23 ANIMAL EVOLUTION— THE INVERTEBRATES Chapter Outline IMPACTS, ISSUES: OLD GENES, NEW DRUGS 23.1 ANIMAL TRAITS AND TRENDS What Is an Animal? Animal Tissues Body Size Revisited Overview of Animal Body Plans 23.2 ANIMAL ORIGINS AND EARLY RADIATIONS 23.3 SPONGES—SUCCESS IN SIMPLICITY General Characteristics Sponge Reproduction 23.4 CNIDARIANS—SIMPLE TISSUES, NO ORGANS General Characteristics Unique Cnidarian Weapons Life Cycles 23.5 FLATWORMS—SIMPLE ORGAN SYSTEMS Structure of a Free‐Living Flatworm Flukes and Tapeworms—The Parasites 23.6 ANNELIDS—SEGMENTS GALORE Simple to Highly Modified Segments Annelid Adaptations—A Case in Point 23.7 THE PLIABLE MOLLUSKS Hiding Out, or Not On the Cephalopod Need for Speed 23.8 ROUNDWORMS 23.9 WHY SUCH SPECTACULAR ARTHROPOD DIVERSITY? Major Groups Key Arthropod Adaptations 23.10 SPIDERS AND THEIR RELATIVES Animal Evolution—The Invertebrates 1 23.11 A LOOK AT THE CRUSTACEANS 23.12 A LOOK AT INSECT DIVERSITY 23.13 UNWELCOME ARTHROPODS 23.14 THE SPINY‐SKINNED ECHINODERMS SUMMARY SELF‐QUIZ CRITICAL THINKING Animal Evolution—The Invertebrates 2