Answers to Mastering Concepts Questions
17.1
1. What characteristics do all animals share?
Animals are multicellular eukaryotes, have cells without walls, are heterotrophs by
ingestion, and have a blastula stage during development.
2. When and in what habitat did animals likely originate?
Animals probably originated in the oceans in Precambrian times. Today’s major groups
of animals arose during Precambrian and Cambrian times.
3. What features were used to build the animal phylogenetic tree?
Animal phylogeny is based on cell and tissue organization, body symmetry and
cephalization, and structures that arise during embryonic development. Molecular
sequences are also important.
4. Distinguish between a coelom and a pseudocoelom.
A coelom is a body cavity that is completely lined with mesoderm, while a pseudocoelom
is a body cavity that is partly lined with mesoderm and partly lined with endoderm.
5. What are the two main types of digestive tract?
Incomplete and complete digestive tracts are the two main kinds of digestive tracts.
6. What advantages does segmentation confer?
Segmentation increases flexibility and allows specialized body functions to be
concentrated in the segments of one region of the body.
17.2
1. How is a sponge’s body different from that of other animals?
A sponge’s body does not contain true tissues. Its body is perforated by pores that house
collar cells that capture its food.
2. How do sponges feed?
Sponges are filter feeders, using flagella on their collar cells to extract organic particles
from water. The collar cells digest the food and share the nutrients with adjacent cells.
3. What are two of the cell types in a sponge?
Two types of cells in a sponge are choanocytes, or collar cells that capture food, and
amoebocytes that form skeletal elements, digest food, divide, and store and transport
nutrients.
4. What is the function of spicules?
Spicules help support and defend a sponge.
5. How do sponges reproduce sexually and asexually?
Sponges reproduce sexually by shedding sperm into the water. The sperm are taken into
the body of another sponge, where they fertilize egg cells. The new sponge develops into
a larva within the body of its parent; larvae are eventually released. Sponges reproduce
asexually by budding or by fragmentation.
6. In what ways are sponges important?
Sponges are ecologically important because they filter seawater and provide habitat for
many other types of organisms. Their economic importance includes the anticancer and
antimicrobial drugs that are derived from them.
17.3
1. What features do all cnidarians share?
All cnidarians have radial symmetry and incomplete digestive tracts. They are named for
their stinging cells.
2. What are some examples of cnidarians?
Cnidarians include jellyfishes, hydras, sea anemones, and coral.
3. How do cnidarians move and defend themselves?
Some cnidarians are sessile, remaining attached to a substrate. Medusas move by
contracting muscles that force water out of the bell. Cnidarians defend themselves with
their stinging cells attached to the tentacles.
4. In what ways are cnidarians important?
Jellyfishes can sting humans. Coral reefs protect coastlines from erosion and store carbon
dioxide in calcium carbonate. Coral reefs also protect many species of fishes,
invertebrates, and other animals.
17.4
1. What features do all flatworms share?
All flatworms have bilateral symmetry, flattened bodies, and incomplete digestive tracts.
2. How does the body shape of a flatworm enhance gas exchange?
The flat body shape of a flatworm maximizes the surface area in contact with the
environment.
3. How do flatworms eat, move, and reproduce?
Free-living flatworms are carnivores and scavengers that eat organic matter, which they
acquire through their pharynx. Parasitic flukes use a pharynx to feed on blood and other
body tissues of their hosts. Tapeworms attach to their host’s intestine with the scolex and
absorb food through the body wall. Flatworms move by contracting muscles against a
hydrostatic skeleton. Flatworms reproduce asexually by fission or sexually inside a host
by producing eggs that are dispersed.
4. In what ways are flatworms important?
Flatworms are important because they cause infections that affect hundreds of millions of
people as well as domesticated and wild animals.
17.5
1. What adaptations do mollusks have in common?
Mollusks have a mantle that secretes a shell in most species, a muscular foot for
movement, a visceral mass containing the digestive and reproductive organs, and a
radula.
2. How do mollusks feed, move, and protect themselves?
Feeding: Mollusks feed either by filtering organic particles out of the water or by rasping
the radula against their food. Cephalopods are predators of fast-moving prey. Movement:
Mollusks move by contracting muscles against the fluid of the coelom. Bivalves move by
contraction of the muscular foot. Cephalopods move by jet propulsion. Protection: Many
mollusks protect themselves with a hard shell; others can evade predators by changing
color and darting away, often in a cloud of ink.
3. In what ways are mollusks important?
Mollusks are important as human food sources, although they can accumulate toxins
produced by dinoflagellates. Snails and slugs damage and consume crops and garden
plants. Some snails harbor life cycle stages of parasitic worms. Introduced species such
as zebra mussels damage the ecology of the Great Lakes.
17.6
1. What features do all annelids share?
Annelids are segmented worms with bilateral symmetry and a complete digestive tract.
2. List examples of animals in each of the two classes of annelids.
Earthworms and leaches are in the oligochaete group and the marine segmented worms
are in the polychaete group.
3. How do annelids feed, exchange gases, and move?
Feeding: Earthworms feed by ingesting soil organic matter, whereas leeches suck blood
or ingest small prey. Polychaetes can be filter feeders or predators. Gas exchange:
Whereas polychaetes have gills for gas exchange, leeches and earthworms exchange
gases by diffusion through the body wall. Movement: Earthworms and leeches move by
contracting circular and longitudinal muscles that surround the fluid-filled coelom.
Polychaetes swim with their parapodia.
17.7
1. What features do all roundworms share?
Roundworms, also known as nematodes, are unsegmented, cylindrical worms with
tapered ends and a complete digestive tract.
2. Contrast the bodies of roundworms, flatworms, and annelids.
Roundworms have a pseudocoelom, compared to the true coelom of annelids and the
absence of any coelom in flatworms. Roundworms have cylindrical, unsegmented bodies,
compared to the segmented bodies of annelids and the flat bodies of flatworms. All types
of worms have elongated bodies with bilateral symmetry.
3. How do nematodes feed and move?
Feeding: Nematodes feed on fungi, bacteria, roots, or almost any other plant part. They
also parasitize mammals by sucking blood or consuming digested food. Movement:
Nematodes move by contracting longitudinal muscles against the fluid-filled
pseudocoelom. They have no circular muscles, so they only thrash from side to side.
4. How are roundworms important?
Roundworms are important because they are intestinal parasites of humans and other
animals. They cause elephantiasis and river blindness. They also cause crop diseases and
attack insect pests. Caenorhabditis elegans is an important model organism used in
biological research.
17.8
1. What features distinguish the arthropods?
Arthropods have an exoskeleton and jointed appendages.
2. What are the main body regions of an arthropod?
The main body regions of an arthropod are the head, thorax, and abdomen.
3. How do arthropods use their jointed appendages?
Arthropods use their jointed appendages for movement, defense, prey capture, sensing
the environment, ornaments, weapons, and copulation.
4. Describe how arthropods feed, respire, excrete metabolic wastes, sense their
environment, move, reproduce, and defend themselves.
Feeding: Arthropods have mouthparts that allow them to feed, either by chewing or
sucking. They feed on almost every imaginable substance. Gas exchange: Land-based
arthropods respire by diffusion of gases through holes in the exoskeleton that lead to
tubules that exchange gases with tissues. Arthropods that live in water respire with gills.
Spiders and scorpions respire with book lungs. Excretion: Arthropods excrete metabolic
wastes using Malpighian tubules that remove nitrogenous wastes. These are deposited
into the end of the digestive tract and ejected though the anus. Senses: Arthropods have
specialized sense organs that allow them to be sensitive to changes in their environments.
These include changes in light, sound, scents, chemicals, potential mates, food proximity,
touch, air currents, and body position. Movement: Arthropods move by contracting
muscles attached to their exoskeletons. Many of these muscles span segments, allowing
them to move. Reproduction: Although there are asexually reproducing species, most
arthropods reproduce sexually, using internal fertilization. Defense: Some arthropods
defend themselves by fleeing from predators, but many can bite, sting, pinch, make
noises, or give off foul odors. Camouflage is another defense, as are wing spots that may
startle predators.
5. What is the function of the exoskeleton?
The exoskeleton is protective and functions in movement. Exoskeletons give shape to the
body.
6. In what ways are arthropods important?
Arthropods are important pollinators of crops. They also spread diseases and can sting
humans. They destroy crops and wooden structures. They make honey, silk, and
beeswax, and people eat shrimp, lobsters, crabs, and crayfish. Spiders eat harmful insects.
Many people are allergic to dust mites.
7. How are chelicerates different from mandibulates?
The difference is in the mouthparts. Chelicerates have clawlike chelicerae, whereas
mandibulates have jawlike mandibles.
8. Give an example of an animal in each subphylum of arthropods.
The five subphyla of arthropods include:
 trilobites – example: trilobites (extinct)
 chelicerates – spiders, scorpions, and their relatives
 mandibulates (myriapods) – centipedes and millipedes
 mandibulates (crustaceans) – lobsters
 mandibulates (insects) – butterflies
17.9
1. What characteristics distinguish the echinoderms?
Echinoderms have a spiny skin, and adults have a radially symmetrical body divided into
five parts. Larvae have bilateral symmetry. The echinoderms also have a water vascular
system.
2. What are some examples of echinoderms?
Sea urchins, sea lilies, sand dollars, sea cucumbers, sea stars, and brittle stars are
examples of echinoderms.
3. In what ways are echinoderms important?
Echinoderms are important in that some people eat them, and some can sting humans.
Some sea stars, like the crown of thorns sea star, harm coral reefs.
17.10
1. What are the four defining characteristics of chordates?
The four defining features of chordates are: a notochord; a dorsal, hollow nerve cord;
pharyngeal pouches or slits; and a postanal tail.
2. Which chordates have a cranium, and which are also vertebrates?
Animals with a cranium include hagfishes and vertebrates. Vertebrates, in turn, include
lampreys, fishes, amphibians, reptiles, and mammals.
3. How did the origin of jaws, lungs, limbs, and the amnion affect the course of vertebrate
evolution?
Jaws enabled animals to exploit new food sources; lungs allowed vertebrates to move out
of water and onto land, obtaining O2 from air; limbs allowed vertebrates to walk on land
and expanded their ecological roles; the amnion allowed eggs to be laid on land instead
of in water.
4. What is the difference between an ectotherm and an endotherm?
The body temperature of an ectotherm varies with the environment, but can be altered by
behaviors such as basking in the sun. In contrast, the body temperature of an endotherm
is maintained at a steady level by metabolic processes and physiological mechanisms
(such as shivering, panting, and sweating).
17.11
1.What are the tunicates and lancelets?
Both tunicates and lancelets are examples of invertebrate chordates. Tunicates are small
sessile marine organisms that resemble bags with two siphons. Lancelets are marine
organisms resembling tiny, boneless, eyeless fishes.
2. Make a table comparing the features of each vertebrate group.
Answers will vary but should include the main characteristics of lampreys, fishes,
amphibians, reptiles, and mammals. References include the tables for each group in the
text and the chordate characteristics summary table.
3. Describe the adaptations that mark the transition from fishes to amphibians, reptiles,
and mammals.
The movement to land required lungs and four legs. Amphibians must lay their eggs in
water, whereas reptiles have a shelled egg. Most mammals give birth to live young, and
all suckle their offspring and have fur or hair.
4. What characteristics place the birds within the reptiles, and how are birds different
from other reptiles?
Birds have similar skeletons to reptiles, as well as an amniotic egg. The presence of
feathers on some dinosaurs is another shared characteristic. Birds are different from other
reptiles because of endothermy and their adaptations to flight.
17.12
1. Name and describe the three groups of contemporary primates. To which group do
humans belong?
The three groups of contemporary primates are prosimians (lemurs and others), simians
(monkeys), and hominoids (apes). Humans are hominoids.
2. What can skeletal anatomy and DNA sequences in existing primates tell us about
human evolution?
The skeletal anatomy of existing primates can help us understand human evolutionary
history and the trends in human evolution. Primate anatomy allows us to infer the diets,
body positions, sizes, and many other aspects of fossil humans and their ancestors.
3. What are the three groups of hominines in the human family tree, and which still exist
today?
The three existing groups of hominines in the human family tree are gorillas, chimps, and
humans. Each group also contains members known only from fossils.
4. Which conditions may have contributed to the evolution of humans?
Conditions that led to the evolution of humans are rooted in plate tectonics. After India
and the Himalayan region collided with Eurasia, the rise of the Himalayas altered
Africa’s climate. It became cooler and some tropical forests became grasslands.
Ancestors of humans began to live on grasslands instead of in trees.
17.13
1. Explain why Tiktaalik is called a “fishapod.”
It had a mix of fish and tetrapod characteristics. It had scales and limbs like a fish, and
lungs and limbs like a tetrapod.
2. How might the ability to crawl on land for short periods have enhanced the
reproductive fitness of Tiktaalik?
The ability to crawl on land for short periods of time would have allowed Tiktaalik to
utilize new food resources in areas without competition, and this would have increased
fitness.
Answers to Write It Out Questions
1. Compare the nine major animal phyla in the order in which the chapter presents them,
listing the new features for each group.
Sponges – multicellularity; Cnidarians – tissues and two germ layers; Flatworms –
bilateral symmetry, three germ layers, protostome, incomplete digestive tract; Mollusks –
mantle, complete digestive tract, coelom; Annelids – segmentation; Roundworms –
pseudocoelom; Arthropods – exoskeleton, jointed appendages; Echinoderms –
deuterostome, water vascular system; Chordates – notochord, hollow nerve cord,
pharyngeal slits, postanal tail.
2. You are visiting the aquarium, and your companion points at an animal in a tank. None
of the signs shows a picture of the animal or its name. What criteria would you use to
assign the animal to a phylum?
You could start by checking for a cranium and the backbone, which would indicate a
vertebrate. If the animal were wormlike and had segmentation, it might be an annelid. If
it were wormlike without segmentation, it might be a roundworm. The presence of an
exoskeleton would indicate an arthropod, and a radula or hinged shell would indicate a
mollusk. You could also check for radial symmetry. If it were in five parts it could be an
echinoderm, and if not it could be a cnidarian. If the animal was flattened, it might be a
flatworm. Anything with a porous body might be a sponge.
3. List the criteria used to distinguish: (a). animals from other organisms; (b) vertebrates
from invertebrates; (c) protostomes from deuterostomes; (d) ectotherms from
endotherms; (e) a tapeworm, a nematode, a slug, an earthworm, a snake, and a caecilian.
(a) Animals are multicellular, eukaryotic, heterotrophic organisms that do not possess cell
walls. Animals undergo a unique embryological development, and their cells secrete and
bind to a nonliving substance known as the extracellular matrix.
(b) Vertebrates possess backbones; invertebrates do not.
(c) In protostomes, the first indentation of the gastrula forms the mouth; in
deuterostomes, the first indentation forms the anus.
(d) Ectotherms do not have an internal mechanism to regulate temperature, whereas
endotherms do.
(e) A tapeworm is a flatworm, which is unsegmented, has an incomplete digestive tract,
and lacks a coelom. A nematode is a roundworm, which is unsegmented, sheds its
cuticle, has a complete digestive tract, and has a pseudocoelom. A slug is a mollusk with
no shell and a broad, flat foot which it uses to crawl along a trail of mucus. An
earthworm is a segmented worm with a coelom and a complete digestive tract. The snake
is a limbless reptile, whereas a caecilian is a limbless amphibian.
4. Distinguish between: (a) radial and bilateral symmetry; (b) blastula and gastrula; (c)
complete and incomplete digestive tract; (d) coelom and pseudocoelom.
(a) In radial symmetry, many planes passing through the body can divide the body into
mirror images. In bilateral symmetry, only one plane divides the animal into mirror
images. Bilaterally symmetrical animals such as ants and zebras have head (anterior) and
tail (posterior) ends, and they typically move “head-first” through their environment.
(b) The early animal embryo begins as a solid ball of cells that quickly hollows out to
form a blastula, a sphere of cells surrounding a fluid-filled cavity. The blastula folds in on
itself to generate the gastrula.
(c) In organisms with an incomplete digestive tract, one opening (the mouth) both takes
in food and ejects wastes. Digestion occurs in the gastrovascular cavity, which distributes
nutrients to all parts of the animal’s body. In animals with a complete digestive tract,
food passes in one direction from mouth to anus.
(e) A coelom is a fluid-filled body cavity that forms completely within the mesoderm. A
pseudocoelom (“false coelom”) is lined partly with mesoderm and partly with endoderm.
5. Segmented animals occur in multiple phyla. How might segmentation benefit an
animal? If segmentation is adaptive, why do unsegmented animals still exist?
Segmentation occurs in the annelids, arthropods, and chordates, adding flexibility and
offering new opportunities for developing specialized body parts. Unsegmented animals
still exist because the lack of segmentation wasn’t selected against during their
evolutionary history. That is, in many habitats, unsegmented animals still have
reproductive success.
6. What is the evidence for the surprisingly close relationship between echinoderms and
chordates?
Both echinoderms and chordates are deuterostomes; molecular evidence also supports the
hypothesis.
7. What are the four distinguishing characteristics of chordates?
Chordates have a notochord, a dorsal hollow nerve cord, pharyngeal gill slits, and a
postanal tail.
8. How do tunicates and lancelets differ from fishes and tetrapods?
Tunicates and lancelets lack a cranium, vertebrae, jaws, and a skeleton.
9. Draw from memory a phylogenetic tree that traces the evolutionary history of
vertebrates. Include the features that mark each branching point in your tree.
The lobe-finned fishes gave rise to the amphibians, which have limbs and lungs, but
straddle the land-water transition. Some amphibians gave rise to reptiles, which have
additional adaptations to dry land (including waterproof skin and an amniote egg).
Different branches of reptiles eventually gave rise to modern birds and mammals.
10. Tunicates look much different than other chordates. Why are they classified in this
phylum?
The tunicates have all four chordate characteristics during the larval stage.
11. If you found an eel-like animal in the ocean, what features would you look for in
deciding whether you had a hagfish or an eel (a true fish)?
The hagfish lacks true jaws and vertebrae; the eel has these features.
12. List three adaptations that enable fishes to live in water, amphibians to live on land,
snakes to live in the desert, and birds to fly.
Fishes have a segmented backbone, gills, and fins. Amphibians can live on land because
of lungs, limbs, and the ability to absorb oxygen through their skin. Snakes can live in the
desert because they have scales, internal fertilization, amniote eggs, well-developed
lungs, sturdy skeletons, and enhanced circulation. Birds can fly because they have
feathers, tapered bodies, hollow bones, a four-chambered heart, and efficient lungs.
13. How does the changing placement of birds in the vertebrate family tree illustrate the
scientific process? Why does this type of research matter?
Scientific hypotheses and theories change to incorporate new data; the emerging evidence
of the similarities between reptiles and birds resulted in changes in the phylogenetic tree.
Refining these relationships helps us to better understand the evolutionary process and
the history of life.
14. How are fishes, amphibians, nonavian reptiles, birds, and mammals important to
humans? How are they important in ecosystems?
Other vertebrates are important to humans because they provide a source of enjoyment,
food, and commercial products. They also play essential roles in ecosystem function such
as indicators of environmental quality or predators that control prey populations.
15. Among plants and vertebrates, some species are well-adapted to dry land, whereas
others require moisture to reproduce. Compare and contrast the adaptations in plants and
vertebrates that have allowed each group to breed in increasingly dry habitats.
Both groups of organisms evolved reproductive mechanisms that do not depend upon
water for the dispersal of sperm: pollen in plants, and internal fertilization in terrestrial
vertebrates. Both groups also produce structures that protect and nourish the developing
embryo: fruits and seeds in plants, and the amniotic egg in reptiles and mammals.
16. Give three examples of interactions between animals classified in different phyla.
Mosquitoes (arthropods) can transmit heartworms (nematodes) to dogs (chordates).
Snails (mollusks) can serve as an intermediate host for a type of liver fluke (flatworm)
that infects mammals. Sea stars (echinoderms) can prey on clams (mollusks).
17. Explain how a sessile or slow-moving lifestyle, such as that of sponges, sea
cucumbers, and tunicates, might select for bright colors and an arsenal of toxic chemicals.
A soft-bodied sessile or slow-moving animal is extremely vulnerable to predators. Those
that produce toxins and warning coloration are likely to have greater reproductive success
than those that do not.
18. Biologists have speculated that the first feathered dinosaurs may have used feathers in
mating displays, to conserve body heat, or in flight. How might you use fossils, living
species, or both to test each hypothesis?
You could begin with detailed observations of the fossilized feather impressions in the
earliest feathered dinosaurs. You could measure the dimensions of the feather, the
number of feathers, or the percentage of body area covered by feathers. Next you could
observe the same variables in existing species that use feathers for mating displays,
conserving body heat, and flight. Initial conclusions could be based on the modern
species with variables most similar to the earliest feathered dinosaurs.
19. Invasive animal species are disrupting ecosystems around the world. Search the
Internet for a list of invasive animal species. Which phyla are represented in the list?
What harm do invasive species do? How important is it to try to eradicate invasive
species?
[Answers will vary.]
20. What can scientists learn by comparing the fossilized skeletons of extinct primates
with the bones of modern species?
The fossilized skeletons can yield important information regarding locomotion, posture,
diet, and brain size.
21. The foramen magnum in Australopithecus africanus is closer to the front of the skull
than in gorillas. What does this observation indicate about A. africanus?
This shows that the spinal cord was oriented under the head like it is in humans and other
bipedal species.
22. Use the Internet to learn about National Geographic’s Genographic Project. What are
the main objectives and components of the project, and how are researchers using the
information they gather to learn more about human evolution?
The Genographic Project is trying to map the migration patterns of humans from our last
common ancestral population to today. It uses DNA samples from hundreds of thousands
of volunteers around the world and analyzes it with sophisticated software programs.
23. In what ways has culture been an important factor in human evolution?
Culture has facilitated the development of language, art, agriculture, and many other
human characteristics. The ability to transmit knowledge from generation to generation
has enabled us to learn from the past and has contributed greatly to our success as a
species.
24. At one time, several species of Homo existed at the same time. Propose at least two
hypotheses that might explain why only Homo sapiens remains.
Perhaps Homo sapiens had greater intelligence and, therefore, greater reproductive
success than the other species. Perhaps Homo sapiens interbred with the other species,
eliminating the distinctions among them. Perhaps Homo sapiens killed off the other
species.
25. How do you predict a scientist would respond to a question about whether humans
“evolved from monkeys”?
Humans did not “evolve” from monkeys. Humans do share a common ancestor with
monkeys (indeed, with all primates), but monkeys are not our closest relatives.
Chimpanzees are our closest relatives, but a chimp did not turn into a human. Instead, a
population of ancestral apes diverged into chimpanzees, humans, and other extinct and
existing lineages.
Answers to Pull It Together Questions
1. Which animals are cephalized?
Cephalized animals include annelids, roundworms, flatworms, arthropods, some
mollusks, and chordates.
2. Which animals have an incomplete digestive tract?
Cnidarians and flatworms have an incomplete digestive tract.
3. Which animals have segmented bodies?
Segmented bodies occur in annelids and arthropods. Chordates also have a segmented
body, but the divisions are much less obvious.
4. Which animals have a coelom or pseudocoelom?
Mollusks, annelids, arthropods, echinoderms, and chordates have a coelom. Nematodes
have a pseudocoelom.
5. Add tunicates, hagfishes, birds, monotremes, vertebrates, marsupials, and placental
mammals to this concept map.
All of these animals would be found under the “Chordates”. You could draw two
branches from the word “Chordates.” A branch labeled “with backbones” would include
“Vertebrates”, which in turn would lead to “Birds”, “Monotremes”, “Marsupials”, and
“Placental mammals”. A branch labeled “without backbones” would include the
“Hagfishes”. Within that vertebrate group you could draw branches for “Birds” and
“Mammals”. Within the “Mammals” you could indicate the egg-laying “Monotremes”,
pouched “Marsupials”, and “Placental Mammals” (which retain the young inside the
female’s body).
6. Which animals are aquatic and which are terrestrial? What advantages were gained by
the first organisms to leave the water?
The sponges, cnidarians, echinoderms, and fishes are all aquatic groups. The reptiles and
birds are all terrestrial. All other groups have some aquatic and some terrestrial forms.
The first organisms to leave the water were able to exploit new and more varied
resources. The first groups on land also had fewer predators until a terrestrial existence
became more widespread.
7. Other than the ones pictured, give an example of a species in each phylum.
[Answers will vary].