19 EVOLUTION OF THE ANIMAL PHYLA
CHAPTER OUTLINE
Introduction to the Animals (p. 404)
19.1
19.2
19.3
General Features of Animals (p. 404; Table 19.1)
A. Animals are all multicellular heterotrophs.
B. Animals lack cell walls and are flexible in form.
C. Most animals are able to move from one place to another to locate food.
1. A remarkable form of movement unique to animals is flying; the amphibians are the only
terrestrial vertebrates never to have a flying member.
D. Most of the 35 animal phyla are inhabitants of the oceans, but the arthropods, mollusks, and
vertebrates are the dominant animal groups on land.
1. About 99% of all animals are invertebrates, and only 42,500 species are vertebrates.
E. Most animals reproduce sexually, and cells formed in meiosis function directly as gametes.
1. With few exceptions, there is no counterpart among animals to the alternation of
generations seen in plants.
F. Embryonic Development
1. The zygote undergoes a series of divisions and first 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.
3. Generally speaking, the pattern of embryological development is characteristic of the
animal kingdom.
G. The cells of most animals are organized into tissues.
The Animal Family Tree (p. 406; Figs. 19.1, 19.2)
A. The Traditional Viewpoint
1. The animals (metazoans) are traditionally divided into 35 distinct phyla.
2. The First Branch: Tissues
a. Parazoa and 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 bilaterally symmetrical animals into those
with a body cavity and those without.
b. Those with a body cavity are split into those with a coelom and those without.
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. New phylogenies based on molecular data split the protostomes up into the
Lophotrochozoans and Ecdysozoans.
a. Lophotrochozoans are animals that grow by adding mass to an existing body.
b. Ecdysozoans grow by molting.
5. Over the next few years, additional molecular data can be anticipated, which will lessen
the confusion of the animal family tree.
Six Key Transitions in Body Plan (p. 408; Fig. 19.3; Table 19.2)
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.
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 Segmentation
1. In protostome animals (flatworms, nematodes, annelids, mollusks, and arthropods), the
blastopore develops into the mouth.
2. In deuterostomes (echinoderms and chordates), the blastopore develops into the anus.
E. The Evolution of Segmentation
1. In many segmented animals, each segment can develop a more or less complete set of
adult organs 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. 411)
19.4
19.5
Sponges: Animals Without Tissues (p. 411; Fig. 19.4)
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. The sponge body consists 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. 414; Figs. 19.5, 19.6, 19.7)
A. Eumetazoans show body symmetry and have tissues.
B. Radially symmetrical eumetazoans have two embryonic layers: an outer ectoderm, which
gives rise to the epidermis, and an inner endoderm, which gives rise to the gastrodermis; a
jellylike layer called the mesoglea forms between the epidermis and gastrodermis.
C. The most primitive members of the subkingdom Eumetazoa are in the phyla Cnidaria and
Ctenophora. These animals show radial symmetry, with body parts arranged 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 an attached
polyp form.
5. In some species of cnidarians, alternation between these two forms is seen during their
life cycle.
F. Ctenophorans
1. Ctenophorans include the comb jellies, the largest animals that use cilia for movement.
The Advent of Bilateral Symmetry (p. 417)
19.6
Solid Worms: Bilateral Symmetry (p. 417; Figs. 19.8-19.12)
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 called flatworms (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 blood flukes of the genus
Schistosoma, which 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. 421)
19.7
Roundworms: The Evolution of a Body Cavity (p. 421; Figs. 19.13, 19.14)
A. An internal body cavity evolved in bilaterally symmetrical animal groups other than the solid
worms.
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 pseudocoel that
develops between endoderm and mesoderm.
2. The pseudocoel serves as a hydrostatic skeleton against which the animal’s muscles can
work.
3.
19.8
19.9
19.10
The largest pseudocoelomate phylum is Nematoda, which includes 20,000 species of
nematodes, eelworms, and other roundworms. 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 that permits specialization of different areas or organs within the tract
E. Nematoda: The Roundworms
1. Nematodes are abundant and found everywhere; many are microscopic and found in soil;
other are parasites of animals and plants.
2. Nematodes are covered with a thick cuticle, which is molted as they grow.
3. About 50 species of nematodes regularly parasitize human beings.
4. Trichinosis is a disease caused by nematodes and is contracted by eating undercooked
pork.
Mollusks: Coelomates (p. 424; Fig. 19.15)
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-foot, a central portion containing most body organs, and a
mantle, which is a heavy coat of tissue attached interiorly to the breathing apparatus, the
gills.
3. Three groups of mollusks are gastropods, bivalves, and cephalopods.
4. These groups differ in their arrangement of the basic mollusk body plan.
5. 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. 426; Fig. 19.16)
A. The next key evolutionary advance, segmentation, arose among the coelomates.
B. A segmented body plan involves a series of identical segments, 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. 427; Figs. 19.17-19.25)
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.
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 (mandibles) 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 mandibles 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, water fleas, pillbugs, and others
that play a crucial role in aquatic ecosystems.
4. Millipedes and centipedes are closely related to insects and have one pair of legs
(centipedes) or two pairs of legs (millipedes) 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 about 1
million known species.
7. Insects have an elaborate head, a thorax with three segments, each giving rise to a pair of
legs and usually two pairs of wings, and a segmented abdomen.
Redesigning the Embryo (p. 434)
19.11
19.12
19.13
Protostomes and Deuterostomes (p. 434; Figs. 19.26)
A. The coelomate animals fall into two categories, depending on the course of their embryologic
development.
B. Most animal embryos start as hollow balls of cells, then indent to form two-layered balls that
open to the outside.
C. In protostomes, the mouth of these embryos develops at or near this opening, which is called
the blastopore.
1. Even noncoelomate animals develop in this manner.
D. In deuterostomes, the anus develops at or near the blastopore, and the mouth develops later
from another part of the embryo.
E. Radial cell division also arises in deuterostomes, while cell division is spiral (spiral cleavage)
in protostomes.
F. In protostomes, differentiation occurs earlier during embryologic development, so the fate of
each cell cannot be altered; in deuterostomes, the first cell divisions produce identical cells,
any of which can develop into an entire organism.
G. Also, the coelom develops differently in protostomes versus deuterostomes.
Echinoderms: The First Deuterostomes (p. 436; Fig. 19.27)
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.
D. Echinoderms begin life as bilaterally symmetrical larvae but end up as radially symmetrical
adults with a unique body design.
E. The nervous system consists of a central ring that gives rise to 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. 438; Figs. 19.28, 19.29)
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 also 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 series of pouches behind the mouth, called the pharyngeal pouches, 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.
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Parazoa (p. 406) The Parazoa are animals without symmetry, tissues, or organs.
Eumetazoa (p. 406) The Eumetazoa are animals that have a definite shape and symmetry.
choanocytes (p. 412) Flagellated cells in sponges.
radial symmetry (p. 408) Symmetrical about a central axis.
ectoderm (p. 414)
endoderm (p. 414)
nematocyst (p. 416) The stinging weapon of the cnidarians.
medusae (p. 416)
polyp (p. 416)
bilateral symmetry (p. 408) Having a right and left half, mirror images of each other.
mesoderm (p. 417)
cephalization (p. 417) The process of developing a head end.
acoelomate (p. 417) Solid body construction.
pseudocoelomate (p. 421) Having a body cavity between the mesoderm and endoderm.
coelom (p. 421) A body cavity that develops entirely within the mesoderm.
coelomates (p. 421) Possessing a coelom.
mantle (p. 424)
radula (p. 424) The rasping tongue-like organ of mollusks.
segmentation (p. 426) The building of a body from a series of similar segments.
exoskeleton (p. 428) Rigid, external skeleton.
protostome (p. 434) Mouth forms before anus.
deuterostome (p. 434) Mouth forms after anus.
endoskeleton (p. 436) An internal skeleton.
spiral cleavage (p. 434)
radial cleavage (p. 434)
endoskeleton (p. 436)
notochord (p. 438) Flexible rod along the back of the embryo in Chordates.