Mastering Concepts
20.1
1. What characteristics do all animals share?
Animals are multicellular, 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, and when did today’s major
groups of animals arise?
Animals probably originated in the oceans in Precambrian times. Today’s major groups
of animals arose during Precambrian and Cambrian times.
3. What evidence do biologists use to construct 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. What are the events of early embryonic development in an animal?
In embryonic development the diploid zygote divides repeatedly by mitosis to form a
blastula, which folds to form the gastrula with either two or three germ layers. These
germ layers develop into the animal’s tissues and organs.
5. Which animals have true tissues, and which have organs?
All animals except sponges have true tissues. All animals except sponges and cnidarians
have organs.
6. What is the difference between radial and bilateral symmetry?
In radial symmetry, many planes can divide the animal into mirror halves. In bilateral
symmetry, only one plane divides the animal into mirror halves.
7. 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.
8. What are the two main types of digestive tract?
Incomplete and complete digestive tracts are the two main kinds of digestive tracts.
9. 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.
10. How does direct development differ from indirect development?
In direct development, the young resemble the adults immediately after hatching or birth.
In indirect development, an animal goes through metamorphosis as it transforms from
larva to adult.
20.2
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. These are taken in to the
body of another sponge where they fertilize egg cells. The new sponge develops within
the body of its parent. 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.
20.3
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 jellyfish, hydra, sea anemones, and coral.
3. What is the difference between a polyp and a medusa?
A polyp is a sessile life cycle stage of a cnidarian. A medusa is the free-swimming life
cycle stage of a cnidarian.
4. How do cnidarians feed, move, and reproduce?
Feeding: Cnidarians are carnivores that feed by catching prey with their tentacles,
stinging it with cnidocytes, and stuffing it into their mouths. Movement: Some cnidarians
are sessile, remaining attached to a substrate. Medusas move by contracting muscles that
force water out of the bell. Reproduction: Sexual reproduction involves shedding sperm
and eggs into the water, where they unite to form a zygote. The zygote develops into a
polyp stage that in turn undergoes asexual reproduction to produce new medusas.
5. In what ways are cnidarians important?
Because cnidarians sting humans they are economically important to tourism. 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.
20.4
1. What features do all flatworms share?
All flatworms have bilateral symmetry and flattened bodies and incomplete digestive
tracts.
2. How does the body shape of a flatworm enhance gas exchange with the environment?
The flat body shape of a flatworm maximizes the surface area in contact with the
environment.
3. Describe the three classes of flatworms.
The three groups of flatworms are parasitic flukes, parasitic tapeworms, and free-living
flatworms.
4. How do free-living flatworms, tapeworms, and flukes eat?
Free-living flatworms are carnivores and scavengers that eat organic matter, which they
acquire through their pharynx. Parasitic flukes use the 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.
5. What types of specialized cells and tissues do flatworms have in their excretory and
nervous systems?
Flatworms have protonephridia that maintain internal water balance and excrete
nitrogenous wastes. They also have concentrations of nerve cells in their anterior ends
connected to longitudinal nerve cords. Some sensory cells detect light.
6. How do flatworms move and reproduce?
Flatworms move by contracting muscles and using cilia that allow them to glide along
secretions of mucus. Many reproduce asexually, whereas others reproduce sexually
inside their hosts.
7. 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.
20.5
1. What structures do mollusks have?
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. What are the four largest classes of mollusks, and where do they live?
The four main groups of mollusks are chitons, bivalves, gastropods, and cephalopods.
Most mollusks live in watery or moist environments, although some snails and slugs live
on land.
3. How do mollusks feed, move, excrete metabolic wastes, reproduce, 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
muscular contraction of the foot. Cephalopods move by jet propulsion. Excretion:
Mollusks have a nephridium that filters blood and produces urine. Reproduction:
Mollusks reproduce sexually, either by external or internal fertilization. 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.
4. 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.
20.6
1. What features do all annelids share?
Annelids are segmented worms with bilateral symmetry and a complete digestive tract.
2. What are the distinguishing features of the two classes of annelids, and where do
members of each group live?
The oligochaetes have a saddle-like thickening, the clitellum, near their head. Each of
their segments has few to no bristles. This group includes both terrestrial and freshwater
species. Polychaetes usually have paddle-like parapodia and many bristles on their
segments. This group contains marine organisms.
3. How do annelids feed, exchange gases, excrete metabolic wastes, move, reproduce,
and defend themselves?
Feeding: Most annelids feed by consuming organic matter. Polychaetes can be filter
feeders or predators. Gas exchange: Whereas polychaetes have gills for gas exchange,
leeches and earthworms exchange gases by diffusion through their body wall. Excretion:
Annelids excrete metabolic wastes using nephridia that filter fluid from the coelom.
Wastes are discharged to the environment through nephridiopores. Movement: Annelids
move by contracting circular and longitudinal muscles that surround the fluid-filled
coelom. Polychaetes swim with their parapodia. Reproduction: Leeches and oligochaetes
reproduce by exchanging sperm. Polychaetes have separate sexes and external
fertilization. Defense: Annelids hide from predators in burrows. Some polychaetes defend
themselves with strong jaws.
4. In what ways are annelids important?
Annelids are important because they aerate and fertilize soil. Physicians sometimes apply
leeches to remove excess blood. Oligochaetes are used for fishing bait and as soil
conditioners.
20.7
1. What features do all roundworms share?
Roundworms, also known as nematodes, are mostly small, unsegmented worms with
tapered ends and a complete digestive tract.
2. What are some examples of roundworms?
Examples of roundworms include: Ascaris, an intestinal roundworm; Trichinella,
transmitted in uncooked pork; and Wuchereria, the cause of elephantiasis.
3. Compare and contrast the roundworm body structure with those of a flatworm and an
annelid.
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 bilateral symmetry.
4. What evidence places roundworms in a clade with arthropods?
The evidence that places roundworms in a clade with arthropods includes molecular data
and the fact that both roundworms and arthropods molt periodically.
5. How do nematodes feed, excrete metabolic wastes, move, and reproduce?
Feeding: Nematodes feed on fungi, bacteria, roots, or almost any other plant part. They
also parasitize mammals by sucking blood or consuming digested food. Excretion:
Nematodes excrete metabolic wastes through interconnected cells that remove
nitrogenous wastes from the pseudocoelom and excrete them through an excretory pore.
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.
Reproduction: Nematodes reproduce sexually. Most species have separate sexes.
6. In what ways 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.
20.8
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 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 four groups of arthropods include:
- trilobites – example: trilobites (extinct)
- chelicerates – spiders, scorpions, and their relatives
- mandibulates (crustaceans) - lobsters
- mandibulates (myriapods) –centipedes and millipedes
- mandibulates (insects) - butterflies
20.9
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. Where do echinoderms live?
Echinoderms live in marine environments.
3. What is a water vascular system?
A water vascular system is a closed system of internal canals connected to suction cuplike feet.
4. What are some examples of echinoderms?
Sea urchins, sea lilies, sand dollars, sea cucumbers, sea stars, and brittle stars are
examples of echinoderms.
5. How do echinoderms eat, respire, excrete metabolic wastes, sense their environment,
move, reproduce, and defend themselves?
Feeding: Some echinoderms are predators that capture their prey with their tube feet and
then stuff it into their mouths. Sea stars pull bivalve shells apart and evert their stomach
into the bivalve’s shell. Digestive enzymes from the stomach digest the bivalve and the
starfish absorbs the digested food. Other echinoderms use their mouthparts to scrape
algae from rocks. Gas exchange: Some echinoderms, such as sea stars, respire by skin
gills. Others, such as sea cucumbers, respire with a respiratory tree. Excretion:
Echinoderms excrete metabolic wastes using their water vascular system. Senses:
Echinoderms sense their environment using their tube feet. Movement: Tube feet enable
echinoderms to move. Reproduction: Echinoderms have separate sexes and reproduce by
shedding eggs and sperm into the environment. When the gametes unite, the zygote
forms and develops into a larva, then an adult. Defense: Echinoderms defend themselves
with their spiny or spiky skin and protective plates fused into a shell. Some produce toxic
chemicals.
6. 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 starfish, harm coral reefs.
20.10
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 are craniates, and which of those are also vertebrates?
Hagfishes and vertebrates are craniates. All craniates except hagfishes are vertebrates.
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. How do the body coverings of fishes, amphibians, nonavian reptiles, birds, and
mammals differ?
The body coverings differ in their degree of protection from desiccation, as well as their
ability to help retain body heat. Fish scales are derived from bone. Amphibians have
moist skin that lacks scales. Reptile scales, bird feathers, and the fur of mammals all are
derived from keratin.
5. 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).
6. How does the number of heart chambers affect the efficiency of oxygen delivery to
body tissues?
The two-chambered heart of fishes is least efficient because oxygenated and
deoxygenated blood are mixed in the heart. The three-chambered heart of amphibians
and most reptiles is more efficient, and the four-chambered heart of mammals, birds, and
crocodiles provides the most separation of oxygen-rich and oxygen-depleted blood.
20.11
1. How do tunicates use their siphons in feeding and gas exchange?
Tunicates are filter feeders. Their siphons draw in a current of water, pass it over the
mucus-covered gill slits of the pharynx, and allow it to exit. Food particles trapped in the
mucus are moved to the digestive system. As the water moves over gill slits, O2 and
carbon dioxide are exchanged.
2. What is the relationship among tunicates, lancelets, and the vertebrate chordates?
The tunicates and lancelets most closely resemble the last common ancestor with the
vertebrate chordates.
20.12
1. How is a hagfish different from a true fish?
Hagfish have a cranium but lack a backbone.
2. How do hagfishes eat and defend themselves?
Hagfishes locate dead and near-dead animals with their tentacles and use their tongues to
scavenge food from their bodies. They defend themselves by secreting slime or by
twisting their flexible bodies.
20.13
1. What features do all fishes share?
All fish are aquatic vertebrates with gills and fins.
2. Which structures in fishes were the precursors to limbs?
Stumpy pectoral and pelvic fins were the precursors of vertebrate limbs.
3. What are the two types of jawless fishes? Which is extinct and which still exists?
The two groups of jawless fishes are the ostracoderms (extinct) and the lampreys
(existing).
4. What are the three groups of jawed fishes?
The three groups of fishes with jaws are placoderms, cartilaginous fishes, and bony
fishes.
5. How were placoderms different from ostracoderms?
Placoderms had jaws, and ostracoderms did not.
6. What are the major types of cartilaginous and bony fishes?
Sharks, rays, and skates are the major kinds of cartilaginous fishes; ray-finned fishes,
lungfishes, and lobe-finned fishes are the major kinds of bony fishes.
20.14
1. What features do all amphibians share?
Amphibians are tetrapods with lungs and skin that lacks scales. They lay their eggs in
water, although adults may be terrestrial.
2. What role does water play in amphibian gas exchange and reproduction?
Water is the medium in which amphibians exchange gases through their skin and in
which they reproduce. Even amphibians that live on land as adults reproduce in water.
3. What features distinguish the three major orders of amphibians?
Frogs and toads lack tails and necks, have wide mouths, and have adaptations that allow
leaping and hopping. Salamanders and newts have tails and four legs. Caecilians lack
limbs and resemble earthworms with backbones and jaws. Caecilians are the only
amphibians that use internal fertilization.
4. What are some reasons for the recent decline in amphibian populations worldwide?
The recent decline in amphibian populations worldwide is linked to destruction of
wetlands, infection by the chytrid fungus, over-collecting for the pet trade, and
environmental pollution.
20.15
1. What features do all reptiles share?
All reptiles have an amniotic egg, scales on some or all of their body, internal
fertilization, and well-developed lungs.
2. How do scales and the amniotic egg adapt reptiles to dry land?
Dry scales form a skin barrier that allows reptiles to conserve body moisture. The
amniotic egg surrounds the embryo in a pocket of fluid within internal membranes and is
covered by a tough shell. These two features minimize water loss and protect the
developing reptile. Unlike an amphibian egg, a reptile egg does not have to be laid in
water.
3. What features characterize each of the orders of nonavian reptiles?
The three groups are the turtles and tortoises; the lizards, snakes, and tuataras; and the
crocodilians. Turtles have vertebrae and ribs fused to the plates of their shells. Lizards
have four limbs, whereas their close relatives the snakes are limbless. Lizards also have
external ear openings and moveable eyelids, which snakes lack. The tuatara is a lizardlike reptile that lives on islands near New Zealand. Crocodilians are aquatic animals with
bodies covered with heavy scales. They also have a four-chambered heart, while other
reptile hearts have three chambers.
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.
5. What are the functions of feathers?
Feathers are adaptations that provide insulation and allow birds to fly. They also function
in mating displays.
6. What adaptations enable birds to fly?
The adaptations that enable birds to fly include a lightweight skeleton with hollow bones,
a streamlined body shape, feathers, powerful flight muscles, a four-chambered heart, and
highly efficient lungs.
20.16
1. Which characteristics define a mammal?
A mammal is endothermic and has hair or fur. Infants drink milk produced by the female.
The skull and teeth also have unique features. Mammals also have a four-chambered
heart, a diaphragm, and a well-developed brain.
2. What evidence suggests that mammals share a common ancestor with reptiles?
Similar DNA sequences, skeletal similarities, and the existence of egg-laying mammals
are characteristics that indicate a shared common ancestor between reptiles and
mammals.
3. How do monotremes, marsupials, and placental mammals differ in how they
reproduce?
Monotremes lay amniotic eggs that hatch; the hatchling crawls to its mother’s nipples,
which secrete milk. Marsupial young are born at an early stage of development; after
birth, they crawl to the pouch (marsupium), where they attach to a nipple and continue to
develop. Placental mammals are born at a much later stage of development. Within the
mother’s uterus they are nourished by a placenta. After they are born, the young are fed
milk from their mother’s mammary glands.
20.17
1. 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.
2. How might the loss of hind limbs enhance the reproductive fitness of a burrowing
animal such as Najash?
Najash would be able to burrow more efficiently, enabling it to conserve energy and find
food more easily.
Write It Out
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 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 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 gastropod 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: radial and bilateral symmetry; blastula and gastrula; direct and
indirect development; complete and incomplete digestive tract; 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) Animals that undergo direct development resemble adults at hatching or birth. An
animal with indirect development may spend part of its life as a larva, which is an
immature stage that does not resemble the adult. These larvae eventually undergo
metamorphosis, in which they change greatly as they mature into adults. Caterpillars and
tadpoles are larvae.
(d) 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. Compare and contrast feeding in a sponge and a sea urchin.
Sponges feed by filtering water through their porous bodies, whereas sea urchins scrape
algae from a substrate such as rocks.
6. 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.
7. Make a chart showing the characteristics of each subphylum of arthropods. Suppose
you could examine a fossilized arthropod; use your chart to describe how you would
assign the fossil to a subphylum.
Chelicerates—two body regions, four or more pairs of legs, chelicerae
Myriapods—many segments, one pair of antennae, many paired appendages
Crustaceans—branched appendages, two pairs of antennae, two or three body regions
Insects—three body regions, three pairs of legs, one pair of antennae
To assign a fossil to a subphylum you would start by counting segments, body regions,
and antennae pairs. If there are more than three segments, the fossil is a myriapod. If
there are three body regions with two pairs of antennae, it is a crustacean; if it has one
pair of antennae and six legs, it is an insect. If there are two body regions with no
antennae, it is a chelicerate; if there are two pairs of antennae, it is a crustacean.
8. 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.
9. What are the four distinguishing characteristics of chordates?
Chordates have a notochord, a dorsal hollow nerve cord, pharyngeal gill slits, and a
postanal tail.
10. How do tunicates and lancelets differ from fishes and tetrapods?
Tunicates and lancelets lack a cranium, vertebrae, jaws, and a skeleton.
11. 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 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.
12. If you found an eel-like animal at the beach, 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.
13. List five adaptations that enable
a. fishes to live in water.
b. amphibians to live on land.
c. snakes to live in the desert.
d. birds to fly.
(a) Segmented backbone, jaws, complex brain, gills, and fins. (b) Lungs, paired limbs,
three-chambered heart, denser skeleton, and improved sight and hearing. (c) Scales,
internal fertilization, amniote eggs, well-developed lungs, and enhanced circulation. (d)
Feathers, tapered bodies, hollow bones, four-chambered heart, and efficient lungs.
14. Tunicates and lancelets did not leave fossil evidence because their bodies lack hard
parts. The hair and mammary glands that distinguish mammals are not hard either, yet
mammals have a rich fossil record. Explain this difference.
Mammals possess hard parts such as teeth and bones, and these hard parts have features
that are unique to mammals.
15. How are a fish’s and a bird’s skeletons similar in structure and function?
Fishes and birds have supportive skeletons of bone and cartilage. Their skeletons include
a cranium, vertebrae, and paired appendages. In both types of animals, muscles attach to
the skeleton to provide movement.
16. Fishes are adapted to life in water, and tetrapods to life on land. Cite two criteria for
assessing which group has been more successful.
One measure would be the number of species in each group; another would be the
number of individuals in each group.
17. Summarize the evidence for the idea that birds are reptiles. How does the changing
placement of birds in the vertebrate family tree illustrate the scientific process? Why does
this type of research matter?
Skeletal similarities, the production of shelled eggs, feathers, and new evidence from
DNA all support the idea that birds are reptiles. 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. Understanding evolutionary
relationships helps to determine better models for studying genes and helps us to better
understand the evolutionary process.
18. If you found a fossil and were not sure whether it was from a reptile or a mammal,
how might you tell the difference?
Mammals have three middle ear bones, whereas reptiles only have two; mammals have a
variety of teeth, whereas reptile teeth follow one pattern; the jaw of mammals is a single
piece; mammals have a single hole on each side of the skull behind the orbits, whereas
most reptiles (except turtles) have two.
19. 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. Also these animals provide humans with a better
understanding of life on Earth. They also play essential roles in ecosystem function such
as indicators of environmental quality or predators that control prey populations.
20. 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.
21. 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).
Sea stars (echinoderms) can prey on clams (mollusks).
22. 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.
23. 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?
One possible answer: 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.
24. Search the Internet to answer this question: Other than Caenorhabditis elegans,
Drosophila melanogaster, and Mus musculus, what are some examples of invertebrate or
vertebrate animals that have contributed to scientists’ knowledge of general biology and
animal biology? What other genomes of animals have scientists sequenced, and what are
some resulting discoveries?
[Answers will vary].
25. Invasive animal species are disrupting ecosystems around the world. Search the
Internet for a list of invasive animal species. What 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].
Pull It Together
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. Draw a concept map that summarizes the chordates, including both invertebrates and
vertebrates.
One possible way to construct the concept map would be to draw two branches from the
word “Chordates.” A branch labeled “with backbones” would include vertebrates, which
in turn would lead to fishes, amphibians, reptiles, and mammals. A branch labeled
“without backbones” would include invertebrates, which in turn would lead to tunicates,
lancelets, and hagfishes.