1. The Evolution of Animals Figures 17.1 – 17.3
CHAPTER 17
The Evolution of Animals
Figures 17.1 – 17.3

2. Zoologists estimate that about a billion billion (1018) individual arthropods populate the Earth
Tapeworms can reach lengths of 20 meters in the human intestine

3. The blue whale, an endangered species that grows to lengths of nearly 30 meters, is the largest animal that has ever existed
A reptile can survive on less than 10% of the calories required by a mammal of equivalent size

4. BIOLOGY AND SOCIETY: INVASION OF THE KILLER TOADS
The incredible diversity of animals
Arose through hundreds of millions of years of evolution
Can be quickly threatened

5. The Australian quoll
Is a catlike creature that preys on many small animals, such as toads
Figure 17.1a

6. A non-native toad
Was introduced from South America in 1935 to fight beetles in sugarcane fields
Caused considerable damage to the ecosystem
Figure 17.1b

7. THE ORIGINS OF ANIMAL DIVERSITY
Animal life began in the Precambrian seas with the evolution of multicellular creatures that ate other organisms

8. What Is an Animal? Animals
Are eukaryotic, multicellular, heterotrophic organisms that obtain nutrients by ingestion
Digest their food within their bodies
Figure 17.2

9. Most animals reproduce sexually and then proceed through a series of developmental stages
Haploid
Sperm
Egg 1 2
Meiosis
Fertilization
Zygote (fertilized egg)
Adult 3
Diploid
Blastula (cross section) 7
Metamorphosis
Digestive tract
Outer cell layer (ectoderm) 4
Primitive gut 6 5
Early gastrula
Larva
Inner cell layer (endoderm)
Later gastrula
Figure 17.3
Opening

10. Most animals have muscle cells and nerve cells that control the muscles

11. The Evolution of Animals Figures 17.4 – 17.15
CHAPTER 17
The Evolution of Animals
Figures 17.4 – 17.15

12. Early Animals and the Cambrian Explosion
Animals probably evolved from a colonial protist that lived in the Precambrian seas
Digestive cavity
Reproductive
cells
Somatic cells 1
Early colony of protists (aggregate of identical cells) 2
Hollow sphere (shown in cross section) 3
Beginning of cell specialization 4
Infolding 5
Gastrula-like “proto-animal”
Figure 17.4

13. At the beginning of the Cambrian period, 545 million years ago, animals underwent a rapid diversification
Figure 17.5

14. What ignited the Cambrian explosion?
Many hypotheses exist

15. Animal Phylogeny
To reconstruct the evolutionary history of animal phyla, researchers must depend on clues from comparative anatomy and embryology

16. Four key evolutionary branch points have been hypothesized

17. 4 3 2 1 Coelom from digestive tube Pseudocoelom True coelom
Sponges
Cnidarians
Flatworms
Roundworms
Mollusks
Annelids
Arthropods
Echinoderms
Chordates
Coelom from cell masses
Coelom from digestive tube 4
Pseudocoelom
True coelom
No body cavity 3
Body cavities
Radial symmetry
Bilateral symmetry 2
True tissues 1
Multicellularity
Figure 17.6

18. The first branch point is defined by the presence of true tissues

19. (b) Bilateral symmetry
The second major evolutionary split is based partly on body symmetry
(a) Radial symmetry
(b) Bilateral symmetry
Figure 17.7

20. Third, the evolution of body cavities led to more complex animals

21. A body cavity
Body covering (from ectoderm)
Tissue-filled region (from mesoderm)
Is a fluid-filled space separating the digestive tract from the outer body wall
May be a pseudocoelom or a true coelom
Digestive tract (from endoderm)
(a) No body cavity (e.g., flatworm)
Pseudocoelom
Body covering (from ectoderm)
Digestive tract (from endoderm)
Muscle layer (from mesoderm)
(b) Pseudocoelom (e.g., roundworm)
Coelom
Body covering (from ectoderm)
Tissue layer lining coelom and suspending internal organs (from mesoderm)
Digestive tract (from endoderm)
Mesentery
(c) True coelom (e.g., annelid)
Figure 17.8

22. Fourth, among animals with a true coelom, there are two main evolutionary branches, which differ in embryonic development

23. MAJOR INVERTEBRATE PHYLA
Invertebrates
Are animals without backbones
Represent 95% of the animal kingdom

24. Sponges Phylum Porifera
Includes sessile animals once believed to be plants
Lack true tissues
Figure 17.9

25. The body of a sponge Resembles a sac perforated with holes
Draws water into a central cavity, where food is collected

26. Choanocyte in contact with an amoebocyte
Pores
Water flow
Skeleton fiber
Central cavity
Choanocyte
Flagella
Amoebocyte
Figure 17.10

27. Cnidarians Phylum Cnidaria
Is characterized by organisms with radial symmetry and tentacles with stinging cells

28. The basic body plan of a cnidarian
Is a sac with a gastrovascular cavity
Has two variations: the sessile polyp and the floating medusa
Mouth/anus
Tentacle
Gastrovascular cavity
Tentacle
Mouth/anus
Polyp form
Medusa form
Figure 17.11

29. Examples of polyps are Hydras, sea anemones, and coral animals
Figure 17.12

30. The organisms we call jellies are medusas

31. Cnidarians are carnivores that use tentacles armed with cnidocytes, or “stinging cells,” to capture prey
Coiled thread
Tentacle
Capsule
“Trigger”
Cnidocyte
Discharge of thread
Prey
Figure 17.13

32. Phylum Platyhelminthes
Flatworms
Phylum Platyhelminthes
Is represented by the simplest bilateral animals
Includes free-living forms such as planarians
Digestive tract (gastrovascular cavity)
Nerve cords
Mouth
Eyespots
Nervous tissue clusters
Figure 17.14

33. Some flatworms are parasitic
Head
Blood flukes are an example
Tapeworms parasitize many vertebrates, including humans
Reproductive structures
Hooks
Sucker
Figure 17.15

34. The Evolution of Animals Figures 17.16 – 17.20
CHAPTER 17
The Evolution of Animals
Figures – 17.20

35. Roundworms Phylum Nematoda
Includes the most diverse and widespread of all animals
Occurs in aquatic and moist terrestrial habitats
Figure 17.16

36. Roundworms exhibit an important evolutionary adaptation, a digestive tube with two openings, a mouth and an anus
A complete digestive tract can process food and absorb nutrients efficiently

37. Mollusks Phylum Mollusca
Is represented by soft-bodied animals, but most are protected by a hard shell
Includes snails, slugs, clams, octopuses, and squids, to name a few

38. The body of a mollusk has three main parts: a muscular foot, a visceral mass, and a mantle
Reproductive organs
Coelom
Mantle
Kidney
Heart
Digestive tract
Mantle cavity
Radula
Shell
Radula
Anus
Gill
Mouth
Nerve cords
Foot
Mouth
Figure 17.17

39. The three major classes of mollusks are
Gastropods, which are protected by a single, spiraled shell
Figure 17.18a

40. Bivalves, protected by shells divided into two halves
Figure 17.18b

41. Cephalopods, which may or may not have a shell
Figure 17.18c

42. Annelids Phylum Annelida Includes worms with body segmentation Anus
Brain
Main heart
Coelom
Digestive tract
Segment walls
Mouth
Accessory hearts
Nerve cord
Blood vessels
Excretory organ
Figure 17.19

43. There are three main classes of annelids
Earthworms, which eat their way through soil
Figure 17.20a

44. Polychaetes, which burrow in the sea floor
Figure 17.20b

45. Leeches, some of which are parasitic
Figure 17.20c

46. The Evolution of Animals Figures 17.21 – 17.26
CHAPTER 17
The Evolution of Animals
Figures – 17.26

47. Arthropods Phylum Arthropoda
Contains organisms named for their jointed appendages
Includes crustaceans, arachnids, and insects

48. General Characteristics of Arthropods
Arthropods are segmented animals with specialized segments and appendages
Cephalothorax
Abdomen
Thorax
Antennae (sensory reception)
Head
Swimming appendages
Walking legs
Pincer (defense)
Mouthparts (feeding)
Figure 17.21

49. The body of an arthropod is completely covered by an exoskeleton

50. There are four main groups of arthropods
Arthropod Diversity
There are four main groups of arthropods
Arachnids, such as spiders, scorpions, ticks, and mites
Figure 17.22

51. Crustaceans, such as crabs, lobsters, crayfish, shrimps, and barnacles
Figure 17.23

52. Millipedes and centipedes
Figure 17.24

53. Insects, most of which have a three-part body
Head
Thorax
Abdomen
Hawk moth
Antenna
Forewing
Eye
Mosquito
Paper wasp
Mouthparts
Hindwing
Grasshopper
Damselfly
Water strider
Ground beetle
Figure 17.25

54. Many insects undergo metamorphosis in their development
(a) Larva (caterpillar)
(b) Pupa
(c) Pupa
(d) Emerging adult
(e) Adult
Figure 17.26