Transcript Ch. 25
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.
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
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
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
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
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
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
Figure 25.1 The animal family tree:
the traditional viewpoint.
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
Figure 25.2 The animal family tree:
A new look.
25.3 Five Key Transitions in Body
Plan
•
the evolution of animals is marked by five
key transitions in body plan
1.
2.
3.
4.
5.
evolution of tissues
bilateral symmetry
body cavity
deuterostome development
segmentation
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
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
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
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
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
Figure 25.3 Evolutionary trends
among the animals.
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
Figure 25.4 Diversity in sponges.
(a)
(b)
Figure 25.5 Phylum Porifera:
sponges.
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
Figure 25.6 Representative
cnidarians.
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
Figure 25.7 Phylum Cnidaria:
cnidarians.
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.
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
Figure 25.9 The life cycle of Obelia,
a marine colonial hydroid.
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
Figure 25.10 How radial and
bilateral symmetry differ.
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
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
Figure 25.11 Body plan of a solid
worm.
Figure 25.12 Flatworms.
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
Figure 25.13 Life cycle of the human
liver fluke, Clonorchis sinensis.
Flukes often require two or more hosts to complete their life cycles.
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
Figure 25.14 Phylum
Platyhelminthes: solid worms.
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
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
Figure 25.15 Diagram of flatworm
anatomy.
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
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
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
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
Figure 25.16 Three body plans for
bilaterally symmetrical animals.
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
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
Figure 25.17 Pseudocoelomates.
(a) Nematodes (phylum Nematoda)
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
Figure 25.18 Phylum Nematoda:
roundworms.
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
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
Figure 25.17 Pseudocoelomates.
(b) Rotifers (phylum Rotifera)
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
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
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
Figure 25.19 Three major groups of
mollusks.
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
Figure 25.20 Phylum Mollusca:
mollusks.
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
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
Figure 25.22 Phylum Annelida:
annelids.
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
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
Figure 25.24 Segmentation in
insects.
Figure 25.25 Phylum Arthropoda:
arthropods.
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
Figure 25.26 Chelicerates and
mandibulates.
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 25.27 Horseshoe crabs.
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
Figure 25.28 Arachnids.
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
Figure 25.29 Crustaceans.
Figure 25.30 Body of a lobster,
Homarus americanus.
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
Figure 25.31 Centipedes and
millipedes.
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
Figure 25.33 Insect diversity.
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
Figure 25.34 Embryonic development
in protostomes and deuterostomes.
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
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
Figure 25.35 Diversity in
echinoderms.
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
Figure 25.36 Phylum
Echinodermata: echinoderms.
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
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.
Figure 25.37 Phylum Chordata:
chordates.
25.13 Chordates: Improving the
Skeleton
• not all chordates are vertebrates
tunicates and lancelets
Figure 25.38 Nonvertebrate chordates.
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 welldifferentiated head
Figure 25.39 A mouse embryo.
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.