Animals are just one branch of the Eukarya domain.

• 11.1 What is an animal?
• 11.2 There are no “higher” or “lower” species.
• 11.3 Four key distinctions divide the animals.

Invertebrates—animals without a backbone—are the most diverse group of animals.

• 11.4 Sponges are animals that lack tissues and organs.
• 11.5 Jellyfishes and other cnidarians are among the most poisonous animals in the world.
• 11.6 Flatworms, roundworms, and segmented worms come in all shapes and sizes.
• 11.7 Most mollusks live in shells.
• 11.8 Are some animals smarter than others?
• 11.9 Arthropods are the most diverse group of all animals.
• 11.10 This is how we do it: How many species are there on earth?
• 11.11 Flight and metamorphosis produced the greatest adaptive radiation ever.
• 11.12 Echinoderms are vertebrates’ closest invertebrate relatives and include sea stars, sea urchins, and sand dollars.

The phylum Chordata includes vertebrates, animals with a backbone.

• 11.13 All vertebrates are members of the phylum Chordata.
• 11.14 The evolution of jaws and fins gave rise to the vast diversity of vertebrate species.
• 11.15 The movement onto land required lungs, a rigid backbone, four legs, and eggs that resist drying.

All terrestrial vertebrates are tetrapods.

• 11.16 Amphibians live a double life.
• 11.17 Birds are reptiles in which feathers evolved.
• 11.18 Mammals are animals that have hair and produce milk.
• 11.19 Humans tried out different lifestyles.
• 11.20 How did we get here? The past 200,000 years of human evolution.