Download 19 EVOLUTION OF THE ANIMAL PHYLA

25 EVOLUTION OF THE ANIMAL PHYLA
EXTENDED LECTURE OUTLINE
LEARNING OBJECTIVES
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Describe the characteristics of animals that differentiate them from the other kingdoms of organisms.
Compare the traditional viewpoint of the diversification of multicellular animals with the molecular
synthesis hypothesis.
List the characteristics of sponges.
Explain the importance of the evolutionary advances seen in the phyla Cnidaria and Ctenophora.
Understand why the advent of bilateral symmetry is important.
Describe the characteristics of acoelomate flatworms.
Explain the advantages of having an internal body cavity.
Describe the characteristics of pseudocoelomate roundworms.
List the traits found in the coelomate mollusks.
Explain why segmentation is important.
Describe the characteristics of the annelids.
Understand why the evolution of jointed appendages and an exoskeleton helped arthropods to be
highly successful.
List the advantages of evolving a deuterostome pattern of development.
Explain why an endoskeleton is advantageous compared to an exoskeleton.
Be able to characterize the echinoderms.
Describe the characteristics of the chordates.
Introduction to the Animals (p. 452)
25.1
Some General Features of Animals (p. 422; Table 25.1)
A. All animals are heterotrophs, many of which are motile, and all depend on autotrophs for
nourishment.
B. Multicellular Heterotrophs
1. Animals are all multicellular heterotrophs.
C. Diverse in Form
1. Most of the 36 animal phyla are inhabitants of the oceans, but the arthropods, mollusks,
and vertebrates are the dominant animal groups on land.
2. About 99% of all animals are invertebrates, and only 42,500 species are vertebrates.
D. No Cell Walls
1. Animals lack cell walls and are flexible in form.
2. The cells of most animals are organized into tissues.
E. Active Movement
1. Most animals are able to move from one place to another to locate food.
2. A remarkable form of movement unique to animals is flying; the amphibians are only
terrestrial vertebrates, never to have a flying member.
F. Sexual Reproduction
1. Most animals reproduce sexually, and cells formed in meiosis function directly as
gametes.
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2.
25.2
25.3
With few exceptions, there is no counterpart among animals to the alternation of
generations seen in plants.
G. Embryonic Development
1. The zygote undergoes a series of divisions and becomes a solid ball of cells, called the
morula.
2. A hollow ball called the blastula develops next, and from this stage develops a gastrula
with a primitive digestive system beginning.
3. Generally speaking, the pattern of embryological development is characteristic of the
animal kingdom.
4. Evolutionary Trends Among the Animals
a. Animals evolved first in the oceans, beginning some 600 million years ago.
b. The precise order in which the 36 animal phyla evolved is not yet known.
The Animal Family Tree (p. 454; Figs. 25.1, 25.2)
A. The Traditional Viewpoint
1. Multicellular organisms are traditionally divided into 36 distinct phyla.
2. The First Branch: Tissues
a. Parazoa & Eumetazoa are very different in structure but are thought to be
monophyletic.
3. The Second Branch: Symmetry
a. Some eumetazoans have radial symmetry while all others have bilateral symmetry.
4. Further Branches
a. Assignment of further branches split the bilateral symmetry animals into those with a
body cavity and those without.
b. Those with a body cavity were split into those with a coelom and those without and
so on.
B. A New Look at the Animal Family Tree
1. New taxonomic comparisons using molecular data have resulted in different conclusions
other than the traditional.
2. The key body form characteristics traditionally used are not the always preserved
characters as once thought.
3. The new field of molecular systematics compares unique sequences within certain genes.
4. Over the next few years, additional molecular data can be anticipated, which will lessen
the confusion of the animal family tree.
C. “Evo-Devo” and the Roots of the Animal Family Tree
1. Origin of Metazoans: 3 prominent hypothesis are the multi-nucleate hypothesis, the
colonial flagellate hypothesis, and the polyphyletic origin hypothesis.
2. Molecular systematics favors the colonial flagellate hypothesis.
3. Biologists have long debated what caused the Cambrian explosion of animal diversity.
a. Many argue the emergence of new body plans was due to the emergence of
predation.
b. Others argue it was geological factors such as the buildup of oxygen and minerals in
the oceans.
c. “Evo-Devo” indicates that the evolution of the Hox developmental gene complex
could have produced the Cambrian explosion.
Key Transitions in Body Plan (p. 456; Fig. 25.3; Table 25.2)
A. Evolution of Tissues
1. Members of the Parazoa, the sponges, lack defined tissues and organs.
2. All other animals are eumetazoans and have distinct tissues and highly specialized cells.
B. Evolution of Bilateral Symmetry
1. Animals with radial symmetry have body parts radiating around a central axis such that
any plane passing through the central axis divides the body into mirror halves.
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2.
Animals with bilateral symmetry can be divided into left and right halves only through the
median longitudinal plane; this allows different organs to be located in different body
parts, and allows more efficient directional movement.
C. Evolution of a Body Cavity
1. The evolution of complex organ systems was dependent upon the evolution of a body
cavity for protection and support.
2. A body cavity provides space for storage of undigested food, more freedom of movement,
and storage of gametes.
D. The Evolution of Deuterostome Development
1. In protostome animals (flatworms, nematodes, annelids, mollusks, and arthropods) the
blastopore develops into the mouth.
2. In deuterostomes, the echinoderms, and the chordates), the blastopore develops into the
anus.
E. The Evolution of Segmentation
1. In many segmented animals, each segment can develop into a complete organism so
damage to one segment may not necessarily be fatal because others can duplicate that
segment’s functions.
2. Segmentation allows more flexibility and more complex movements.
The Simplest Animals (p. 460)
25.4
25.5
Sponges: Animals Without Tissues (p. 460; Figs. 25.4, 25.5)
A. Two subkingdoms exist within the Kingdom Animalia: Parazoa have no symmetry, are not
differentiated into tissues or organs, and consist mostly of the sponges, while Eumetazoa show
symmetry and have tissues and organs.
B. Members of the Parazoa, the sponges, are simple, asymmetrical animals whose bodies are not
organized into tissues but whose cells recognize each other.
C. These members of the phylum Porifera are groups of specialized cells housed in a gel-like
matrix.
D. Sponges live anchored to the bottom of ocean surfaces and filter water, eating tiny organisms.
E. Cells called choanocytes possess flagella that beat rhythmically, drawing water into the
sponge cavity.
F. There are 5,000 or so species of these filter feeders.
Cnidarians: Tissues Lead to Greater Specialization (p. 462; Figs. 25.6, 25.7, 25.8, 25.9)
A. Eumetazoans show body symmetry and have tissues.
B. During embryologic development, three germ layers are present in this group: ectoderm on the
outside, which gives rise to the outer integuments and to nervous systems; middle mesoderm,
which develops into muscle and bone tissues, and endoderm on the inside, giving rise to
digestive systems.
C. The most primitive members of the subkingdom Eumetazoa are in the phyla Cnidaria and
Ctenophora. These animals show radial symmetry, like the spokes of a wheel around an axle,
and are grouped as Radiata.
D. A major evolutionary innovation among the radiates is the extracellular digestion of food.
E. Cnidarians
1. Cnidarians are represented by the hydra, jellyfish, corals, and sea anemones.
2. Cnidarians are carnivores that possess stinging nematocysts to harpoon and capture prey.
3. They have an interior gut cavity that has evolved to enable extracellular digestion, which
is digestion within a cavity rather than within individual cells.
4. Two body forms are seen in cnidarians, a free-floating medusae form, and a polyp, or
attached, form.
5. In some species of cnidarians, alternation between these two forms is seen during their
life cycle.
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F.
Ctenophorans
1. Ctenophorans include the comb jellies, the largest animals that use cilia for movement.
The Advent of Bilateral Symmetry (p. 465)
25.6
Solid Worms: Bilateral Symmetry (p. 465; Figs. 25.10, 25.11, 25.12, 25.13, 25.14, 25.15)
A. In more advanced groups of metazoans, bilateral symmetry evolved, which allows
cephalization, or development of a head end, plus provides for different body surfaces (i.e.,
ventral, or belly-side; dorsal, or opposite to belly-side; anterior, or front; and posterior, or
back).
B. The bilaterally symmetrical eumetazoans have three germ layers, ectoderm, mesoderm, and
endoderm.
C. The simplest of the bilaterally symmetrical animals are the solid worms.
D. Solid worms, such as the flatworms of the phylum Platyhelminthes, are numerous, comprising
20,000 species.
E. In this group organs evolved; organs are groups of different tissues functioning together as
units.
F. Solid worms have a digestive tract but no other body cavity, and are thus acoelomate (without
a body cavity).
G. Flatworms
1. Flatworms are simple in design yet possess a head, and range in size from a millimeter to
several meters in length.
2. Flatworms may be free-living or parasitic; free-living forms are carnivores or scavengers.
3. Flukes and tapeworms make up the two classes of parasitic flatworms.
4. Important human parasites include the liver fluke, and three species of Schistosoma that
cause schistosomiasis.
H. Characteristics of Flatworms
1. Flatworms cannot feed continuously.
2. They have a gut that branches and functions both in digestion and food distribution; food
particles enter and exit the single opening to the gut.
3. Flatworms have a simple excretory system with cilia and flame cells.
4. Flatworms lack a circulatory system and have a very simple nervous system.
5. The reproductive systems of flatworms are complex, and most flatworms are
hermaphroditic.
The Advent of a Body Cavity (p. 469)
25.7
Roundworms: The Evolution of a Body Cavity (p. 469; Figs. 25.16, 25.17, 25.18)
A. An internal body cavity evolved in animal groups other than the solid worms and
asymmetrical animals.
B. This was an important improvement because a cavity allows for circulation, greater
movement, and independent organ function.
C. Kinds of Body Cavities
1. Acoelomates have no body cavity.
2. Pseudocoelomates have a body cavity called a pseudocoel located between the mesoderm
and endoderm.
3. Animals with a coelom have a fluid-filled cavity that develops entirely within the
mesoderm.
4. The problem of diffusion in coelomates is solved by the evolution of a circulatory system.
5. In an open circulatory system, the blood passes from vessels into sinuses, mixes with
body fluid, and reenters the vessels in another position.
6. In a closed circulatory system, the blood is separate from the body fluid and can be
controlled separately.
D. Roundworms: Pseudocoelomates
1. Seven phyla of animals are pseudocoelomates, or animals with a pseudocoelom that
develops between endoderm and mesoderm.
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2.
25.8
25.9
25.10
These animals have a one-way digestive tract that permits specialization of different areas
or organs within the tract.
3. A second pseudocoelomate group, the rotifers (phylum Rotifera), are small aquatic
creatures with a crown of cilia on their heads to propel them through the water.
4. All pseudocoelomates lack a defined circulatory system but most have a one-way
digestive tract.
E. Nematoda: The Roundworms
1. The phylum Nematoda, which includes the nematodes, has the most species of the seven
pseudocoelomate phyla.
2. Nematodes are microscopic worms that are abundant in soil.
3. Nematodes have a hydrostatic skeleton, lack flagella or cilia, and undergo simple
development.
4. About 50 species of nematodes regularly parasitize human beings.
5. Trichinosis is a disease caused by nematodes and is contracted by eating undercooked
pork.
Mollusks: Coelomates (p. 472; Figs. 25.19, 25.20)
A. Coelomates
1. Coelomates are animals that possess a true coelom, or fluid-filled body cavity contained
completely within the mesoderm.
2. Having this arrangement allows for the development of specialized tissues during the
embryonic period by a process called primary induction.
3. During this process, there is a complete interaction of the three germ layers, not seen
before in pseudocoelomate or acoelomate animals.
4. This interaction led to the evolution of complex digestive tracts.
B. Mollusks
1. The mollusks are the most primitive coelomates, but are highly diverse, with over
110,000 species.
2. All mollusks have a head, a central portion containing most body organs, and a foot
region.
3. A heavy coat of tissue, the mantle, contains the breathing apparatus, the gills.
4. Three groups of mollusks are gastropods, bivalves, and cephalopods.
5. These groups differ in their arrangement of the basic mollusk body plan.
6. One of the most characteristic features of the mollusks is a radula, a rasping tongue-like
organ used to scrape food off rocks or to puncture prey.
Annelids: The Rise of Segmentation (p. 474; Figs. 25.21, 25.22)
A. The next key evolutionary advance, segmentation, arose among the coelomates.
B. Segmentation involved a series of identical segments in a body plan, each physically separated
from the next, and each of which can evolve independently to produce a new structure and
function.
C. The annelid worms, phylum Annelida, were the first segmented animals.
D. Annelids show repeated segments, segments that are specialized for a particular function, and
connections between segments, such as a circulatory system.
Arthropods: Advent of Jointed Appendages (p. 476; Figs. 25.23–25.33)
A. The evolution of segmentation enabled the innovation of jointed appendages.
B. Jointed Appendages
1. Development of jointed appendages in the arthropods, phylum Arthropoda, led to an
explosion of different kinds of species.
2. Without jointed appendages, it is impossible to walk, fly, or sense environmental change
with antennae.
C. Rigid Exoskeleton
1. Arthropods also developed a rigid, chitinous exoskeleton on the outsides of their bodies,
which provided a point for muscle attachment.
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2.
One drawback of an exoskeleton was that it prevented arthropods from reaching the
immense sizes seen in some of the early vertebrates that had internal skeletons.
3. Because of their body design, arthropods are very successful, and two-thirds of all named
species are arthropods.
D. Chelicerates
1. Arthropods that lack jaws are called chelicerates, and include spiders, mites, and
scorpions.
2. The largest of the three classes of chelicerates are the mostly terrestrial Arachnida, with
57,000 species of spiders, ticks, mites, and scorpions.
E. Mandibulates
1. Arthropods possessing mouthparts that evolved from anterior appendages are called
mandibulates.
2. Examples of the mandibulates include the crustaceans, centipedes, millipedes, and
insects.
3. The crustaceans are a large, diverse group of mostly aquatic organisms, including 35,000
species of crabs, shrimps, lobsters, crayfish, barnacles, waterfleas, pillbugs, and others
that play a crucial role in aquatic ecosystems.
4. Millipedes and centipedes are closely related to insects and have one pair (centipedes) or
two (millipedes) pairs of legs per body segment.
5. Most centipedes are carnivores with venomous fangs, while most millipedes are
herbivores.
6. The insects, class Insecta, are the most successful group of arthropods, with 90,000
species in North America alone.
7. Insects have an elaborate head, a thorax with three segments, each giving rise to a pair of
legs, usually a total of two pairs of wings, and a segmented abdomen.
Redesigning the Embryo (p. 482)
25.11
25.12
Protostomes and Deuterostomes (p. 482; Fig. 25.34)
A. The coelomate animals fall into two categories, depending on the course of their embryologic
development.
B. Some embryos start as hollow balls of cells, then indent to form two-layered balls that open to
the outside.
C. The mouth of these embryos develops at or near this opening, which is called the blastopore;
animals that develop this way, mouth first, are called protostomes.
1. Even noncoelomate animals develop in this manner.
D. Further down the line, after the arthropods, another pattern of embryologic development
evolved; animals that develop by this second pattern are called deuterostomes.
E. In this pattern, the anus develops at or near the blastopore, and the mouth develops later from
another part of the embryo.
F. Radial cell division also arises in deuterostomes, while cell division is spiral (spiral cleavage)
in protostomes.
G. Deuterostomes divide by radial cleavage; this is significant because in deuterostomes, if a cell
gets separated from other cells early in embryologic development, the cell can still develop
into an entire organism.
H. In protostomes, differentiation occurs earlier during embryologic development, so the fate of
each cell cannot be altered.
I. Deuterostome development allows for more possibilities for evolution.
Echinoderms: The First Deuterostomes (p. 484; Figs. 25.35, 25.36)
A. The first deuterostomes were the echinoderms, which are marine animals of the phylum
Echinodermata.
B. They possess an endoskeleton made up of hard plates below the skin.
C. The 6,000 species of echinoderms include starfish, sea urchins, and sand dollars among
others.
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25.13
D. Echinoderms begin life as bilaterally symmetrical larvae but end up as radially symmetrical
adults with a unique body design.
E. All echinoderms have a nervous system consisting of a central ring that gives rise to five
branches.
F. They also have a water hydraulic system that, with the aid of tube feet, allows them to attach
themselves and move along the bottom of the sea.
Chordates: Improving the Skeleton (p. 487; Figs. 25.37, 25.38, 25.39)
A. General Characteristics of Chordates
1. The chordates are deuterostome coelomates that developed after the echinoderms and
possess an even more advanced endoskeleton.
2. Chordates have a dorsal notochord that allows for back and forth swimming motions.
3. A nerve cord follows this notochord and branches out to different portions of the body.
4. A group of slits behind the mouth, called the pharyngeal slits, is also present during at
least part of the life of all chordates.
5. A postanal tail is present during embryonic development in all chordates and beyond in
many species.
B. Vertebrates
1. Most chordates are vertebrates that have a well-defined head and a backbone, plus an
internal skeleton made up of bone or cartilage.
2. Bone is a partially flexible, hard substance made up of calcium salts and protein that
comprises the skeletons of vertebrates.
KEY TERMS
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Parazoa (p. 454) The Parazoa is the subkingdom of animals made up of animals without symmetry,
tissues, or organs.
Eumetazoa (p. 454) The Eumetazoa is the subkingdom of animals that have shape and symmetry.
choanocytes (p. 460) Flagellated cells in sponges.
radial symmetry (p. 456) Symmetrical about a central axis.
nematocyst (p. 464) The stinging weapon of the cnidarians.
bilateral symmetry (p. 456) Having a right and left half, mirror images of each other.
cephalization (p. 465) The process of developing a head end.
acoelomate (p. 465) Solid body construction.
pseudocoelomate (p. 469) Having a body cavity between the mesoderm and endoderm.
coelom (p. 469) Having a body cavity develop entirely within the mesoderm.
coelomates (p. 469) Possessing a coelom.
radula (p. 472) The rasping tongue-like organ of mollusks.
segmentation (p. 456) The building of a body from a series of similar segments.
exoskeleton (p. 476) Rigid, external skeleton.
protostome (p. 456) Mouth forms before anus.
deuterostomes (p. 456) Mouth forms after anus.
endoskeleton (p. 484) An internal skeleton.
notochord (p. 487) Flexible rod along the back of the embryo of Chordates.
LECTURE SUGGESTIONS AND ENRICHMENT TIPS
1.
Demonstration Aquarium. Set up a saltwater aquarium for student viewing containing as many
different kinds of animal phyla as possible. Try to include such organisms as sponges, anemones,
starfish, sea urchins or other echinoderms, and crustaceans that some students may never have seen
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2.
3.
before. A number of biological supply companies, such as Carolina Biological Supply and Ward's
Biology (see Appendix), offer a wide variety of live creatures if a local aquarium shop does not have
what you need.
Insects Frozen in Time. Have students read D. Grimaldi's article entitled “Captured in Amber,”
Scientific American, April 1996, pages 84–91. This is an opportunity for students to read a scientific
article written for a lay audience to gain some appreciation for the nature of scientific literature.
Discuss how amber has preserved insects from 25 million years ago and some of the secrets of insect
evolution that have been uncovered from this material.
Human Parasites. Nematodes, the pseudocoelomates of the animal kingdom, are responsible for a
number of parasitic conditions in humans. While these conditions are somewhat rare in the United
States, discussing them helps students become aware of what living elsewhere can encompass.
Roundworms, pinworms, and hookworms are all nematodes. Two of the roundworm parasites of
humans are Ascaris and Trichinella spirilis. The large parasitic roundworm Ascaris has 20–35 cm-long
adults that reside in their host's intestines, including those of humans. Reproduction occurs there, and
Ascaris eggs are passed along with feces. Eggs must mature in the soil and thus are restricted to
survival only in warmer climates. A new host picks up this parasite when eggs are swallowed due to
poor sanitation. Once inside the host's body, larvae hatch from their eggs, and tunnel through the
intestinal wall to the liver and eventually to the lungs. They molt in the lungs and crawl up the trachea.
Once they reach the throat, they tend to be swallowed and once again end up in the intestinal tract.
There they mature into adult worms and reproduce, completing their life cycle. Ask your students what
effect having a load of Ascaris worms in the intestines would have on the individual. Trichinella
spirilis is the cause of a disease known as trichinosis (not to be confused with trichomoniasis, the
sexually transmitted disease). For humans, the most common way to get trichinosis is by eating rare
pork infected with encysted larvae. When these larvae mature in human intestines, they tunnel through
the intestinal wall and produce offspring that travel in the bloodstream. Eventually they reach skeletal
muscles and encyst there.
CRITICAL-THINKING QUESTIONS
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3.
Explain why bilateral symmetry was so important an evolutionary advance in the animals.
What evolutionary change solved the problem of diffusion among coelomates?
Discuss the reasons why arthropods are the most numerous and successful animals on Earth.
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