11  •  Animal Diversification 445

Visibility in motion

Animals are just one branch of the Eukarya domain. 446

11.1

What is an animal? 446

11.2

There are no “higher” or “lower” species. 447

11.3

Four key distinctions divide the animals. 448

Invertebrates—animals without a backbone—are the most diverse group of animals. 452

11.4

Sponges are animals that lack tissues and organs. 452

11.5

Jellyfishes and other cnidarians are among the most poisonous animals in the world. 454

11.6

Flatworms, roundworms, and segmented worms come in all shapes and sizes. 456

11.7

Most mollusks live in shells. 459

11.8

Are some animals smarter than others? 462

11.9

Arthropods are the most diverse group of all animals. 463

11.10

This is how we do it: How many species are there on earth? 465

11.11

Flight and metamorphosis produced the greatest adaptive radiation ever. 467

11.12

Echinoderms are vertebrates’ closest invertebrate relatives and include sea stars, sea urchins, and sand dollars. 468

The phylum Chordata includes vertebrates, animals with a backbone. 470

11.13

All vertebrates are members of the phylum Chordata. 470

11.14

The evolution of jaws and fins gave rise to the vast diversity of vertebrate species. 473

11.15

The movement onto land required lungs, a rigid backbone, four legs, and eggs that resist drying. 474

All terrestrial vertebrates are tetrapods. 476

11.16

Amphibians live a double life. 476

11.17

Birds are reptiles in which feathers evolved. 477

11.18

Mammals are animals that have hair and produce milk. 479

11.19

Humans tried out different lifestyles. 481

11.20

How did we get here? The past 200,000 years of human evolution. 483

StreetBIO: KNOWLEDGE YOU CAN USE

Where are you from? “Recreational genomics” and the search for clues to your ancestry in your DNA 486