CHAPTER 18 The Evolution of Animal Diversity

1. CHAPTER 18The Evolution of Animal Diversity Modules 18.1 – 18.4

2. What Am I? • Of some 1.5 million species of organisms known to science, over two-thirds are animals • Humans have a long history of studying animal diversity • But do we always know what an animal is when we see one?

3. Imagine you were the first person to encounter this duck-billed platypus • It has webbed feet, a bill, a furry body, mammary glands, and it lays eggs • With all of its varying characteristics, what would you think it is?

4. Biologists often encounter classification problems when evolution creates organisms with similar characteristics • Marsupials, such as this Tasmanian tiger, are a good example

5. ANIMAL EVOLUTION AND DIVERSITY 18.1 What is an animal? • Animals are eukaryotic, multicellular heterotrophs that ingest their food • They lack cell walls • They also have unique intercellular junctions Figure 18.1A

6. HAPLOID Meiosis Sperm Eggs 1 Metamorphosis 2 • Most animals are diploid except for haploid eggs and sperm 7 Adult • Animals proceed through a well-defined life cycle Digestivetract Zygote(fertilized egg) 3 DIPLOID Larva 6 Outer cell layer(ectoderm) Inner cell layer(endoderm) Blastula(cross section) Opening 4 5 Later gastrula(cross section) Early gastrula(cross section) Figure 18.1B

7. 18.2 The animal kingdom probably originated from colonial protists • Cells in these protists gradually became more specialized and layered Digestivecavity Reproductivecells Somaticcells 3 1 2 Early colony of protists;aggregate ofidentical cells Hollow sphere(shown incross section) Beginningof cellspecialization(cross section) 4 5 Infolding(cross section) Gastrula-like“protoanimal”(cross section) Figure 18.2

8. Ecological, geologic, or genetic factors may have caused the Cambrian explosion in animal diversity • Cambrian fossils can be classified as ancient representatives of the familiar animal phyla

9. INVERTEBRATES 18.3 Sponges have a relatively simple, porous body • Organisms in the phylum Porifera are among the simplest animals • Many sponges are radially symmetrical • Their parts are arranged around a central axis Central axis Figure 18.3A, B

10. Choanocyte incontact withan amoebocyte • Flagellated choanocytes filter food from the water passing through the porous body Pores WATERFLOW Skeletalfiber Centralcavity Choanocyte Flagella Amoebocyte Figure 18.3C

11. They probably evolved from multicellular choanoflagellates, the group that most likely gave rise to the animal kingdom • The sponge lineage arose very early Single cell Stalk Figure 18.3D, E

12. 18.4 Cnidarians are radial animals with stinging threads • Cnidarians are the simplest animals with tissues • These animals exist in two radially symmetrical forms • Polyps, such as hydra, corals, and sea anemones Figure 18.4A, C

13. Medusas, the jellies Figure 18.4B

14. The tentacles, controlled by nerves, then push the food through the mouth into a gastrovascular cavity • In the cavity, the food is digested and then distributed • In cnidarians, only two cell layers are produced during gastrulation • Cnidocytes on their tentacles sting prey

15. Capsule(nematocyst) Coiledthread Tentacle “Trigger” Dischargeof thread Prey CNIDOCYTE Figure 18.4D

16. 18.5 Most animals are bilaterally symmetrical • These animals have mirror-image right and left sides • They also have a head with sensory structures • They move headfirst through their environment Dorsal Posterior Anterior Ventral Figure 18.5

17. 18.6 Flatworms are the simplest bilateral animals • Phylum Platyhelminthes • Planarians have a simple nervous system consisting of a brain, sense organs, and branching nerves • As in cnidarians, the mouth of a flatworm is the only opening for its gastrovascular cavity Digestive tract(gastrovascularcavity) Nerve cords Mouth Eyespots Nervoustissue clusters Figure 18.6A Bilateral symmetry

18. Mature flukes in bloodvessels of intestine Male Humanhost Female • Flukes and tapeworms are parasitic flatworms with complex life cycles 1 Sexual reproductionof flukes in human;fertilized eggs passout in feces 6 Larva penetratesskin andblood vessels 2 Eggs hatchin water 5 Larva thatinfects human 3 Larvathatinfectssnail 4 Asexual reproductionof flukes in snail Snail host Figure 18.6B

19. A tapeworm Units withreproductivestructures Head Hooks Sucker Figure 18.6C

20. 18.7 Most animals have a body cavity • Sponges, cnidarians, and flatworms lack a body cavity Tissue-filled region(from mesoderm) Body covering(from ectoderm) Digestive tract(from endoderm) Figure 18.7A

21. This cavity is a fluid-filled space between the digestive tract and the body wall • The cavity aids in movement, cushions organs, and it may help in circulation • Nearly all other animals have a body cavity

22. Body covering(from ectoderm) Muscle layer(from mesoderm) Pseudocoelom Digestive tract(from endoderm) Body covering(from ectoderm) Coelom Tissue layerlining coelomand suspendinginternal organs(from mesoderm) Digestive tract(from endoderm) Figure 18.7B, C

23. 18.8 Roundworms have a pseudocoelom and a complete digestive tract • Nematodes have a body cavity not completely lined by mesoderm • Like most animals, they possess a complete digestive tract • This is a tube with a mouth and an anus

24. Trichinella juvenile Muscle tissue • Many nematodes are free-living • Others are parasites Figure 18.8A, B

25. 18.9 Diverse mollusks are variations on a common body plan • Phylum Mollusca is a large and diverse phylum that includes • gastropods, such as snails and slugs Figure 18.9B, C

26. bivalves, such as clams and scallops Figure 18.9D

27. cephalopods, such as squids and octopuses Figure 18.9E, F

28. The mantle may secrete a shell which encloses the visceral mass • Mollusks have a true coelom and a circulatory system • Many mollusks feed with a rasping radula • All mollusks have a muscular foot and a mantle

29. VISCERAL MASS Coelom Reproductiveorgans Kidney Heart Digestivetract MANTLE Shell Mantlecavity Radula RADULA Anus Gill Mouth FOOT Nervecords Mouth Figure 18.9A

30. 18.10 Many animals have a segmented body • Segmentation is the subdivision of some or most of the body into a series of repeated parts, or segments Anus Brain Mainheart Coelom Digestivetract Segmentwalls Mouth Accessoryheart Nerve cord Figure 18.10A Excretory organ Blood vessels

31. Segmentation probably evolved as an adaptation for movement Figure 18.10B, C

32. 18.11 Earthworms and other annelids are segmented worms • The segmented bodies of annelids (phylum Annelida) give them added mobility for swimming and burrowing • An earthworm eats its way through soil

33. Polychaetes search for prey on the seafloor or live in tubes and filter food particles Figure 18.11A, B

34. Most leeches are free-living carnivores, but some suck blood Figure 18.11C

35. 18.12 Arthropods are the most numerous and widespread of all animals • Arthropods (phylum Arthropoda) are segmented animals with exoskeletons and jointed appendages

36. Cephalothorax Abdomen Thorax Head Antennae(sensoryreception) Swimmingappendages Walking legs Pincer (defense) Mouthparts (feeding) Figure 18.12A

37. Horseshoe crabs are ancient marine arthropods • In terms of numbers, distribution, and diversity, they are the most successful phylum of animals Figure 18.12B

38. Most arachnids are terrestrial and carnivorous Figure 18.12C

39. Crustaceans are nearly all aquatic Figure 18.12D

40. Millipedes and centipedes make up a fourth group of arthropods Figure 18.12E

41. 18.13 Insects are the most diverse group of organisms • Insects are the most numerous and successful arthropods • They have a three-part body consisting of • head, thorax, and abdomen • three sets of legs • wings (most, but not all insects) • The development of many insects includes metamorphosis

42. Thorax Abdomen Head Antenna Forewing Eye Hindwing Mouthparts Figure 18.13A

43. Their young resemble adults, but are smaller with different body proportions • The insects you see here undergo incomplete metamorphosis Figure 18.13B, C

44. The insects you see here undergo complete metamorphosis Figure 18.13D, E

45. The larvae look very different from the adults • Adults are specialized for dispersal and reproduction • They have larvae specialized for eating and growing Haltere Figure 18.13F, G

46. 18.14 Echinoderms have spiny skin, an endoskeleton, and a water vascular system for movement • Phylum Echinodermata includes organisms such as sea stars and sea urchins • These organisms are radially symmetrical as adults Figure 18.14B, C

47. The water vascular system has suction-cup-like tube feet used for respiration and locomotion Anus Spines Stomach TUBE FEET CANALS Figure 18.14A

48. 18.15 Our own phylum, Chordata, is distinguished by four features • Organisms in this phylum are segmented animals with four distinctive features • Dorsal hollow nerve cord • Stiff notochord • Pharyngeal slits behind the mouth • Muscular post-anal tail

49. These are marine invertebrates • The simplest chordates are tunicates and lancelets POST-ANAL TAIL DORSAL, HOLLOWNERVE CORD PHARYNGEALSLITS Musclesegments Mouth NOTOCHORD LARVA Figure 18.15A

50. Lancelets DORSAL, HOLLOWNERVE CORD NOTOCHORD Head Mouth Water exit Anus Pharynx POST-ANALTAIL PHARYNGEALSLITS Digestivetract Segmentalmuscles Figure 18.15B