1. Evolution of the Animal Phyla
Chapter 19

2. 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
have unique tissues

3. The Animal Family Tree Animals consist of 35 very different phyla.
To judge which phyla are more closely related, taxonomists traditionally have compared anatomical features and aspects of embryological development.
The end result are phylogenies, which are basically like family trees.

4. 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.
Although very different, the Parazoa and Eumetazoa are thought to have evolved from a common ancestor.

5. The Animal Family Tree Eumetazoans are divided into two groups:
Radiata have radial symmetry and two embryological layers, an outer ectoderm and an inner endoderm.
This body plan is called diploblastic.
Bilateria have bilateral symmetry and a third embryological layer, the mesoderm, that occurs between the ectoderm and the endoderm.
This body plan is called triploblastic.

6. The Animal Family Tree
Traditionally, the Bilateria have been subdivided based on traits that were important to the evolutionary history of phyla.
For example, the presence or absence of a body cavity.
The traditional animal phylogeny relies on the either-or nature of categories.

7. The animal family tree: The traditional viewpoint
Coelom forms from
mesoderm cell mass
Mix of
characteristics
Coelom forms from
embryonic gut
No coelom
Pseudocoel No
tissues
Radial
symmetry
Pseudo-
coelomates
Acoelomates
Coelomates
Mix of
characteristics
Parazoa
Radiata
Protostomes
Deuterostomes
Sponges
Cnidarians
Ctenophorans
Flatworms
Nematodes
Mollusks
Annelids
Arthropods
Lophophorates
Echinoderms
Chordates

8. 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 unique gene sequences as means to detect relatedness.
New molecular data has resulted in a variety of new phylogenies, most of which divide protostomes into:
Lophotrochozoans
Ecdysozoans

9. The animal family tree: A new look
Grow by increasing body mass,
locomotion is ciliary, trochophore larvae
Grow by molting
Coelom forms from
embryonic gut No
tissues
Radial
symmetry
Lophotrochozoa
Ecdysozoa
Parazoa
Radiata
Protostomes
Deuterostomes
Sponges
Cnidarians
Ctenophorans
Lophophorates
Flatworms
Mollusks
Annelids
Nematodes
Arthropods
Echinoderms
Chordates

10. Six Key Transitions in Body Plan
The evolution of animals is marked by six key transitions in body plan:
tissues
bilateral symmetry
body cavity
segmentation
molting
deuterostome development

11. Six 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, the Eumetazoa, possess tissues.

12. Six Key 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 around a central axis.
Any plane passing through the central axis divides the organism in halves that are approximate mirror images.
Bilateral symmetry is a body plan with distinct right and left halves that are mirror images.
The plan allows for specialization among body regions and more efficient movement.

13. Six 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.

14. Six 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.

15. Six Key Transitions in Body Plan
The subdivision of the body into segments is another key transition in the animal body plan.
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.

16. Evolutionary trends among the animals
Parazoa
Eumetazoa
Radiata
Bilateria
Acoelomates
Pseudocoelomates
Coelomates
Protostomes
Deuterostomes
Segmented
Segmented
Mollusca
Annelida
Arthropoda
Chordata
Evolutionary trends among the animals
Rotifera (rotifers)
Lophophorates
Echinodermata
Porifera (sponges)
Platyhelminthes (flatworms)
Nematoda (roundworms)
Jointed appendages
exoskeleton, molting
Radiata (Cnidaria and Ctenophora)
Notochord,
segmentation,
jointed
appendages
Segmentation
Molting ?
Deuterostome development,
endoskeleton
Pseudocoel
Coelom
Body cavity
Radial symmetry
Bilateral symmetry
No true tissues
Tissues
Multicellularity
Ancestral protist

17. TABLE 19.2 THE MAJOR ANIMAL PHYLA Approximate Number of Named Species
Phylum
Typical Examples
Key Characteristics
Arthropoda
(arthropods)
Insects, crabs,
spiders, millipedes
Most successful of all animal phyla; chitinous exoskeleton
covering segmented bodies with paired, jointed appendages;
most insect groups have wings; nearly all are freshwater or
terrestrial
1,000,000
Mollusca
(mollusks)
Snails, clams,
octopuses,
nudibranchs
Soft-bodied coelomates whose bodies are divided into three
parts: head-foot, visceral mass, and mantle; many have shells;
almost all possess a unique rasping tongue called a radula; most
are marine or freshwater but 35,000 species are terrestrial
110,000
Chordata
(chordates)
Mammals, fish,
reptiles, birds,
amphibians
Segmented coelomates with a notochord; possess a dorsal nerve
cord, pharyngeal pouches, and a tail at some stage of life; in
vertebrates, the notochord is replaced during development by the
spinal column; most are marine, many are freshwater, and 20,000
species are terrestrial
56,000
Platyhelminthes
(flatworms)
Planaria,
tapeworms,
flukes
Segmented coelomates with a notochord; possess a dorsal nerve
cord, pharyngeal pouches, and a tail at some stage of life; in
vertebrates, the notochord is replaced during development by the
spinal column; most are marine, many are freshwater, and 20,000
species are terrestrial
20,000
Nematoda
(roundworms)
Ascaris, pinworms,
hookworms, Filaria
Pseudocoelomate, unsegmented, bilaterally symmetrical worms;
tubular digestive tract passing from mouth to anus; tiny; without
cilia; live in great numbers in soil and aquatic sediments; some are
important animal parasites
20,000
Annelida
(segmented worms)
Earthworms,
marine worms,
leeches
Coelomate, serially segmented, bilaterally symmetrical worms;
complete digestive tract; most have bristles called setae on each
segment that anchor them during crawling; marine, freshwater,
and terrestrial
12,000

18. Copyright © The McGraw-Hill Companies, Inc
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
TABLE 19.2
(CONTINUED)
Approximate
Numberof
NamedSpecies
Phylum
Typical Examples
Key Characteristics
Cnidaria
(cnidarians)
Jellyfish, hydra,
corals, sea
anemones
Soft, gelatinous, radially symmetrical bodies whose digestive
cavity has a single opening; possess tentacles armed with
stinging cells called cnidocytes that shoot sharp harpoons called
nematocysts; almost entirely marine
10,000
Echinodermata
(echinoderms)
Sea stars, sea
urchins, sand
dollars, sea
cucumbers
Soft, gelatinous, radially symmetrical bodies whose digestive
cavity has a single opening; possess tentacles armed with
stinging cells called cnidocytes that shoot sharp harpoons called
nematocysts; almost entirely marine
6,000
Porifera
(sponges)
Barrel sponges,
boring sponges,
basket sponges,
vase sponges
Asymmetrical bodies without distinct tissues or organs; saclike
body consists of two layers breached by many pores; internal
cavity lined with food-filtering cells called choanocytes; most
marine (150 species live in freshwater)
5,150
Lophophorates
(Bryozoa, also called
moss animalsor
Ectoprocta)
Bower bankia,
Plumatella, sea
mats, sea moss
Microscopic, aquatic deuterostomes that form branching
colonies, possess circular or U-shaped row of ciliated tentacles
for feeding called a lophophore that usually protrudes through
pores in a hard exoskeleton; Bryozoa are also called Ectoprocta
because the anus, or proct, is external to the lophophore; marine
or freshwater
4,000
Rotifera
(wheel animals)
Rotifers
Small, aquatic pseudocoelomates with a crown of cilia around the
mouth resembling a wheel; almost all live in freshwater
2,000

19. Sponges: Animals Without Tissues
Sponges are 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.

20. Sponges: Animals Without Tissues
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.

21. Cnidarians: Tissues Lead to Greater Specialization
Radiata are radially symmetrical and 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.

22. Cnidarians: Tissues Lead to Greater Specialization
A major evolutionary advance in the Radiata is extracellular digestion of food.
Digestion begins outside of cells in a gut cavity called the gastrovascular cavity.
This form of digestion allows animals to digest an animal larger than itself.

23. 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 harpoonlike barb, called a nematocyst, which cnidarians use to spear prey and retract it towards the tentacle
The nematocyst can discharge so explosively that it is capable of piercing the hard shell of a crab

24. Cnidarians: Tissues Lead to Greater Specialization
Cnidarians have two basic body forms:
Medusae are a floating form.
Polyps are a sessile form.
Epidermis
Gastrovascular
cavity
Medusa
Mouth
Polyp
Tentacles
Gastrodermis
Mesoglea

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

26. The life cycle of Obelia, a marine colonial hydroid
Medusae
Ovary
Testis
Feeding
polyp
Sexual
reproduction
Medusa bud
Eggs
Sperm
Reproductive
polyp
Zygote
Blastula
Free-swimming
planula larva
Mature
colony
Young colony and
asexual budding
Settles down to
start new colony

27. 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.

28. Solid Worms: Bilateral Symmetry
Most bilaterally symmetrical animals have evolved a definitive head end.
This process is termed cephalization.

29. Solid Worms: Bilateral Symmetry
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.

30. 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.
Flatworms are the simplest animals in which organs occur.

31. 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.

32. Solid Worms: Bilateral Symmetry
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, parasitic flatworms do not need eyespots.
This loss of features that lack adaptive purpose for parasitism is sometimes called degenerative evolution.

33. Life cycle of the human liver fluke, Clonorchis sinensis
Raw, infected fish
is consumed by
humans or other
mammals
Metacercarial
cysts in fish
muscle 3
Liver
Flukes often require two or more hosts to complete their life cycles
Bile duct
Adult fluke 2 1
Egg containing
miracidium
Miracidium hatches
after being eaten
by snail
Cercaria
Redia
Sporocyst

34. 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

35. Solid Worms: Bilateral Symmetry
Those flatworms that have a digestive cavity, have an incomplete gut, one with only one opening.
The gut branches throughout the body and is involved in both digestion and excretion.
The parasitic flatworms lack a gut entirely and absorb food directly through their body walls.

36. 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, either a nerve net or longitudinal nerve cords with cross connections.
Free-living forms have eyespots to distinguish light from dark.

37. 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

38. Roundworms: The Evolution of a Body Cavity
Pseudocoelomate
Mesoderm
Ectoderm
Acoelomate
Coelomate
Endoderm
Pseudocoel
Coelomic
cavity
There are three basic kinds of body plans found in bilaterally symmetrical animals:
Acoelomates have no body cavity.
Pseudocoelomates have a body cavity (called a pseudocoel) located between the mesoderm and the endoderm.
Coelomates have a body cavity (called a coelom) that develops entirely within the mesoderm.

39. Roundworms: The Evolution of a Body Cavity
The largest pseudocoelomate phylum is Nematoda, containing about 20,000 species.
The members of this phylum include nematodes, eelworms, and other roundworms.
Nematodes are unsegmented, cylindrical worms covered by a flexible cuticle that is molted as they grow.
Nematodes move in a whiplike fashion.

40. 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 is transmitted to humans who eat undercooked pork.
Intestinal roundworms, Ascaris lumbricoides, live in human intestines.

41. Roundworms: The Evolution of a Body Cavity
Another pseudocoelomate phylum is Rotifera.
Rotifers are small, aquatic organisms that have a crown of cilia at their heads.
The cilia help in both locomotion and feeding.

42. Mollusks: Coelomates
The body of a mollusk (Phylum Mollusca) 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.

43. Mollusks: Coelomates There are three major groups of mollusks:
Gastropods—include the snails and slugs.
Bivalves—include clams, oysters, and scallops.
Cephalopods—include the octopuses and squids.

44. Mollusks: Coelomates
Mollusks have a unique feeding structure, called a radula.
The radula is a rasping tonguelike organ that bears rows of pointed, backward-curving teeth.

45. Mollusks: Coelomates
In most mollusks, the outer surface of the mantle secretes a protective shell.
The shell has multiple layers comprised of protein, calcium, and pearl.

46. Annelids: The Rise of Segmentation
One of the early innovations in 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.

47. 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.
There are three body plan characteristics:
Repeated segments
Specialized segments
Connections

48. Arthropods: Advent of Jointed Appendages
The most successful of all animal groups is the Phylum Arthropoda, consisting of 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.

49. 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.
Head
Thorax
Abdomen
Antennae
Mouth

50. Arthropods: Advent of Jointed Appendages
Chelicerates are arthropods that lack jaws.
They include spiders, mites, scorpions, and horseshoe crabs.
Mandibulates are arthropods with jaws, called mandibles.
They include the crustaceans, insects, centipedes, and millipedes.
Antenna
Eye
Mandible
Eyes
Pedipalp
Chelicera

51. 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.

52. Arthropods: Advent of Jointed Appendages
The class Arachnida has 57,000 named species of arachnids, including spiders, ticks, mites, scorpions, and daddy longlegs.
Arachnids have a pair of chelicerae, a pair of pedipalps, and four pairs of walking legs.

53. Arthropods: Advent of Jointed Appendages
Crustaceans belong to the subphylum Crustacea and comprise a diverse group of mandibulates.
There a 35,000 species of crustaceans described including species of crabs, shrimps, lobsters, crayfish, water fleas, pillbugs, and sowbugs.

54. Arthropods: Advent of Jointed Appendages
Most crustaceans have two pairs of antennae, three pairs of chewing appendages, and various numbers of legs.

55. Arthropods: Advent of Jointed Appendages
Crustaceans pass through a larval stage called the nauplius.

56. 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.

57. 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

58. 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.
Coelom
Anus
Archenteron
Mesoderm
Mouth
Mouth forms from
blastopore
Anus forms
from blastopore
outpockets to
form coelom
Blastopore
Blastula
32 cells
16 cells
8 cells
4 cells
2 cells
1 cell
Deuterostomes
Protostomes
Mesoderm splits

59. 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
Occurs later in deuterostome cleavage than in protostome cleavage.
Origin of the coelom.

60. Echinoderms: The First Deuterostomes
Echinoderms are deuterostomes that belong to the phylum Echinodermata.
Echinoderm means “spiny skin” and refers to the endoskeleton of calcium-rich ossicles just beneath the echinoderm’s skin.
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.

61. Echinoderms: The First Deuterostomes
A key adaptation of echinoderms is the water vascular system that aids movement.
This system is fluid-filled and composed of a central ring canal from which five radial canals extend out into the arms.
From each radial canal, tiny vessels extend through short side branches into thousands of tiny, hollow tube feet.
Echinoderms can extend the tube feet, attach them to the ocean floor, and pull against them to move.

62. Chordates: Improving the Skeleton
Chordates are deuterostomes that belong to the phylum Chordata.
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.

63. Chordates: Improving the Skeleton
The approximately 56,000 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.

64. Chordates: Improving the Skeleton
Not all chordates are vertebrates.
Tunicates and lancelets are chordates.

65. Chordates: Improving the Skeleton
Vertebrate chordates differ from tunicates and lancelets in two important respects.
Vertebrates have a backbone that replaces the role of the notochord.
Vertebrates have a distinct and well-differentiated head.