1. Evolution of the Animal Phyla
The Living World
Fifth Edition
George B. Johnson
Jonathan B. Losos
Chapter 25
Evolution of the Animal Phyla
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

2. 25.1 General Features of Animals
animals share many important characteristics, such as they
are heterotrophs
are multicellular and lack cell walls
can move from place to place
have diverse forms and habitats
reproduce, mostly, by sexual reproduction
have a common pattern of development
unique tissues

3. 25.2 The Animal Family Tree
the multicellular animals 35 very different phyla
to judge which phyla are more closely related, taxonomists compare anatomical features and aspects of embryological development
the end result are phlyogenies, which are basically like family trees
the main branches of the phylogenies make possible the evolutionary history of animals

4. 25.2 The Animal Family Tree
the kingdom Animalia is traditionally divided into two main branches based on tissue presence
Parazoa possess neither tissues nor organs and have no discernible symmetry
they are represented mostly by the phylum Porifera, the sponges
Eumetazoa have a definite shape and symmetry and, in most cases, tissues organized into organs and organ systems

5. 25.2 The Animal Family Tree
although very different, the Parazoa and Eumetazoa are thought to have evolved from a common ancestor
the shared ancestor was probably a choanoflagellate
the choanoflagellate lived over 700 million years ago and was a colonial, flagellated protist

6. 25.2 The Animal Family Tree
within the Eumetazoan phylogeny, the family tree branches on the basis of the type of embryological layering
Radiata have two embryological layers, an outer ectoderm and an inner endoderm
this body plan is called diploblastic
Bilateria have a third embryological layer, the mesoderm, that occurs between the ectoderm and the endoderm
this body plan is called triploblastic

7. 25.2 The Animal Family Tree
additional branches to the phylogenetic tree were assigned by identifying traits that were important to the evolutionary history of phyla
for example, the presence or absence of a body cavity
the traditional phylogeny of taxonomists relies on the either-or-nature of categories

8. Figure 25.1 The animal family tree: the traditional viewpoint.

9. 25.2 The Animal Family Tree
the traditional animal phylogeny is being revised because some of the important characters may not be conserved to the extent previously thought
molecular systematics offers a means to construct phylogenic trees using clusters of genes as means to detect relatedness
this new approach has resulted into significant refinements of the traditional phylogeny
for example, the protostomes have a more complex evolutionary history

10. Figure 25.2 The animal family tree: A new look.

11. 25.3 Five Key Transitions in Body Plan
the evolution of animals is marked by five key transitions in body plan
evolution of tissues
bilateral symmetry
body cavity
deuterostome development
segmentation

12. 25.3 Five Key Transitions in Body Plan
the presence of tissues is the first key transition in the animal body plan
only the Parazoa, the sponges, lack defined tissues and organs
these animals exist as aggregates of cells with minimal intercellular coordination
all other animals besides members of the Parazoa possess tissues
they belong to the Eumetazoa

13. 25.3 Five Transitions in Body Plan
virtually all animals other than sponges have a definite shape and symmetry
radial symmetry is a body plan in which all parts of the body are arranged along a central axis
if a plane passing through the central axis divides the organism in halves, the halves will be mirror images
bilateral symmetry is body plan with distinct right and left halves that are mirror images
the plan allows for specialization among body regions

14. 25.3 Five Key Transitions in Body Plan
the evolution of a body cavity was an important step in animal evolution
this internal space allowed for the support of organs, distribution of materials, and coordination of development
for example, the digestive tract can be larger and longer

15. 25.3 Five Key Transitions in Body Plan
bilateral animals can be divided into two groups based on differences in the basic pattern of development
protostomes include the flatworms, nematodes, mollusks, annelids, and arthropods
deuterostomes include the echinoderms and the chordates
deuterostomes evolved from protostomes more than 630 million years ago

16. 25.3 Five Key Transitions in Body Plan
the subdivision of the body into segments is a key transition to the animal body plan that occurs early on during development
in highly segmented animals, each segment can develop a more or less complete set of adult organ systems
each segment can function as a separate locomotory unit

17. Figure 25.3 Evolutionary trends among the animals.

18. 25.4 Sponges: Animals Without Tissues
sponges, members of the phylum Porifera
their bodies a little more than masses of specialized cells embedded in a gel-like matrix
clumps of cells disassociated from a sponge can give rise to new sponges
the body of a sponge is perforated by many pores
choanocytes are flagellated cells that line the body cavity of the sponge and draw in water through the pores
the sponge is a filter feeder which traps any food particles

19. Figure 25.4 Diversity in sponges.
(a)
(b)

20. Figure 25.5 Phylum Porifera: sponges.

21. 25.5 Cnidarians: Tissues Lead to Greater Specialization
the Radiata include two phyla
Cnidaria comprises the hydra, jellyfish, corals and anemones
Ctenophora comprises the comb jellies
the members of the Radiata have a body plan that allows them to interact with their environment on all sides
a major evolutionary advance in the Radiata is extracellular digestion of food
digestion begins outside the body in a gut cavity called, the gastrovascular cavity
this form of digestion allows animals to digest an animal larger than itself

22. Figure 25.6 Representative cnidarians.

23. 25.5 Cnidarians: Tissues Lead to Greater Specialization
cnidarians (phylum Cnidaria) are carnivores that capture prey with tentacles that ring their mouths
these tentacles and, sometimes, the body surface, bear stinging cells called cnidocytes
within each cnidocyte is a harpoon-like barb, called a nematocyst, which cnidarians use to spear their prey and they retract towards the tentacle
the nematocyst can discharge so explosively that it is capable of piercing the hard shell of a crab

24. Figure 25.7 Phylum Cnidaria: cnidarians.

25. 25.5 Cnidarians: Tissues Lead to Greater Specialization
cnidarians have two basic body forms
medusae are the floating form
polyps are the sessile form
Figure 25.8 The two basic body forms of cnidarians.

26. 25.5 Cnidarians: Tissues Lead to Greater Specialization
medusae are often called “jellyfish,” because of their gelatinous interior, or “stinging nettles,” because of their nematocyts
polyps are pipe-shaped animals that usually attach to rock
in corals, the polyps secrete a deposit of calcium carbonate in which they live

27. Figure 25.9 The life cycle of Obelia, a marine colonial hydroid.

28. 25.6 Solid Worms: Bilateral Symmetry
body symmetry differs among the Eumetazoa
radial symmetry means that multiple planes cutting the organism in half will produce mirror images
bilateral symmetry means that only one plane can cut the organism in half to produce mirror images

29. Figure 25.10 How radial and bilateral symmetry differ.

30. 25.6 Solid Worms: Bilateral Symmetry
most bilaterally symmetrical animals have evolved a definitive head end
this process is termed cephalization
the bilaterally symmetrical eumetazoans produce three embryonic layers
ectoderm will develop into the outer coverings of the body and the nervous system
mesoderm will develop into the skeleton and muscles
endoderm will develop into the digestive organs and intestine

31. 25.6 Solid Worms: Bilateral Symmetry
the solid worms are the simplest of all bilaterally symmetrical animals
the largest phylum of these worms is the Phylum Platyhelminthes, which includes the flatworms
flatworms lack any internal cavity other than the digestive tract
this solid condition is called acoelomate
they have separate organs, including a uterus and testes

32. Figure 25.11 Body plan of a solid worm.

33. Figure Flatworms.

34. 25.6 Solid Worms: Bilateral Symmetry
most flatworms are parasitic but some are free-living
flatworms range in size from less than a millimeter to many meters long
there are two classes of parasitic flatworms
flukes
tapeworms
the parasitic lifestyle has resulted in the eventual loss of features not used or needed by the parasite
for example, the parasites lack cilia in the adult stage and do not need eye spots
this loss of features that lack adaptive purpose for parasitism is sometimes called degenerative evolution

35. Figure 25.13 Life cycle of the human liver fluke, Clonorchis sinensis.
Flukes often require two or more hosts to complete their life cycles.

36. 25.6 Solid Worms: Bilateral Symmetry
tapeworms are a classic example of degenerative evolution
the body of a tapeworm has been reduced to two primary functions
eating
reproduction

37. Figure 25.14 Phylum Platyhelminthes: solid worms.

38. 25.6 Solid Worms: Bilateral Symmetry
if flatworms have a digestive cavity, then it is incomplete
the gut branches throughout the body and is involved in both digestion and excretion
they are capable of performing some extracellular digestion
the parasitic flatworms lack a gut entirely and absorb food directly through their body walls

39. 25.6 Solid Worms: Bilateral Symmetry
flatworms have an excretory system
the system consists of a network of fine tubules that run through the body
enlarged flame cells (cilia-lined bulbs) are located on the side branches of the tubules
the cilia move water and excretory substances into the tubules and then into exit pores
the primary function of the flame cells is in the regulation of water balance
most of the metabolic wastes are excreted through the gut

40. Figure 25.15 Diagram of flatworm anatomy.

41. 25.6 Solid Worms: Bilateral Symmetry
flatworms lack a circulatory system and all cells must be within diffusion distance of oxygen and food
flatworms have a simple nervous system
they use sensory pits or tentacles along the sides of the head to detect food, chemical, and movement
free-living forms have eyespots to distinguish light from dark

42. 25.6 Solid Worms: Bilateral Symmetry
reproduction in flatworms is complex
most flatworms are hermaphroditic, meaning that each individual contains both male and female reproductive structures
some flatworms have a complex succession of distinct larval stages
some flatworms are capable of asexual regeneration

43. 25.7 Roundworms: The Evolution of a Body Cavity
a key transition in the evolution of the animal body plan was the evolution of the body cavity
the evolution of an internal body cavity helped improve the animal body design in three areas
circulation
movement
organ function

44. 25.7 Roundworms: The Evolution of a Body Cavity
there are three basic kinds of body plans found in bilaterally symmetrical animals
acoelomates have no body cavity
pseudocoelomates have a body cavity located between the mesoderm and the endoderm
coelomates have a body cavity (called a coelom) that develops entirely within the mesoderm

45. Figure 25.16 Three body plans for bilaterally symmetrical animals.

46. 25.7 Roundworms: The Evolution of a Body Cavity
seven phyla of bilaterally symmetrical animals have a pseudocoelom
the pseudocoelom serves as a hydrostatic skeleton, a skeleton that gains its rigidity from fluids kept under pressure
all pseudocoelomates lack a circulatory system
most pseudocoelomates have a complete digestive tract

47. 25.7 Roundworms: The Evolution of a Body Cavity
the phylum Nematoda contains the greatest number of species among the phyla that are pseudocoelomates
the members of this phylum include nematodes, eelworms, and other roundworms
they are unsegmented, cylindrical worms covered by a flexible cuticle that is molted as they grow
nematodes move in a whip-like fashion

48. Figure 25.17 Pseudocoelomates. (a) Nematodes (phylum Nematoda)

49. 25.7 Roundworms: The Evolution of a Body Cavity
the mouth of a nematode is often equipped with piercing organs called stylets
food passes through a muscular chamber called the pharynx
reproduction in nematodes is usually sexual with, usually, separate sexes
their development is simple and adults have very few cells
Caenorhabditis elegans is a roundworm important to genetic and developmental studies

50. Figure 25.18 Phylum Nematoda: roundworms.

51. 25.7 Roundworms: The Evolution of a Body Cavity
some nematodes are parasitic in humans, cats, dogs, and animals of economic importance
heartworm in dogs is caused by a nematode
trichinosis is an infection caused by the nematode Trichinella and transmitted to humans who eat undercooked pork
intestinal roundworms, Ascaris lumbricoides, live in human intestines

52. 25.7 Roundworms: The Evolution of a Body Cavity
another phylum consisting of animals with a pseudocoelomate body plan is the phylum Rotifera
rotifers are small, aquatic organisms that have a crown of cilia at their heads
the cilia help in both locomotion and feeding

53. Figure 25.17 Pseudocoelomates. (b) Rotifers (phylum Rotifera)

54. 25.8 Mollusks: Coelomates
coelomate animals are more successful than pseudocoelomates because of the nature of embryonic development
primary induction is a process in animal development in which one of the three primary embryonic tissues interacts with another
the interaction requires physical contact
in coelomates, contact is made possible between mesoderm and endoderm
this interaction permits localized portions of the digestive tract to become highly specialized

55. 25.8 Mollusks: Coelomates
the mollusks, members of the phylum Mollusca, are the only coelomates without segmented bodies
the basic body of a mollusk is comprised of three regions
a head-foot
a visceral mass containing the body’s organs
a mantle that envelopes the visceral mass and is associated with the gills

56. 25.8 Mollusks: Coelomates there three major groups of mollusks
gastropods—include the snails and slugs
bivalves—include clams, oysters, and scallops
cephalopods—include the octopi and squids

57. Figure 25.19 Three major groups of mollusks.

58. 25.8 Mollusks: Coelomates
mollusks have a unique feeding structure, called a radula
the radula is a rasping tongue-like organ that bears rows of pointed, backward-curving teeth
in most mollusks, the outer surface of the mantle secretes a protective shell
the shell has multiple layers comprised of protein, calcium, and pearl

59. Figure 25.20 Phylum Mollusca: mollusks.

60. 25.9 Annelids: The Rise of Segmentation
one of the early innovations to body plan to arise among the coelomates was segmentation
segmentation is the building of a body from a series of similar segments
this body plan offers a lot of flexibility in that small changes to segments can produce a new kind of segment with different functions
the first segmented animals to evolve were the annelid worms, phylum Annelida

61. 25.9 Annelids: The Rise of Segmentation
the basic body plan of an annelid is a tube within a tube
the digestive tract is suspended within the tube of the coelom
the tubes run from mouth to anus
derived from this basic organization are three characteristics
repeated segments
specialized segments
connections

62. Figure 25.22 Phylum Annelida: annelids.

63. 25.10 Arthropods: Advent of Jointed Appendages
the most successful of all animal groups is the phylum Arthropoda, comprising the arthropods
these animals have jointed appendages
in addition to joints, arthropods have an exoskeleton made of chitin
the muscles of arthropods attach to the interior of this outer shell
the shell offers protection against predators and water loss

64. 25.10 Arthropods: Advent of Jointed Appendages
chitin cannot support much weight
arthropod size is limited as a result
arthropod bodies are segmented like annelids
segments often fuse into functional groups in the adult stage

65. Figure 25.24 Segmentation in insects.

66. Figure 25.25 Phylum Arthropoda: arthropods.

67. 25.10 Arthropods: Advent of Jointed Appendages
chelicerates are arthropods that lack jaws
they include spiders, mites and scorpions
mandibulates are arthropods with jaws, called mandibles
they include the crustaceans, insects, centipedes and millipedes

68. Figure 25.26 Chelicerates and mandibulates.

69. 25.10 Arthropods: Advent of Jointed Appendages
the chelicerate fossil record goes back 630 million years
a surviving type of chelicerate from this period is the horseshoe crab
Figure Horseshoe crabs.

70. 25.10 Arthropods: Advent of Jointed Appendages
the class Arachnida has 57K named species and includes the spiders, ticks, mites, scorpions, and daddy longlegs
arachnids have a pair of chelicerae, a pair of pedipalps, and four pairs of walking legs

71. Figure Arachnids.

72. 25.10 Arthropods: Advent of Jointed Appendages
crustaceans belong to the phylum Crustacea and comprise a diverse group of mandibulates
there a 35K species of crustacea described including species of crabs, shrimps, lobsters, crayfish, water fleas, pillbugs, and sowbugs
most crustaceans have two pairs of antennae, three pairs of chewing appendages, and various numbers of legs
all crustaceans pass through a larval stage called the nauplius

73. Figure Crustaceans.

74. Figure 25.30 Body of a lobster, Homarus americanus.

75. 25.10 Arthropods: Advent of Jointed Appendages
millipedes and centipedes have bodies that consist of a head region followed by numerous similar segments
centipedes have one pair of legs per segment while millipedes have two
centipedes are all carnivorous while millipedes are herbivorous

76. Figure 25.31 Centipedes and millipedes.

77. 25.10 Arthropods: Advent of Jointed Appendages
insects belong to the Class Insecta and are the largest group of arthropods
they are the most abundant eukaryotes on the earth
insects have three body sections
head
thorax
abdomen

78. Figure 25.33 Insect diversity.

79. 25.11 Protostomes and Deuterostomes
there are two major kinds of coelomate animals representing two distinct evolutionary lines
protostomes
the mouth develops from or near the blastopore
deuterostomes
the anus forms from or near the blastopore; the mouth forms on another part of the blastula

80. Figure 25.34 Embryonic development in protostomes and deuterostomes.

81. 25.11 Protostomes and Deuterostomes
deuterostomes also differ from protostomes in three other fundamental ways
the pattern of cleavage
protostomes have spiral cleavage while deuterostomes have radial cleavage
fating of cells
it occurs later in deuterostome cleavage than in protostome cleavage
origin of the coelom

82. 25.12 Echinoderms: The First Deuterostomes
echinoderms belong to the phylum Echinodermata
echinoderm means “spiny skin” and refers to the calcium-rich ossicles that protude just beneath the echinoderm’s skin
they are entirely marine animals and include sea stars, sea urchins, sand dollars, and sea cucumbers
all are bilaterally symmetrical as larvae but become radially symmetrical as adults

83. Figure 25.35 Diversity in echinoderms.

84. 25.12 Echinoderms: The First Deuterostomes
a key adaptation of echinoderms is the water vascular system
this system is a fluid-filled and composed of a central ring canal around which five radial canals extend out into the arms
from each radial canal short side branches extend to form thousands of tiny, hollow tube feet
most echinoderms reproduce sexually but asexual regeneration is also common

85. Figure 25.36 Phylum Echinodermata: echinoderms.

86. 25.13 Chordates: Improving the Skeleton
chordates belong to the phylum Chordata and are deuterostome coelomates
they exhibit a truly internal endoskeleton with muscles attached to an internal rod, called a notochord
this innovation opened the door to large body sizes not possible in earlier animal forms

87. 25.13 Chordates: Improving the Skeleton
the approximately 56K species of chordates share four principal features
notochord
nerve cord
pharyngeal pouches
postanal tail
all chordates have all four of these characteristics at some time in their lives.

88. Figure 25.37 Phylum Chordata: chordates.

89. 25.13 Chordates: Improving the Skeleton
not all chordates are vertebrates
tunicates and lancelets
Figure Nonvertebrate chordates.

90. 25.13 Chordates: Improving the Skeleton
vertebrate chordates differ from tunicates and lancelets in two important respects
vertebrates have a backbone
this replaces the role of the notochord
vertebrates have a distinct and well-differentiated head

91. Figure A mouse embryo.

92. Inquiry & Analysis
How many of the species exhibit variation in the comprehensive character index?
How does the magnitude of this variation within species compare with the variation seen between species?
Graph of the history of evolutionary change among bryozoa.