla. Arthropods made up about 1.24 million of them, roughly 80%, and insects alone about 66% of all animals (see Zhang). That makes arthropods by far the largest phylum.

The same count gave about 117 000 molluscs, 29 000 flatworms, 25 000 nematodes, 21 000 echinoderms and 17 000 annelids (see Zhang).

Counts disagree because new species are named every year and sources count differently. UCMP gives more than 15 000 known nematodes and suggests half a million may be undiscovered (see UCMP). It says the true number of arthropods is probably in the tens of millions (see UCMP). Treat any species count as an estimate with a date on it.

Vertebrates go back more than 500 million years, to the late Cambrian (see UCMP). Each new feature below opened up a new way of life.

• Jaws. The first vertebrates had no jaws, like living hagfish and lampreys. Jawed vertebrates appeared about 100 million years later, in the Silurian. Vertebrate skull and jaw bones come from neural crest cells, found only in vertebrates (see UCMP).
• Limbs on land. Tetrapods, the four-footed vertebrates, began moving onto land about 360 million years ago. Their closest living relatives are the lungfish (see UCMP).
• The amniotic egg. A shell keeps the egg from drying out, and membranes inside handle food, gas exchange and waste. Turtles, lizards, birds and mammals are all amniotes (see UCMP).
• Feathers. Archaeopteryx, about 150 million years old, had feathers and a wishbone like a bird, but also teeth, a long bony tail and clawed fingers like a dinosaur (see UCMP).
• Hair and milk. Mammals are hairy, milk-producing and warm-blooded. The earliest mammals laid eggs, and the monotremes still do (see UCMP) (see UCMP).

The Evolution lesson shows how shared features like these count as evidence, and History of Life follows life onto land. For what animals do with these bodies, see Animal Behavior.

Naming and sorting animals took centuries of collecting, a few famous fossils and, lately, DNA.