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KEY
.... Figure 33.1 What function do the red whorls of this
organism have?
CONCEPTS
33.1 Sponges are basal animals that lack true tissues
33.2 Cnidarians are an ancient phylum of
eumetazoans
33.3 lophotrocholoans, a clade identified by
the surrounding water. The tentacles emerge from a tube of
calcium carbonate secreted by the worm that protects and supports its soft body. Light-sensitive structures on the tentacles
can detect the shadow cast by a predator, triggering the worm
to contract muscles that rapidly withdraw the tentacles into
the tube.
Christmas tree worms are invertebrates-animals that
lack a backbone. Invertebrates account for 95% of known animal species. They occupy almost every habitat on Earth, from
the scalding water released by deep-sea hydrothermal vents to
the rocky, frozen ground of Antarctica. Adaptation to these
varied environments has produced an immense diversity of
forms, ranging from a species consisting simply ofa flat bilayer
of cells to other species with silk-spinning glands, pivoting
spines, dozens of jointed legs, or tentacles covered with suction cups, just to list a few.
In this chapter, we'll take a tour ofthe invertebrate world, using the phylogenetic tree in Figure 33.2 as a guide. Figure 33.3
on the next three pages, surveys 23 invertebrate phyla. As representatives of invertebrate diversity, many of those phyla are
explored in more detail in the rest of this chapter.
molecular data, have the widest range of
animal body forms
33.4 Ecdysoloans are the most species-rich animal
group
33.5 Echinoderms and chordates are deuterostomes
t first glance, you might mistake the organism shown
A
in Figure 33.1 for some type of seaweed. But this colorful inhabitant of coral reefs is actually an animal,
not an alga. Specifically, it is a species of segmented worm
known as a Christmas tree worm (Spirobranchus giganteus).
The two tree-shaped whorls are tentacles, which the worm
uses for gas exchange and for filtering small food particles from
... Figure 33.2 Review of animal
phylogeny. Except for sponges (basal
animals in phyla Calcarea and Silicea) and a
few other groups, all animals have tissues and
are in the clade Eumetazoa Most animals are
In the diverse clade Bilatena
ANCESTRAL
PROTIST
Cnidaria
LOPhWOChO"'~
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Common
ancestor of
all animals
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JIJ.
....
Calcarea
andSiliceaW
'cd,,,,,,, •
Deuterostomia
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• Figure 33,3
Exploring Invertebrate Diversity
Kingdom Animalia encompasses 1.3 million known species, and estimates oftotal species range far higher.
Of the 23 phyla surveyed here, those illustrated with smaller-sized "preview" photographs are discussed
more fully in this chapter or another.
Animals in these phyla are informally
called "sponges." Sponges are sessile
animals that lack true tissues. They live as
suspension feeders, trapping particles that
pass through the internal channels of their
bodies (see Concept 33.1).
A sponge
Cnidaria (10,000 species)
Cnidarians include corals, jellies, and
hydras. These animals have a diplobastic,
radially symmetrical body plan that
includes a gastrovascular cavity with a
single opening that serves as both mouth
and anus {see Concept 33.2).
The single known species in
this phylum, Trichoplax
adhaercns, does not even look
like an animal. It consists of a
few thousand cells arranged
in a double-layered plate.
Trichoplax can reproduce by
dividing into two individuals
or by budding off many
multicellular individuals.
[-;;" ~.
A plawzoan (lM)
V(
A jelly
Acoel flatworms (lM)
Acoel flam'orms have a simple nervous
system and a saclike gut, and thus had
been placed in ph}~um Platyhelminthes.
Molecular analyses, however, indicate
Acoela is a separate lineage that diverged
before the three main bilaterian clades (see
Concept 32.4).
A denophore, or comb jelly
Ctenophores (comb jellies) are
diploblastic and radially
symmetrical like cnidarians,
suggesting that both phyla
diverged from other animals
very early. Comb jellies make up
much ofthe ocean's plankton.
They have many distinctive
traits, including eight "combs"
ofcilia that propel the animals
through the water. \X'hen a small
animal contacts the tentacles of
some comb jellies, spedalized cells burst open, covering the prey
with sticky threads.
LOPHOTROCHOZOANS
~helminthes (20,000 species)
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A marine flatworm
Flatworms (including tapeworms,
planarians, and flukes) have bilateral
symmetry and a central nervous system
that processes information from sensory
structures. They have no body cavity or
organs for circulation (see Concept 33.3).
Despite their microscopic size, rotifers
have specialized organ systems, including
an alimentary canal {digestive tract). They
feed on microorganisms suspended in
water {see Concept 33.3).
A rotifer (LM)
Brachiopods, or lamp shells, may be easily
mistaken for clams or other molluscs.
However, most brachiopods have a unique
stalk that anchors them to their substrate
(see Concept 33.3).
Ectoprocts {also known as bryozoans) live
as sessile colonies and are covered by a
tough exoskeleton (see Concept 33.3).
Ectoprocts
A brachiopod
Continued on next page
CHAPTER THIRTY·THREE
Invertebrates
667
• Figure 33.. (continued)
••
• Invertebrate Diversity
Acanthocephalans (from the
The only known species of
cycliophoran, Symbion pandora,
was discovered in 1995 on the
Greek acanthias, prickly, and
cephalo, head) are called spinyheaded worms because of the
curved hooks on the proboscis at
the anterior end of their body. All
species are parasites. Some
acanthocephalans manipulate
their intermediate hosts {generally
An acanthocephalan (lM)
arthropods) in ways that increase
their chances of reaching their final hosts (generally vertebrates).
For example, acanthocephalans that infect New Zealand mud
(nibs force their hosts to move to more visible areas on the beach,
where the crabs are more likely to be eaten by birds, the worms'
final hosts.
mouthparts ofa lobster. This
tiny, vase-shaped creature has a
A (ydiophoran (colorized SEM)
unique body plan and a
particularly bizarre life cycle.
Males impregnate females that
are still developing in their
mothers' bodies. The fertilized
females then escape, settle
elsewhere on the lobster, and release their offspring. The offspring
apparently leave that lobster and search for another one to which
they attach.
I I I
Proboscis worms, or ribbon
worms, swim through water or
Molluscs (including snails, clams, squids,
and octopuses) have a soft body that in
burrow in sand, extending a
unique proboscis to capture
prey. Like flatworms, they lack
a true coelom. However, unlike
flatworms, nemerteans have an
many species is protected by a hard shell
(see Concept 33.3).
alimentary canal and a closed
circulatory system in which the
blood is contained in vessels
and hence is distinct from fluid
•.
ies)
~~J
An octopus
Annelids, or segmented worms, are
distinguished from other worms by their
body segmentation. Earthworms are the
most familiar annelids, but the phylum
also includes marine and freshwater
species (see Concept 33.3).
A ribbon worm
in the body cavity.
A marine annelid
ECDYSOZOA
Loriciferans (from the Latin lorica,
corset, andferre, to bear) are tiny
animals that inhabit the deep-sea
bottom. A loriciferan can telescope
it.s head, neck, and thorax: in and
out of the lorica, a pocket formed
by six plates surrounding the
abdomen. Though the natural
history ofloricifenms is mostly a
mystery, at least some species
likely eat bacteria.
668
UNIT fiVE
A lorkiferan (LM)
The Evolutionary History of Biological Diversity
A priapulan
Priapulans are worms with a
large, rounded proboscis at the
anterior end. (They are named
after Priapos, the Greek god of
fertility, who was symbolized by
a giant penis.) Ranging from
0.5 mm to 20 em in length, most
species burrow through seafloor
sediments. Fossil evidence
suggest.s that priapulans were
among the major predators
during the Cambrian period.
Tardigrades (colorized SEM)
Tardigrades (from the Latin
tardus, slow, and grOOus, step) are
sometimes caUed water bears for
their rounded shape, stubby
appendages, and lumbering,
bearlike gait. Most tardigrades are
less than 0.5 mm in length. Some
live in oceans or fresh water, while
others live on plants or animals.
AJ;, many as 2 million tardigrades
can be found on a square meter of
Onychophorans, also called
velvet worms, originated during
the Cambrian explosion (see
Chapter 32). Originally, they
thrived in the ocean, but at
some point they succeeded in
colonizing land. Today they live
only in humid forests.
Onychophorans have fleshy
antennae and several dozen
pairs of saclike legs.
An onychophoran
moss. Harsh conditions may cause tardigmdes to enter a state of
dormancy; while dormant, they can survive temperatures as low as
- 272'C, dose to absolute zero!
I I I
•.
Roundworms are enormously abundant
and diverse in the soil and in aquatic
habitats; many species parasitize plants
and animals. The most distinctive feature
of roundworms is a tough cuticle that
coats the body (see Concept 33.4).
The vast majority of known animal species,
including insects, crustaceans, and
arachnids, are arthropods. All arthropods
have a segmented exoskeleton and jointed
appendages (see Concept 33.4).
A roundworm
A scorpion
(an arachnid)
DEUTER05TOMIA
An acorn worm
like echinoderms and chordates,
hemichordates are members of the
deuterostome clade (see Chapter 32).
Hemichordates share some traits with
other chordates, such as gill slits and a
dorsal nerve cord. The largest group of
hemichordates are the enteropneusts, or
acorn worms. Acorn worms are marine
and generally live buried in mud or under
rocks; they may grow to more than 2 m
in length.
Chordata (52,000 species)
A sea urchin
Echinoderms, such as sand dollars, sea stars,
and sea urchins, are aquatic animals in the
deuterostome clade that are bilaterally
symmetrical as larvae but not as adults.
They move and feed by using a network of
internal canals to pump water to different
parts of their body (see Concept 33.5).
More than 90% of all known chordate species
have backbones (and thus are vertebrates).
However, the phylum Chordata also includes
three groups of invertebrates: lancelets,
tunicates, and hagfishes. See Chapter 34 for a
full discussion of this phylum.
t
A tunicate
CHAPTER THIRTY·THREE
Invertebrates
669
r;;:~:7 ?r=·~sal animals that
lack true tissues
Animals in the phyla Calcarea
and Silicea are known infor·
mally as "sponges:' (Previously,
all sponges were placed in a
lI..iIIiiii. Dl!IJterostomia
single phylum, Porifera, now
thought to be paraphyletic based on molecular data.) Among
the simplest of animals, sponges are sedentary and were mistaken for plants by the ancient Greeks. They range in size from
a few millimeters to a few meters and live in both fresh and maCalcarea and Silicea
C",dafla
Lophotrochozw
Ecdywzw
rine waters. Sponges are suspension fceders: They capture
food particles suspended in the water that passes through their
body, which in some species resembles a sac perforated with
pores. Water is drawn through the pores into a central cavity,
the spongococ1, and then flows out of the sponge through a
larger opening called the osculum (Figure 33.4). More complex sponges have folded body walls, and many contain
branched water canals and several oscula.
Sponges are basal animals; that is, they represent a lineage
that originates near the root of the phylogenetic tree of animals. Unlike nearly all other animals, sponges lack true tissues,
groups of similar cells that act as a functional unit and are isolated from other tissues by membranous layers. However, the
sponge body does contain several different cell types. For example, lining the interior of the spongocoel are flagellated
choanocytes, or collar cells (named for the membranous collar around the base of the flagellum). The similarity between
choanocytes and the cells of choanoflagellates supports molecular evidence suggesting that animals evolved from a
choanoflagellate-like ancestor (see Figure 32.3).
The body ofa sponge consists oftwo layers ofcells separated
by a gelatinous region called the mesohyl. Wandering through
the mesohyl are cells called amoebocytes, named for their use
ofpseudopodia. Amoebocytes have many functions. They take
up food from the water and from choanocytes, digest it, and
carry nutrients to other cells. They also manufacture tough
skeletal fibers within the mesohy!. In some groups of sponges,
these fibers are sharp spicules made from calcium carbonate or
silica. Other sponges produce more flexible fibers composed of
a protein called spongin; you may have seen these pliant skeletons being sold as fluffy brown bath sponges.
Most sponges are hermaphrodites, meaning that each individual functions as both male and female in sexual reproduction by producing sperm and eggs. Almost all sponges
exhibit sequential hermaphroditism, functioning first as one
sex and then as the other.
o
Choanocytes. The spongocoel is lined
with flagellated cells called choanocytes.
By beating flagella, the choanocytes create
a current that draws water in through the
pores and out through the osculum.
L
\ • Coli"
/~'
Spongocoel. Water ~
passing through pores
enters a cavity called the
spongocoel.
oepidermis
Pores. Water enters the
through pores
formed by doughnutshaped cells that span
the body wall.
olayerEpidermis.
The outer
consists of tightly
packed epidermal cells.
othisMesohyl.
The wall of
sponge consists of
two layers of cells separated
by a gelatinous matrix. the
mesohyl ("middle matter
r
r--s:
r
Phagocytosts of
food particles
I
A
Water /"
\
•
flow
/"i.~
/'
... Figure 33.4 Anatomy of a sponge.
fiVE
The movement of a choanocyte's
flagellum also draws water through its
collar of fingerlike projections. Food
particles are trapped in the mucus
coating the projections, engulfed by
phagocytosis, and either digested or
transferred to amoebocytes.
~
) •
UNtT
Amoebocyte
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W
670
Choanocyte
Osculum
Azure vase sponge (Callyspongia
plicifera)
o
Flagellum
The Evolutionary History of Biological Diversity
1
fI Amoebocytes. These cells can
transport nutrients to other cells of
the sponge body, produce materials
for skeletal fibers (spicules), or
become any type of sponge cell
as needed.
Sponge gametes arise from choanocytes or amoebocytes.
Eggs reside in the mesohyl, but sperm are carried out of the
sponge by the water current. Cross-fertilization results from
some of the sperm being drawn into neighboring individuals.
Fertilization occurs in the mesohyl, where the zygotes develop
into flagellated, swimming larvae that disperse from the parent sponge. After settling on a suitable substrate, a larva develops into a sessile adult.
Sponges produce a variety of antibiotics and other defensive compounds. Researchers are now isolating these compounds, which hold promise for fighting human diseases. For
example, a compound called cribrostatin isolated from marine
sponges can kill penicillin-resistant strains of the bacterium
Streptococcus. Other sponge-derived compounds are being
tested as possible anticancer agents.
CONCEPT
CHECK
Polyp
Body
stalk
.. Figure 33.5 Polyp and medusa forms of cnidarians. The
body wall of a cnidarian has two layers of cells: an outer layer of
epidermis (from ectoderm) and an inner layer of gastrodermiS (from
endoderm), Digestion begins in the gastrovascular cavity and is
completed inside focxl vacuoles in the gastrodermal cells, Flagella on
the gastrodermal cells keep the contents of the gastrovascular cavity
agitated and help distribute nutrients. Sandwiched between the
epidermis and gastrodermis is a gelatinous layer, the mesoglea,
33.1
I. Describe how sponges feed.
2. -',1@il IA Some molecular evidence suggests that
the sister group of animals is not the choanoflagellates. but rather a group of parasitic protists. Mesomycetozoa. Given that these parasites lack collar
cells, can this hypothesis be correct? Explain.
For suggested answers, see Appendix A.
All animals except sponges
and a few other groups belong
Lophotrochozoa
to the clade Eumetazoa, aniEcdysozw
mals with true tissues (see
Deuterostom'a
Chapter 32). One of the oldest
lineages in this clade is the phylum Cnidaria. Cnidarians have
diversified into a wide range of sessile and motile forms, including hydras, corals, and jellies (commonly called "jellyfish").
Yet most cnidarians still exhibit the relatively simple, diploblastic, radial body plan that existed some 570 million years ago.
The basic body plan ofa cnidarian is a sac with a central digestive compartment, the gastrovascular cavity. A single
opening to this cavity functions as both mouth and anus. There
are m'o variations on this body plan: the sessile polyp and the
motile medusa (Figure 33.5). Polyps are cylindrical forms that
adhere to the substrate by the aboral end of their body (the end
opposite the mouth) and extend their tentacles, waiting for
prey. Examples of the polyp form include hydras and sea
anemones. A medusa is a flattened, mouth-down version of
the polyp. It moves freely in the water by a combination of passive drifting and contractions of its bell-shaped body. Medusae
include free-swimming jellies. The tentacles of a jelly dangle
from the oral surface, which points downward. Some cnidarians exist only as polyps or only as medusae; others have both a
polyp stage and a medusa stage in their life cycle.
Cnidarians are carnivores that often use tentacles arranged
in a ring around their mouth to capture prey and push the food
into their gastrovascular cavity, where digestion begins. Any
undigested remains are expelled through the mouth/anus.
The tentacles are armed with batteries of cnidocytcs, cells
unique to cnidarians that function in defense and prey capture
(Figure 33,6). Cnidocytes contain cnidae (from the Greek
cnide, nettle), capsule-like organelles that are capable of exploding outward and that give phylum Cnidaria its name. Specialized cnidae called nematocysts contain a stinging thread
Calcarea and Silicea
Cnidaria
~
Tentacle
Cuticle
of prey
Thread
Thread
discharges
•
I
Cnldocyte
Thread
(coiled)
.. Figure 33.6 A cnidocyte of a hydra. This type of cnidocyte
contains a stinging capsule, the nematocyst, which contains a coiled
thread, When a '"trigger" is stimulated by touch or by certain chemicals,
the thread ShOOIS out, puncturing and injecting poison into prey,
CHAPTER THIRTY·THREE
Invertebrates
671
that can penetrate the body wall of the cnidarian's prey. Other
kinds of cnidae have long threads that stick to or entangle
small prey that bump into the cnidarian's tentacles.
Contractile tissues and nerves occur in their simplest forms
in cnidarians. Cells of the epidermis (outer layer) and gastro~
dermis (inner layer) have bundles of microfilaments arranged
into contractile fibers (see Chapter 6). The gastrovascular cavity acts as a hydrostatic skeleton against which the contractile
cells can work. When a cnidarian closes its mouth, the volume
of the cavity is fixed, and contraction of selected cells causes
the animal to change shape. Movements are coordinated by a
nerve net. Cnidarians have no brain, and the noncentralized
nerve net is associated with sensory structures that are distributed radially around the body. Thus, the animal can detect
and respond to stimuli from all dire<tions.
As summarized in Table 33.1, the phylum Cnidaria is di~
vided into four major classes: Hydrozoa, Scyphozoa, Cubozoa,
and Anthozoa (Figure 33.7).
T"'33.1 Classes of Phylum Cnidaria
Class and Examples
Main Characteristics
Hydrozoa (Portuguese
man-of-wars, hydras,
Obelia, some corals; see
Figures 33.7a and 33.8)
Scyphozoa (jellies, sea
nettles; see Figure 33.7b)
Most marine, a few freshwater;
both polyp and medusa stages
in most species; polyp stage
often colonial
All marine; polyp stage absent
or reduced; free-swimming;
medusae up to 2 m in diameter
All marine; box-shaped
medusae; complex eyes; potent
venom
Cubozoa (box jellies, sea
wasps; see Figure 33.7c)
Anthozoa (sea anemones,
most corals, sea fans; see
Figure 33.7d)
(3) These
..
All marine; medusa stage
completely absent; most sessile;
many colonial
~
colonial polyps are members of
class Hydrozoa.
UNIT fiVE
Most hydrozoans alternate between polyp and medusa forms,
as in the life cycle of Obelia (Figure 33.8). The polyp stage, a
colony of interconnected polyps in the case of Obelia, is more
conspicuous than the medusa. Hydras, among the fewcnidarians found in fresh water, are unusual hydrowans in that they exist only in polyp form. \Vhen environmental conditions are
favorable, a hydra reproduces asexually by budding, fonning outgrowths that pinch off from the parent and live independently
(see Figure 13.2). \Vhen conditions deteriorate, hydras can reproduce sexually, forming resistant zygotes that remain dormant until conditions improve.
Scyphozoans
The medusa generally is the predominant stage in the life cycle
ofthe class Scyphozoa. The medusae of mostspe<ies live among
the plankton as jellies. Most coastal scyphozoans go through a
stage as small polyps during their life cycle, whereas those that
live in the open ocean generally lack the polyp stage altogether.
(ubozoans
As their name (which means "cube animals") suggests, cubozoans have a box-shaped medusa stage. Cubozoans can be distinguished from scyphozoans in other significant ways, such
as having complex eyes embedded in the fringe of their
medusae. They are comparatively strong swimmers and as a
result are less likely to be stranded on shore. Cubozoans,
which generally live in tropical oceans, are often equipped
with highly toxic cnidocytes. The sea wasp (Chironexfleckeri),
a cubozoan that lives off the coast of northern Australia, is
one of the deadliest organisms: Its sting causes intense pain
and can lead to respiratory failure, cardiac arrest, and death
within minutes. The poison of sea wasps isn't universally
fatal, however; sea turtles have defenses against it, allowing
them to eat the cubozoan in great quantities.
(b) Many jellies (class Scyphozoa) Cc) The sea wasp (Chironex
are bioluminescent. Food
fleekefl) is a member of class
captured by nematocyst(ubozoa. Its poison, which
bearmg tentacles is trans·
can subdue fish and other
ferred to specialized oral arms
large prey. is more potent
(that lack nematocysts) for
than cobra venom.
transport to the mouth .
.&. Figure 33.7 Cnidarians.
672
Hydrozoans
The Evolutionary History of Biological Diversity
Cd) Sea anemones and other
members of class Anthozoa
exist only as polyps.
epolyps,
Some of the colony's
equipped with
OMedusae swim
off, grow, and
reproduce sexually.
Gather polyps, specialized
for reproduction, lack
tentacles and produce tiny
medusae by asexual budding.
tentacles, are specialized
for feeding.
o
Acolony of
interconnected
polyps (inset,
LM) results
from asexual
reproduction
by budding.
j;
1--------1>.
Medusa
:r~erm
SEXUAL
Egg
REPRODUCTION
Portion of
a colony
of polyps
ASEXUAL
REPRODUCTION
(BUDDING)
De~eloping
polyp
I~
K,y
oandThedevelops
planula eventually settles
into a new polyp.
"The zygote de~elops into a
solid ciliated larva called a planula.
•
Haploid (nJ
Diploid (2n)
... Figure :n.8 The life cycle of the hydrozoan Obelia. The polyp stage is asewal, and the
medusa stage is sexual; these two stages alternate, one producing the other, Do not confuse this
with the alternation of generations that occurs in plants and some algae: In Obelia, both the polyp
and the medusa are diploid organisms, Typical of animals. only the single-celled gametes are haploid
By contrast. plants ha~e a multicellular haploid generation and a multicellular diploid generation
Suppose that Obelia medusae and gametes were haploid. but all other stages were
diploid. What aspects of its actual life cycle would have to change for this to occur?
-'moil.
Anthozoans
Sea anemones (see Figure 33.7d) and corals belong to the class
Anthozoa (meaning "flower animals n ). These cnidarians occur
only as polyps. Corals live as solitary or colonial forms, and
many species secrete a hard external skeleton of calcium carbonate. Each polyp generation builds on the skeletal remains of
earlier generations, constructing "rocksn with shapes characteristic of their species. It is these skeletons that we usually
think of as coral.
Coral reefs are to tropical seas what rain forests are to tropical land areas: They provide habitat for a wealth ofother species.
Unfortunately, like rain forests, coral reefs are being destroyed
at an alarming rate by human activity. Pollution and overfishing
are major threats, and global warming (see Chapter 55) may also
be contributing to their demise by raising seawater temperatures above the narrow range in which corals thrive.
CONCEPT
CHECK
33.2
1. Compare and contrast the polyp and medusa forms
of cnidarians.
2. Describe the structure and function of the stinging
cells for which cnidarians are named.
3. -','!lO"g If the common ancestor of cnidarians
were an open-ocean jelly, what might you infer about
evolutionary trends in the relative importance of the
polyp and medusa stages?
For suggested answers, see Appendix A.
CHAPTER TH'RTY·THREE
Invertebrates
673
r~:;:::;c~:Z~~ns, a clade identified
by molecular data, have the widest
range of animal body forms
C~k~re~ ~nd Slll(e~
Crud~ri~
lophotro<hozoa
Ecdys.olO<l
Dl'uterostomia
The vast majority of animal
species belong to the dade
Bilateria, whose members ex-
T." 33.2
Classes of Phylum Platyhelminthes
Class and Examples
Main Characteristics
Turbellaria (mostly free· living
flatworms, such as Dugesia:
see Figures 33.9 and 33.10)
Most marine, some freshwater, a few terrestrial;
predators and scavengers;
body surface ciliated
Monogenea (monogeneans)
Marine and freshwater para·
sites; most infect external
SurfdCes of fishes; life history
simple; ciliated larva starts
infection on host
Trematoda [trematodes, also
calle<! flukes; see Figure 33.11)
Parasites, mostly of verte·
brates; two suckers attach to
host; most life cycles include
intermediate and final hosts
Cestoda [tapeworms;
see Figure 33.12)
Parasites of vertebrates;
scolex attaches to host;
proglottids produce eggs and
break off after fertilization; no
head or digestive system; life
cycle with one or more intermediate hosts
hibit bilateral symmetry and
triploblastic development (see
Chapter 32). Most bilaterians are also coelomates. \Vhile the se·
quence ofbilaterian evolution is a subject of active investigation,
most researchers think that the most recent common ancestor of
Hving bilaterians probably existed in the late Proterozoic eon
(about 575 million years ago). Most major groups of bilaterians
first appeared in the fossil record during the Cambrian explosion.
As you read in Chapter 32, molecular evidence suggests that
there are three major clades of bilaterally symmetrical animals:
Lophotrochozoa, Ecdysozoa, and Deuterostomia. This section
will focus on the first of these clades, the lophotrochozoans.
Concepts 33.4 and 33.5 will explore the other two clades.
Although the clade Lophotrochozoa was identified by molecular data, its name comes from features found in some ofits mem·
bers. Some lophotrochozoans develop a structure called a
lophophore, acrown ofciliated tentacles that functions in feeding,
while others go through a distinctive stage called the trochophore
larva (see Figure 3213). Other members ofthe group have neither
of these features. Few other unique morphological features are
widely shared within the group-in fact, the lophotrochozoans
are the most diverse bilaterian clade in terms of body plan. This
diversity in form is reflected in the number of phyla classified in
the group: Lophotrochozoo includes about 18 phyla, more than
twice the number in any other clade ofbilaterians.
Well now introduce six lophotrochozoan phyla: the flatworms, rotifers, ectoprocts, brachiopods, moUuscs, and annelids.
exchange and the elimination of nitrogenous waste (ammonia)
can occur by diffusion across the body surface. Flatworms have
no organs spedalized for gas exchange, and their relatively sim·
pIe excretory apparatus functions mainly to maintain osmotic
balance with their surroundings. This apparatus consists of
protoncphridia, networks of tubules with ciliated structures
called flame bulbs that pull fluid through branched ducts opening to the outside (see Figure 44.11). Most flarn'orms have a gastrovascular cavity with only one opening. Though flatworms
lack a circulatory system, the fine branches of the gastrovascular cavity distribute food directly to the animal's cells.
Flahvorms are divided into four classes (Table 33.2); Turbellaria (mostly free-living flarn'OTms), Monogenea (monogeneans),
Trematoda (trematodes, or flukes), and Cestoda (tapeworms).
Flatworms
Turbellarians
Flatworms (phylum Platyhelminthes) live in marine, freshwater, and damp terrestrial habitats. In addition to free· living
forms, flarn'orms include many parasitic species, such as
flukes and tapeworms. Flatworms are so named because they
have thin bodies that are flattened dorsoventrally (between
the dorsal and ventral surfaces); platyhelminth means "flat
worm." (Note that worm is not a formal taxonomic name but
a general term for animals with long, thin bodies.) The smallest flatworms are nearly microscopic free-living species, while
some tapeworms are more than 20 m long.
Although flatworms undergo triploblastic development,
they are acoelomates (animals that lack a body cavity). Their flat
shape places all their cells close to water in the surrounding environmentor in their gut. Because ofthis proximity to water, gas
Turbellarians are nearly all free-living and mostly marine
(Figure 33.9). The best-known freshwater turbellarians are
members ofthe genus Dugesia, commonly called planarians.
Abundant in unpolluted ponds and streams, planarians prey
on smaller animals or feed on dead animals. They move by using cilia on their ventral surface, gliding along a film of mucus
they secrete. Some other turbellarians also use their muscles
to swim through water with an undulating motion.
A planarian's head is equipped with a pair of light·sensitive
eyespots and lateral flaps that function mainly to detect specific
chemicals. The planarian nervous system is more complex and
centralized than the nerve nets of cnidarians (Figure 33.10).
Experiments have shown that planarians can learn to modify
their responses to stimuli.
674
UNIT fiVE
The Evolutionary History of Biological Diversity
... Figure 33.10 Anatomy of
a planarian. a turbellarian.
Pharynx. The mouth is at the
tip of a muscular pharynK.
Digestive juices are spilled
onto prey, and the pharym
sucks small pieces of food
into the gastrovascular cavity,
where digestion continues.
Digestion is completed within
the cells lining the gastrovascular cavity, which has many
fine subbranches that provide
an e>rt.ensive surface area.
Undigested wastes
are egested
through the mouth.
Gastrovascular
caVity
... Figure 33.9 A marine flatworm (class Turbellarial.
';;"';Io,-Eyespots
Some planarians can reproduce asexually through fission.
The parent constricts roughly in the middle of its body, separating into a head end and a tail end; each end then regenerates
the missing parts. Sexual reproduction also occurs. Planarians
are hermaphrodites, and copulating mates typically crossfertilize each other.
Ventral nerve cords. From
the ganglia, a pair of
ventral nerve cords runs
the length of the body.
o Mature
flukes live in the blood vessels of the human
intestine A female fluke fits into a groove running
Monogeneans and Trematodes
Monogeneansand trematodes live as parasites in or on other animals. Many have
suckers thatattach to the internal organs or
outer surfaces of the host animal. A tough
covering helps protect the parasites within
their hosts. Reproductive organs occupy
nearly the entire interior ofthese worms.
As agroup, trematodes parasitize awide
range ofhosts, and most species have complex life cycles with alternating sexual and
asexual stages. Manytrematodes require an
intermediate host in which larvae develop
before infecting the final host (usuallya vertebrate), where the adult worms live. For example, trematodes that parasitize humans
spend part of their lives in snail hosts
(Figure 33.11). Around the world, some
200 million people are infected with blood.
flukes (Schistosoma) and suffer from schistosomiasis, a disease whose symptoms include pain, anemia, and dysentery.
Living within different hosts puts demands on trematodes that free-living animals don't face. Ablood. fluke, for instance,
must evade the immune systems of both
snails and humans. By mimicking the surface proteins of its hosts, the blood fluke
creates a partial immunological camou-
Ganglia. At the anterior end
of the worm, near the main sources
of sensory input, is a pair of ganglia,
dense clusters of nerve cells.
the length of the larger male's body, as shown in
the lM at right.
o These
larvae penetrate ~
the skin and blood
vessels of humans
working in fields
irrigated with water
contaminated with
infected
human
feces,
AI"
>----<
1 mm
e Blood
flukes reproduce
sexually in the human
host. The fertilized eggs
exit the host in feces.
o Ifpond
the feces reach a
or other source
of water, the eggs
develop into ciliated
larvae. These larvae
infect snails, the
intermediate hosts,
o within
AseKual reproduction
a snail results
In
another type of motile
larva, which escapes
from the snail host.
... Figure 33.11 The life cycle of a blood fluke (Schistosoma manson;), a trematode.
Snails eat algae, whose growth is stimulated by nutrients found in fertilizer. How
would the contamination of irrigation water with fertilizer likely affect the occurrence of
schistosomiasis? Explain,
_I,J:t.iI!.
CHAPTER THIRTY·THREE
Invertebrates
675
flage for itself. It also releases molecules that manipulate the
hosts' immune systems into tolerating the parasite's existence.
These defenses are so effective that individual blood flukes can
survive in humans for more than 40 years.
Most monogeneans, however, are external parasites of fish.
The monogenean life cycle is relatively simple; a ciliated, freeswimming larva initiates the infection of a host fish. Although
monogeneans have been traditionally aligned with
the trematodes, some structural and chemical evidence suggests they are more closely related to
tapeworms.
Tapeworms
Tapeworms (class Cestoda) are also parasitic
(figure 33.12). The adults live mostly inside
vertebrates, including humans. In many tapeworms, the anterior end, or scolex, is armed
with suckers and often hooks that the worm
uses to attach itself to the intestinal lining of
its host. Tapeworms lack a mouth and gas·
trovascular cavity; they absorb nutrients released by digestion in the host's intestine.
Absorption occurs across the tapeworm's
body surface.
Posterior to the scolex is a long ribbon of
units called proglottids, which are little more
than sacs of sex organs. After sexual reproduction, proglottids loaded with thousands
of fertilized eggs are released from the posterior end of a tapeworm and leave the host's
body in feces. In one type of life cycle, infected feces contaminate the food or water of
intermediate hosts, such as pigs or cattle,
and the tapeworm eggs develop into larvae
that encyst in muscles of these animals. A human acquires the
larvae by eating undercooked meat contaminated with cysts,
and the worms develop into mature adults within the human.
Large tapeworms can block the intestines and rob enough nutrients from the human host to cause nutritional deficiencies.
Doctors use an oraUy administered drug, niclosamide, to kill the
adult worms.
Roti!ers
Rotifers (phylum Rotifera) are tiny animals that inhabit freshwater, marine, and damp soil habitats. Ranging in size from
about SO Ilm to 2 mm, rotifers are smaller than many protists
but nevertheless are multicellular and have specialized organ
systems (Figure 33,13). in contrast to cnidarians and flatworms, which have a gastrovascular cavity, rotifers have an
alimentary canal, a digestive tube with a separate mouth and
anus. Internal organs lie within the pseudocoelom, a body cavitythat is not completely lined by mesoderm (see Figure 32.8b).
Fluid in the pseudocoelom serves as a hydrostatic skeleton
(see Chapter SO). Movement of a rotifer's body distributes the
fluid throughout the body, circulating nutrients.
The word rotifer, derived from Latin, means "wheel-bearer,"
a reference to the crown of cilia that draws a vortex of water
into the mouth. Posterior to the mouth, a region of the digestive tract called the pharynx bears jaws called trophi that grind
up food, mostly microorganisms suspended in the water.
Rotifers exhibit some unusual fonns of reproduction. Some
species consist only of females that produce more females from
wU'ertilized eggs, a type ofreproduction caUed parthenogenesis.
Other species produce two types of eggs that develop by
parthenogenesis. One type forms females while the other type
(produced when conditions deteriorate) develops into simplified males that cannot even feed themselves. These males survive only long enough to fertilize eggs, which form resistant
'I---'7Proglottids with
reproductive strUdures
Scolex
.... figure 33.12 Anatomy of a tapeworm. The inset shows a
close-up of the scolex (colorized SEM),
676
UNIT fiVE
The Evolutionary History of Biological Diversity
.... Figure 33.13 A rotifer. These pseudocoelomates, smaller than
many protists, are generally more anatomically complex than
flatworms (lM).
zygotes that can survive when a pond dries up. When conditions are favorable, the zygotes break dormancy and develop
into a new female generation that reproduces by parthenogenesis until conditions become unfavorable again.
It is puzzling that so many rotifer species survive without
males. The vast majority of animals and plants reproduce sexually at least some of the time, and sexual reproduction has
certain advantages over asexual reproduction. For example,
species that reproduce asexually tend to accumulate harmful
mutations in their genomes faster than sexually reproducing
species. As a result, asexual species should experience higher
rates of extinction and lower rates of speciation.
Seeking to understand this unusual group, Nobel Prizewinning biologist Matthew Meselson, of Harvard Vniversity,
has been studying a class of asexual rotifers named Bdel1oidea.
Some 360 species ofbdel1oid rotifers are known, and all ofthem
reproduce by parthenogenesis without any males. Paleontologists have discovered bdel10id rotifers preserved in 35-mil1ionyear-old amber, and the morphology of these fossils resembles
only the female form, with no evidence of males. By comparing
the DNA ofbdelloids with that of their closest sexually reproducing rotifer relatives, Meselson and his colleagues concluded
that bdel10ids have likely been asexual for much longer than
35 million years. Howthese animals manage to flout the general
rule against long-lived asexuality remains a puzzle.
Lophophorates: Ectoprocts and Brachiopods
Bilaterians in the phyla Ectoprocta and Brachiopoda are among
those known as lophophorates. These animals have a
/ophophore, a crown of ciliated tentacles that surround the
mouth (see Figure 32. 13a). As the cilia draw water toward the
mouth, these tentacles trap suspended food particles. Other
similarities, such as a V-shaped alimentary canal and the absence of a distinct head, reflect these organisms' sessile existence. In contrast to flatworms, which lack a body cavity, and
rotifers, which have a pseudocoelom, lophophorates have a
true coelom that is completely lined by mesoderm (see
Figure 32.8a).
Ectoprocts (from the Greek ectv, outside, and procta, anus)
are colonial animals that superficially resemble clumps of
moss. (In fact, their common name, bryozoans, means ~moss
animals.~) In most species, the colony is encased in a hard
exoskeleton (external skeleton) studded with pores through
which the lophophores extend (Figure 33.14a). Most ectoproct species live in the sea, where they are among the most
widespread and numerous sessile animals. Several species are
important reef builders. Ectoprocts also live in lakes and rivers.
Colonies of the freshwater ectoproct Pectinate/ILl magnifica
grow on submerged sticks or rocks and can grow into a gelatinous, ball-shaped mass more than 10 em across.
Brachiopods, or lamp shells, superficially resemble clams
and other hinge-shelled molluscs, but the two halves ofthe bra-
(a) Ectoprocts, such as this sea mat (b) Brachiopods have a hinged
(Membranipora membranacea),
shell. The two parts of the
are coloniallophophorates.
shell are dorsal and ventral.
... Figure 33.14 Lophophorates.
chiopod shell are dorsal and ventral rather than lateral, as in
clams (Figure 33.14b). All brachiopods are marine. Most live
attached to the seafloor by a stalk, opening their shell slightly to
allow water to flow through the lophophore. The living brachiopods are remnants of a much richer past that included
30,000 species in the Paleozoic and Mesozoic eras. Some living
brachiopods, such as those in the genus Lingula, are nearly
identical to fossils of species that lived 400 million years ago.
Molluscs
Snails and slugs, oysters and clams, and octopuses and squids
are all molluscs (phylum Mollusca). Most molluscs are marine, though some inhabit fresh water, and some snails and
slugs live on land. Molluscs are soft-bodied animals (from the
Latin mol/uscus, soft), but most secrete a hard protective shell
made ofcalcium carbonate. Slugs, squids, and octopuses have
a reduced internal shell or have lost their shell completely during their evolution.
Despite their apparent differences, all molluscs have a similar body plan (Figure 33.15, on the next page). Molluscs are
coelomates, and their bodies have three main parts: a muscular foot, usually used for movement; a visceral mass containing most of the internal organs; and a mantle, a fold of tissue
that drapes over the visceral mass and secretes a shell (ifone is
present). In many molluscs, the mantle extends beyond the
visceral mass, producing a water-filled chamber, the mantle
cavity, which houses the gills, anus, and excretory pores.
Many molluscs feed by using a straplike rasping organ called a
radula to scrape up food.
Most molluscs have separate sexes, and their gonads (ovaries
or testes) are located in the visceral mass. Many snails, however,
are hermaphrodites. The life cycle of many marine molluscs includes aciliated larval stage, the trochophore (see Figure 32.13b),
CHAPTER THIRTY·THREE
Invertebrates
677
Heart. Most molluscs have an open circulatory
system. The dorsally located heart pumps
circulatory fluid called hemolymph through arteries
into sinuses (body spaces). The organs of the
mollusc are thus continually bathed in hemolymph.
Nephridium. Excretory organs
called nephridia remove metabolic
wastes from the hemolymph.
Visteral mass.
Coelom
The long digestive tract is
coiled in the visceral mass.
Intestine
Gonads
Mantle
cavity
(..."....~~!r:H
Anus
The nervous system
consists of a nerve
ring aroundfrom
the
esophagus,
which nerve cords
extend.
Gill
I~-==i:::::.,.-_":::!~!!~=E~~~;:'\
Mouth
Foot
Nerve
cords
Esophagus
Radula. The mouth
region in many
mollusc species
contains a rasp-like
feeding organ
called a radula. This
belt of backwardcurved teeth
repeatedly thrusts
outward and then
retracts into the
mouth, scraping
and scooping like a
backhoe.
... Figure 33.15 The basic body plan of a mollusc.
T."J:U Major Classes of Phylum Mollusca
Class and Examples
Main Characteristics
Polyplacophora (chitons;
see Figure 33.16)
Marine; shell with eight plates;
foot used for locomotion;
radula; no head
Gastropoda (snails, slugs;
see Figures 33.17 and 33.18)
Marine, freshwater, or terrestrial; head present; a symmetrical body, usually with a coiled
shell; shell reduced or absent;
foot for locomotion; radula
Bivalvia (dams, mussels,
scallops, oysters; see
Figures 33.19 and 33.20)
Marine and freshwater; flattened shell with two valves;
head reduced; paired gills; no
radula; most are suspension
feeders; mantle forms siphons
Cephalopoda (squids,
octopuses, cuttlefishes,
chambered nautiluses;
see Figure 33.21)
Marine; head surrounded by
grasping tentddes, usually
with suckers; shell external,
internal, or absent; mouth with
or without radula; locomotion
by jet propulsion using siphon
formed from foot
which is also characteristic of marine annelids (segmented
worms) and some other 10photrochozoans.
The basic body plan of molluscs has evolved in various ways
in the phylum's eight classes. We'll examine four ofthose classes here (Table 33.3): Polyplacophora (chitons), Gastropoda
(snails and slugs), Bivalvia (clams, oysters, and other bivalves),
and Cephalopoda (squids, octopuses, cuttlefishes, and cham~
bered nautiluses).
678
UNIT
fiVE
The Evolutionary History of Biological Diversity
.... Figure 33.16 A chiton. Note the eight-plate shell charaderistic
of molluscs in the class Polyplacophora.
Chitons
Chitons have an oval-shaped body and a shell divided into
eight dorsal plates (Figure 33.16). The chiton's body itself,
however, is unsegmented. You can find these marine animals
clinging to rocks along the shore during low tide. If you try to
dislodge a chiton by hand, you will be surprised at how well its
foot, acting as a suction cup, grips the rock. A chiton can also
use its foot to creep slowly over the rock surface. Chitons use
their radula to scrape algae off the rock surface.
Gastropods
About three-quarters of all living species of molluscs are gastropods (Figure 33.17). Most gastropods are marine, but
there are also many freshwater species. Some gastropods have
adapted to life on land, including garden snails and slugs.
A distinctive characteristic of class Gastropoda is a developmental process known as torsion. As a gastropod embryo develops, its visceral mass rotates up to 180', causing the animal's
anus and mantle cavity to wind up above its head (Figure 33.18).
After torsion, some organs that were bilateral may be reduced in
size, while others may be lost on one side of the body. Torsion
should not be confused with the formation of a coiled shell,
which is an independent developmental process.
Most gastropods have a single, spiraled shell into which the
animal can retreat when threatened. The shell is often conical
but is somewhat flattened in abalones and limpets. Many gastropods have a distinct head with eyes at the tips of tentacles.
Gastropods move literally at a snail's pace by a rippling motion
of their foot or by means of cilia, often leaving a trail of slime in
their wake. Most gastropods use their radula to graze on algae or
plants. Several groups, however, are predators, and their radula
has become modified for boring holes in the shells ofother molluscs or for tearing apart prey. In the cone snails, the teeth ofthe
radula act as poison darts that are used to subdue prey.
Terrestrial snails lack the gills typical of most aquatic gastropods. Instead, the lining of their mantle cavity functions as
a lung, exchanging respiratory gases with the air.
Bivalves
The moUuscs of class Bivalvia include many species of clams,
oysters, mussels, and scallops. Bivalves have a shell divided into
two halves (Figure 33.19). The halves are hinged at the middorsal line, and powerful adductor muscles draw them tightly
together to protect the animal's soft body. Bivalves have no distinct head, and the radula has been lost. Some bivalves have
eyes and sensory tentacles along the outer edge oftheir mantle.
The mantle cavity of a bivalve contains gills that are used for
gas exchange as well as feeding in most species (Figure 33.20).
Most bivalves are suspension feeders. They trap fine food particles in mucus that coats their gills, and cilia then convey those
particles to the mouth. Water enters the mantle cavity through
an incurrent siphon, passes over the gills, and then exits the
mantle cavity through an excurrent siphon.
... Figure 33.19 A bivalve. This scallop has many eyes (dark blue
spots) peering out from each half of its hirlged shell.
(a) A land snail
Hinge area
Coelom
,~;;ti::~G~",~
~
Heart
Adductor
muscle
ArluS
... Figure 33.17
Gastropods.
(b) A sea slug. Nudibranchs. or sea slugs, lost
their shell during their evolution.
11~~~~r1~~
.~
Stomach
Excurrent
iphon
Intestine
Mouth
Foot
... Figure 33.18 The results of torsion in a gastropod. Because
of torsion (twisting of the visceral mass) durirlg embryonic development. the
digestive tract is coiled and the anus is near the anterior end of the animal.
~;~:~=:t::;~
Mantle
cavity
Gonad
Gill
Water
flow
Incurrent
siphon
... Figure 33.20 Anatomy of a clam. Food particles suspended
in water that enters through the incurrent siphon are collected by the
gills and passed via cilia to the mouth.
CHAPTER THIRTY·THREE
Invertebrates
679
Most bivalves lead sedentary lives, a characteristic suited to
suspension feeding. Sessile mussels secrete strong threads that
tether them to rocks, docks, boats, and the shells of other ani~
mals. However, clams can pull themselves into the sand or
mud, using their muscular foot for an anchor, and scallops can
skitter along the seafloor by flapping their shells, rather like the
mechanical false teeth sold in novelty shops.
Cephalopods
Cephalopods are active predators (Figure 32.21). They use
their tentacles to grasp prey, which they then bite with beaklike jaws and immobilize with a poison found in their saliva.
The foot of a cephalopod has become modified into a muscular excurrent siphon and part of the tentacles. Squids dart
about by drawing water into their mantle cavity and then firing a jet of water through the excurrent siphon; they steer by
pointing the siphon in different directions. Octopuses use a
similar mechanism to escape predators.
A mantle covers the visceral mass of cephalopods, but the
shell is reduced and internal (in squids and cuttlefishes) or
missing altogether (in many octopuses). One small group of
shelled cephalopods, the chambered nautiluses, survives today.
Cephalopods are the only molluscs with a closed circulatory system. They also have well-developed sense organs and
.. Octopuses are considered
among the most
Intelligent invertebrates.
• Squids are speedy
carnivores with
beak-like Jaws and
well·developed eyes.
a complex brain. The ability to learn and behave in a complex
manner is probably more critical to fast-moving predators
than to sedentary animals such as clams.
The ancestors of octopuses and squids were probably
shelled molluscs that took up a predatory lifestyle; the shell
was lost in later evolution. Shelled cephalopods called
ammonites, some of them as large as truck tires, were the
dominant invertebrate predators of the seas for hundreds of
millions of years until their disappearance during the mass
extinction at the end of the Cretaceous period, 65.5 million
years ago (see Chapter 25).
Most species ofsquid are less than 75 cm long, but some are
considerably larger. The giant squid (Architeulhis dux) was for
a long time the largest squid known, with a mantle up to 2.25 m
long and a total length of 18 m. In 2003, however, a specimen
of the rare species Mesvnychoteuthis hami/toni was caught
near Antarctica; its mantle was 2.5 m long. Some biologists
think that this specimen was a juvenile and estimate that
adults of its species could be ty,'ice as large! Unlike A. dux,
which has large suckers and small teeth on its tentacles, M.
hamilton! has two rows ofsharp hooks at the ends ofits tentacles that can deliver deadly lacerations.
It is likely that A. dux and M. hamilton! spend most of their
time in the deep ocean, where they may feed on large fishes.
Remains of both giant squid species have been found in the
stomachs of sperm whales, which are probably their only natural predator. In 2005, scientists reported the first observations of A. dux in the wild, photographed while attacking
baited hooks at a depth of 900 m. M. hamiltvni has yet to be
observed in nature. Overall, these marine giants remain
among the great mysteries of invertebrate life.
Annelids
Annelida means "little rings; referring to the annelid body's re~
semblance to a series of fused rings. Annelids are segmented
Table lJ.4 Classes of Phylum Annelida
.. Chambered
nautiluses are
the only living
cephalopods with
an external shell
... Figure 33.21 Cephalopods.
680
UNIT fiVE
The Evolutionary History of Biological Diversity
Class and Examples
Main Characteristics
Oligochaeta (freshwater, marine,
and terrestrial segmented
worms; see Figure 33.22)
Reduced head; no parapodia,
but chaetae present
Polychaeta (mostly marine
segmented worms; see
Figure 33.23)
Many have a well-developed
head; each segment usually
has parapodia with many
chaetae; free-living
Hirudinea (leeches; see
Figure 33.24)
Body usually flattened, with
reduced coelom and segmentation; chaetae usually absent;
suckers at anterior and posterior ends; parasites, predators, and SCAvengers
worms that live in the sea, in most freshwater habitats, and in
damp soil. Annelids are coelomates, and they range in length
from less than 1 mm to more than 3 m, the length ofa giant Australian earthworm.
The phylum Annelida can be divided into three classes
(Table 33,4, on the facing page): Oligochaeta (the earthworms and their relatives), Polychaeta (the polychaetes), and
Hirudinea (the leeches),
Each segment is surrounded by longitudinal muscle, which in
turn is surrounded by circular muscle. Earthworms coordinate
the contraction of these two sets of muscles to move
(see Figure 50.33). These muscles work against the noncompressible coelomic fluid, which acts as a hydrostatic skeleton.
Epidermis
Oligochaetes
Oligochaetes (from the Greek oligos, few, and cllaite, long hair)
are named for their relatively sparse chaetae, or bristles made of
chitin. TIlis class of segmented worms includes tlle earthworms
and a variety of aquatic species. Figure 33.22 provides a guided
tour ofthe anatomy of an earthwonn, which is representative of
annelids. Earthworms eat their way through the soil, extracting
Coelom. The coelom
of the earthworm is
partitioned by septa.
Septum
(partition
between
segments)
Many of the internal
structures are repeated
within each segment of
the earthworm.
Metanephridium. Each
segment of the worm
contains a pair of
excretory tubes, called
meta nephridia, with
ciliated funnel-shaped
openings called
nephrostomes. The
meta nephridia remove
wastes from the blood
and coelomic fluid
through exterior pores.
longitudinal
muscle
Dorsal--f!'lII
Chaetae. Each segment
vessel
has four pairs of
chaetae, bristles that I-~===:::
provide traction for
burrowing.
Fused
nerve
cords
Nephrostome
Venlral
vessel
Tiny blood
vessels are
abundant in the
earthworm's
--ii---Iskin, which
functions as its
respiratory organ. The blood
contains oxygencarrying
hemoglobin.
Clitellum
Metanephridium
Intestine
Giant Australian earthworm
Cerebral ganglia. The earthworm
nervous system features a brainlike pair of cerebral ganglia above
and in front of the pharynx. A
ring of nerves around the pharynx
connects to a subpharyngeal
ganglion, from which a fused pair
of nerve cords runs posteriorly.
L---"""""":~~~
Mouth
The circulatory system, a network of vessels,
is closed. The dorsal and ventral vessels are
linked by segmental pairs of vessels. The
dorsal vessel and five pairs of vessels that
circle the esophagus are muscular and
pump blood through the circulatory system.
Ventral nerve cords with segmental
ganglia. The nerve cords penetrate the
septa and run the length of the anima!,
as do the digestive tract and
longitudinal blood vessels.
... Figure 33.22 Anatomy of an earthworm, an oligochaete.
CHAPTER THIRTY·THREE
Invertebrates
681
nutrients as the soil passes through the alimentary canal Undigested material, mixed with mucus secreted into the canal, is
eliminated as fecal castings through the anus. Farmers value
earthworms because the animals till and aerate the earth, and
their castings improve the texture of the soil. (Charles Darwin
estimated that a single acre of British farmland contains about
SO,coo earth....,orms, producing 18 tons ofcastings per year.)
Earthworms are hermaphrodites, but they cross-fertilize.
Two earthworms mate by aligning themselves in opposite direclions in such a way that they exchange sperm (see figure 46.1),
and then they separate. The received sperm are stored temporarily while an organ called the c1itellum secretes a cocoon
of mucus. The cocoon slides along the worm, picking up the
eggs and then the stored sperm. The cocoon then slips off the
worm's head and remains in the soil while the embryos develop. Some earthworms can also reproduce asexually by fragmentation followed by regeneration.
Polychaeles
Each segment of a polychaete has a pair of paddle-like or
ridge-like structures called parapodia rnear feet") that function in locomotion (Figure 33.23). Each parapodium has numerous chaetae, so polychaetes usually have many more
chaetae per segment than do oligochaetes. In many polychaetes, the parapodia are richly supplied with blood vessels
and also function as gills.
Polychaetes make up a large and diverse class, most of
whose members are marine. A few species drift and swim
among the plankton, many crawl on or burrow in the seafloor,
and many others live in tubes. Some tube-d.....ellers, such as the
fan worms, build their tubes by mixing mucus with bits ofsand
and broken shells. Others, such as Christmas tree worms (see
Figure 33.1 l, construct tubes using only their own secretions.
Leeches
The majority of leeches inhabit fresh water, but there are also
marine species as well as terrestrial leeches found in moist
vegetation. Leeches range in length from about 1 to 30 cm.
.. Figure 33.24 A leech. A
nurse applied thIS medJonalleech
(Hirudo l'nf'didnaJisl to a patient's
SCH"e thumb 10 drain blood from a
hematoma (an abnormal
accumulation of blood around an
II'Iternal inJury).
Many are predators that feed on other invertebrates, but some
are parasites that suck blood by attaching temporarily to other
animals, including humans (Figure 33.24). Some parasitic
species use bladelike jaws to slit the skin oftheir host, whereas
others secrete enzymes that digest a hole through the skin.
The host is usually oblivious to this attack because the leech
secretes an anesthetic. After making the incision, the leech secretes another chemical, hirudin, which keeps the blood ofthe
host from coagulating near the incision. The parasite then
sucks as much blood as it can hold, often more than ten times
its own weight. After this gorging, a leech can last for months
without another meal.
Until this century, leeches were frequently used for bloodletting. Today they are used to drain blood that accumulates in tissues following certain injuries or surgeries. Researchers have
also investigated the potential use of hirudin to dissolve unwanted blood dots that form during surgery or as a result of
heart disease. Several recombinant forms of hirudin have been
developed, two ofwhich were recently approved fordinical use.
As a group, Lophotrocholoa encompasses a remarkable
range ofbody plans, as illustrated by members ofsuch phyla as
Rotifera, Ectoprocta, Mollusca, and Annelida. Next we'll explore the diversity of Ecdysoloa, a dominant presence on
Earth in terms of sheer number of speCies.
CONCEPT CHECI(
n.)
I. Explain how tapeworms can survive without a
.. Figure 33.23 A
polychaete. HesioIyra berg;
lives on the seafloor around
deep-sea hydrothermal vents.
coelom, a mouth, a digestive system, or an excretory
system.
2. How does the modification of the molluscan foot in
gastropods and cephalopods relate to their respecti\'e
lifestyles?
3. Annelid anatomy can be described as -a tube within a
tube:" Explain.
4. _lili','Ii* Relatively few free-living lophotrcchozoans live on land, above the surface of the soil.
Focusing on gravity, hypothesize why this is so.
for suggested answers. see Appendix A.
682
UNIT
fiVE
The Evolutionary History of Biological Diversity
r::;;:::o~~'~re the most
species-rich animal group
r;;==;
Although defined primarily by
molecular evidence, the dade
Lopholnxhowa
Ecdysozoa includes animals
Ecdysozoa
that shed a tough external coat
Deuterostorma
(cuticle) as they grow; in fact,
the group derives its name from this process, which is called
molting, or ecdysis. Ecdysozoa consists of about eight animal
phyla and contains mOTe known species than all other protist,
fungus, plant, and animal groups combined. Here we'll focus
Calcarea and Silicea
Cmdana
on the two largest e<dysozoan phyla, the nematodes and
arthropods, which are among the most successful and abun-
dant of all animal groups.
Nematodes
Some of the most ubiquitous animals, nematodes (phylum
Nematoda), or roundworms, are found in most aquatic habitats,
in the soil, in the moist tissues of plants, and in the body fluids
and tissues of animals. In contrast to annelids, nematodes do
not have segmented bodies. The cylindrical bodies of nematodes range from less than 1 mm to more than a meter in length,
often tapering to a fine tip at the posterior end and to a more
blunt tip at the anterior end (Figure 33.25). A nematode's body
is covered by a tough cuticle; as the worm grows, it periodically
sheds its old cuticle and secretes a new, larger one. Nematodes
have an alimentary canal, though they lack a circulatory system.
Nutrients are transported throughout the body via fluid in the
pseudocoelom. The body wall muscles are all longitudinal, and
their contraction produces a thrashing motion.
Nematodes usually reproduce sexually, by internal fertilization. In most species, the sexes are separate and females are
larger than males. A female may deposit 100,000 or more fertilized eggs (zygotes) per day. The zygotes of most species are
resistant cells that can survive harsh conditions.
Multitudes ofnematodes live in moist soil and in decomposing organic matter on the bottoms of lakes and oceans. While
25,000 species are known, perhaps 20 times that number actually exist. It has been said that if nothing but nematodes remained on Earth, they would still preserve the outline of the
planet and many onts features. These free-living worms play an
important role in decomposition and nutrient cycling, but little
is known about most species. One species of soil nematode,
Caellorhabditis elegalls, however, is very well studied and has
become a model research organism in biology (see Chapter 21).
Ongoing studies on C. elegans are revealing some of the mechanisms involved in aging in humans, among other findings.
Phylum Nematoda includes many significant agricultural
pests that attack the roots ofplants. Other species of nematodes
parasitize animals. Humans are hosts to at least 50 nematode
species, including various pinworms and hookworms. One notorious nematode is Trichinella spiralis, the worm that causes
trichinosis (Figure 33.26). Humans acquire this nematode by
eating raw or undercooked pork or other meat (including wild
game such as bear or walrus) that has juvenile worms encysted
in the muscle tissue. \'(!ithin the human intestines, the juveniles
develop into sexually mature adults. Females burrow into the
intestinal muscles and produce more juveniles, which bore
through the body or travel in lymphatic vessels to other organs,
including skeletal muscles, where they encyst.
Parasitic nematodes have an extraordinary molecular
toolkit that enables them to redirect some of the cellular functions of their hosts and thus evade their immune systems.
Plant-parasitic nematodes inject molecules that induce the
development of root cells, which then supply nutrients to the
parasites. Trichinella controls the expression of specific
muscle-cell genes that code for proteins that make the cell elastic enough to house the nematode. Additionally, the infected
muscle cell releases signals that attract blood vessels, which
Encysted juveniles
Muscle tissue
... Figure 33.26 Juveniles of the parasitic nematode
.. Figure 33.25 Afree-living nematode (colorized SEM),
Trichinella spiralis encysted in human muscle tissue (LM).
CHAPTER THIRTY·THREE
Invertebrates
683
then supply the nematode with nutrients. These extraordinary
parasites have been dubbed "animals that act like viruses."
• FI
Arthropods
Hox genes?
33.28
•
Did the arthropod body plan result from new
18
Zoologists estimate that there are about a billion billion (10 )
arthropods living on Earth. More than 1 million arthropod
species have been described, most ofwhich are insects. In fact,
two out of every three known species are arthropods, and
members ofthe phylum Arthropoda can be found in nearly all
habitats of the biosphere. By the criteria of species diversity,
distribution, and sheer numbers, arthropods must be reo
garded as the most successful of all animal phyla.
Arthropod Origins
Biologists hypothesize that the diversity and success of
arthropods is related to their body plan-their segmented
bodies, hard exoskeleton, and jointed appendages (arthropod
means "jointed feet"). The earliest fossils with this body plan
are from the Cambrian explosion (535-525 million years
ago), indicating that the arthropods are at least that old.
Along with arthropods, the fossil record of the Cambrian
explosion contains many species of lvbopods, an extinct group
from which arthropods may have evolved. Lobopods such as
Hallucigenia (see Figure 25.4) had segmented bodies, but most
of their body segments were identical to one another. Early
arthropods, such as the trilobites, also showed little variation
from segment to segment (Figure 33.27). As arthropods continued to evolve, the segments tended to fuse and become
fewer, and the appendages became specialized for a variety of
functions. These evolutionary changes resulted not only in
great diversification but also in an efficient body plan that permits the division oflabor among different body regions.
What genetic changes led to the increasing complexity of
the arthropod body plan? Arthropods today have two unusual
Hox genes, both of which influence segmentation. To test
whether these genes could have driven the evolution of increased body segment diversity in arthropods, Sean Carroll
(see pages 534-535) and colleagues studied Hox genes in onychophorans, dose relatives of arthropods (Figure 33.28).
EXPERIMENT
How did the highly successful arthropod body
plan arise? One hypothesis suggests that it resulted from the origin (by a gene duplication event) of two unusual Hox genes found
in arthropods: Ultrabithorax (Vbx) and abdominal-A (abd-A). To
test this hypothesis, Sean Carroll, of the University of Wisconsin,
Madison, and colleagues turned to the onychophorans, a group of
invertebrates closely related to arthropods. Unlike many living
arthropods, onychophorans have a body plan in which most body
segments are identical to one another, Thus, Carroll and colleagues reasoned that if the origin of the Ubx and abd-A Hox
genes drove the evolution of body segment diversity in arthropods, these genes probably arose on the arthropod branch of the
evolutionary tree:
-
Other
ecdysoloans
Origin of ubx and
abd-A Hoxg~?
r--"'I-.-
Arthropods
Common ancestor Of/ ...- - - - - Onychophorans
onychophorans and arthropods
As the hypothesis depided above suggests, Ubx and abd-A
would not have been present in the common ancestor of arthropods and onychophorans. and hence. onychophorans should not
have these genes. To find out whether this was the case. Carroll
and colleagues eKamined the Hox genes of the onychophoran
Acanrhokara kaputensis.
RESULTS
The onychophoran A. kaputensis has all
arthropod Hox genes, including Ubx and abd·A,
Red indicates the
body regions of this
onychophoran embryo
in which Ubx or
abd~A genes were
expressed. (The
inset shows this
area enlarged.)
Ant", antenna
J '" jaws
L1-L15 '" body segments
.. Figure 33.27 A trilobite
fossil. Trilobites were common
denlzens of the shallow seas
throughout the PaleozoIC era but
disappeared with the great
Permian extindions about 250
million years ago. Paleontologists
have described about 4,000
trilobIte species.
CONCLUSION Since A. kaputensis, an onychophoran, has the
arthropod Hox genes, the evolution of increased body segment diversity in arthropods must not have been related to the origin of neY\'
Hax genes,
SOURCE
J K. Grenier. 5
C~rroll
et
~I,
Evolution 01 tfle entire
~rthropod Hox gene >et predated the ongln and r~diatlon 01 the
onjl(hophor~rv'~rthropod d~de. CUflf'flt Biology 7547-553 (1997),
.i,lI:tU!£W
If Carroll and colleagues had found that A kaputen·
sis did nor have the Ubx and abd-A Hox genes, how would their
conclusion have been affected? Explain.
684
UNIT
fiVE
The Evolutionary History of Biological Diversity
Their results indicate that arthropod body plan diversity did
not arise from the acquisition of new Hox genes. Instead, the
evolution of body segment diversity in arthropods may have
been driven by changes in the sequence or regulation of existing Hox genes. (See Chapter 25 for a discussion of how
changes in form can result from changes in the sequence or
regulation of developmental genes such as Hox genes.)
ter. Arthropods began to diversify on land following the colonization of land by plants in the early Paleozoic. Evidence includes a 428-million-year-old fossil of a millipede found in
2004 by an amateur fossil hunter in Scotland. Fossilized tracks
of other terrestrial arthropods date from about 450 million
years ago.
Arthropods have well-developed sensory organs, including
eyes, olfactory (smell) receptors, and antennae that function in
both touch and smell. Most sensory organs are concentrated
at the anterior end of the animal.
Like many molluscs, arthropods have an open circulatory
system, in which fluid called hemolymph is propelled by a
heart through short arteries and then into spaces called sinuses surrounding the tissues and organs. (The term blood is
generally reserved for fluid in a dosed circulatory system.) Hemolymph reenters the arthropod heart through pores that are
usually equipped with valves. The hemolymph-filled body sinuses are collectively called the hemocoel, which is not part of
the coelom. Although arthropods are coelomates, in most
species the coelom that forms in the embryo becomes much
reduced as development progresses, and the hemocoel becomes the main body cavity in adults. Despite their similarity,
the open circulatory systems of molluscs and arthropods
probably arose independently.
A variety of specialized gas-exchange organs have evolved in
arthropods. These organs allow the diffusion ofrespiratory gases
in spite of the exoskeleton. Most aquatic species have gills with
thin, feathery extensions that place an extensive surface area in
contact with the surrounding water. Terrestrial arthropods generally have internal surfaces specialized for gas exchange. Most
insects, for instance, have tracheal systems, branched air ducts
leading into the interior from pores in the cuticle.
General Characteristics ofArthropods
Over the course ofevolution, the appendages of some arthropods have become modified, specializing in functions such as
walking, feeding, sensory reception, reproduction, and defense. Figure 33.29 illustrates the diverse appendages and
other arthropod characteristics of a lobster.
The body of an arthropod is completely covered by the cuticle, an exoskeleton constructed from layers of protein and the
polysaccharide chitin. The cuticle can be thick and hard over
some parts of the body and paper-thin and flexible over others,
such as the joints. The rigid exoskeleton protects the animal and
provides points ofattachment for the muscles that move the appendages. But it also means that an arthropodcannotgrowwithout occasionally shedding its exoskeleton and producing a larger
one. This molting process is energetically expensive. A molting
or recently molted arthropod is also vulnerable to predation and
other dangers until its new, soft exoskeleton hardens.
\Vhen the arthropod exoskeleton first evolved in the sea, its
main functions were probably protection and anchorage for
muscles, but it later additionally enabled certain arthropods to
live on land. The exoskeleton's relative impermeability to water helped prevent desiccation, and its strength solved the
problem of support when arthropods left the buoyancy ofwa-
... Figure 33.29 External anatomy of
an arthropod. Many of the distlndive
Cephalothorax
Abdomen
features of arthropods are apparent in this
dorsal view of a lobster. along with some
uniquely crustacean characteristics. The body
is segmented. but this charaderistic is obvious
only In the abdomen The appendages
(including antennae. pincers. mouthparts,
walking legs, and swimming appendages) are
Jointed, The head bears a pair of compound
(multilens) eyes, each situated on a movable
stalk, The whole body, including appendages.
is covered by an exoskeleton
Swimming appendages
(one pair located
under each
abdominal segment)
U"-.
Walking legs
Pincer (defense)
Mouthparts (feeding)
CHAPTER THIRTY·THREE
Invertebrates
685
T.'.:n.5 Subphyla of Phylum Arthropoda
Subphylum
and Examples
Main Characteristics
Cheliceriformes
(horseshoe cnlbs,
spiders, scorpions,
ticks, mites; see
Figures 33.30-33.32)
Myriapoda (millipedes
and centipedes; see
Figures 33.33 and 33.34)
Body having one or two main
parts; six pairs of appendages
(chelicerae, pedipalps, and four
pairs ofwalking legs); mostly
terrestrial or marine
Distinct head bearing antennae
and chewing mouthparts;
terrestrial; millipedes are her-
bivorous and have two pairs of
walking legs per trunk segment;
centipedes are carnivorous and
have one pair of walking legs per
trunk segment and poison claws
on first body segment
Hexapoda (insects,
springtails; see
Figures 33.35-33.37)
Crustacea (crabs,
lobsters. crayfishes,
shrimps; see
Figures 33.29 and 33.38)
Body divided into head, thorax,
and abdomen; antennae present;
mouthparts modified for chewing,
sucking, or lapping; three pairs of
legs and usually two pairs of
wings; mostly terrestrial
.... Figure 33.30 Horseshoe crabs (Umulus polyphemus).
Common on the Atlantic and Gulf coasts of the United States, these
"living fossils" have changed little in hundreds of millions of years.
They are surviving members of a rich diversity of cheliceriforms that
once filled the seas.
Body of two or three parts;
antennae present; chewing
mouthparts; three or more
pairs of legs; mostly marine and
freshwater
Morphological and molecular evidence suggests that living
arthropods consist offour major lineages that diverged early in
the evolution of the phylum (Table 33.5): cheliceriforrns (sea
spiders, horseshoe crabs, scorpions, ticks, mites, and spiders);
myriapods (centipedes and millipedes); hexapods (insects
and their wingless, six~legged relatives); and crustaceans
(crabs, lobsters, shrimps, barnacles, and many others).
.. Scorpions have pedipalps
that are pincers specialized for defense and the
capture of food. The tip of
the tail bears a poisonous
stinger.
Cheliceriforms
Cheliceriforms (subphylum Cheliceriformes; from the Greek
c1wilQs, lips, and cheir, arm) are named for clawlike feeding appendages called chelicerae, which serve as pincers or fangs.
Cheliceriforms have an anterior cephalothorax and a poste~
rior abdomen. They lack antennae, and most have simple eyes
(eyes with a single lens).
The earliestcheliceriforms were curyptcrids, or water scor·
pions. These marine and freshwater predators grew up to 3 m
long; it is thought that some species could have walked on land,
much as land crabs do today. Most ofthe marine cheliceriforms,
including all of the eurypterids, are extinct. Among the marine
cheliceriforms that survive today are the sea spiders (pycnogonids) and horseshoe crabs (Figure 33.30).
The bulk ofmodem cheliceriforms are arachnids, a group that
includes scorpions, spiders, ticks, and mites (Figure 33.31). TIcks
686
UNIT fiVE
The Evolutionary History of Biological Diversity
.. Dust mites are ubiquitous
scavengers in human
dwellings but are harmless
except to those people who
are allergic to them
(colorized SEM),
<Ill
.... Figure 33.31 Arachnids.
Web-building spiders are
generally most active
during the daytime.
and many mites are among a large group of
Stomach
Intestine
parasitic arthropods. Nearly all ticks are
Heart
bloodsucking parasites that live on the
Digestive
bodysurfaces ofreptiles or mammals. Pargland
asitic mites live on or in a wide variety of
vertebrates, invertebrates, and plants.
Ovary
Arachnids have a cephalothorax that
has six pairs ofappendages: the chelicerae;
a pair ofappendages calledpedipalps that
function in sensing, feeding, or reproduction; and four pairs of walking legs
(Figure 33.32). Spiders use their fangAnus
like chelicerae, which are equipped with
poison glands, to attack prey. As the cheSpinnerets
Chelicera
Sperm
licerae pierce the prey, the spider secretes
Silk gland
receptacle
digestive juices onto the prey's torn tis... Figure 33.32 Anatomy of a spider.
sues. The food softens, and the spider
sucks up the liquid meal.
In most spiders, gas exchange is carried out by book lungs, stacked platelike structures contained
in an internal chamber (see Figure 33.32). The extensive surface area of these respiratory organs is a structural adaptation
that enhances the exchange of O2 and CO 2 between the hemolymph and air.
A unique adaptation of many spiders is the ability to catch
insects by constructing webs of silk, a liquid protein produced
by specialized abdominal glands. The silk is spun by organs
called spinnerets into fibers that then solidify. Each spider engineers a web characteristic of its species and builds it perfectly
on the first try. This complex behavior is apparently inherited.
Various spiders also use silk in other ways: as droplines for
rapid escape, as a cover for eggs, and even as ~gift wrap~ for food
that males offer females during courtship. Many small spiders
also extrude silk into the air and let themselves be transported
... Figure :n.:n A millipede.
by wind, a behavior known as uballooning~
Brain
Poison
gland
Pedipalp
Myriapods
Millipedes and centipedes belong to the subphylum Myriapoda,
the myriapods. All living myriapods are terrestrial. The myriapod head has a pair of antennae and three pairs of appendages
modified as mouthparts, including the jaw-like mandibles.
Millipedes (class Diplopoda) have a large number of
legs, though fewer than the thousand their name implies
(Figure 33.33). Each trunk segment is formed from two
fused segments and bears two pairs of legs. Millipedes eat
decaying leaves and other plant matter. They may have been
among the earliest animals on land, living on mosses and
primitive vascular plants.
Unlike millipedes, centipedes (class Chilopoda) are carnivores. Each segment ofa centipede's trunk region has one pair
oflegs (Figure 33.34). Centipedes have poison claws on their
foremost trunk segment that paralyze prey and aid in defense.
... Figure 33.34 A centipede.
CHAPTER THIRTY·THREE
Invertebrates
687
The insect body has three regions: head,
thorax, and abdomen. The segmentation
of the thorax and abdomen are obvious,
but the segments that form the head are fused.
Abdomen
..
Thorax
Head
...............
Compound eye
~::::::>Antennae
Heart. The insect heart
drives hemolymph
through an open
circulatory system.
~~~
Cerebral ganglion. The two nerve
cords meet in the head, where the
ganglia of several anterior segments
are fused into a cerebral ganglion
(brain, colored white below). The
antennae, eyes, and other sense
organs are concentrated on the head.
Anus
Malpighian tubules.
Metabolic wastes are
removed from the
Vagina
excretorYf.--_====~~:::)~1!~~~7~~1"~'il~~~
hemolymph
organs
called byMalpighian
tubules, which are
out-pocketings of the
digestive tract.
Tracheal tubes. Gas exchange in insects is
accomplished by a tracheal system of branched.
chitin-lined tubes that infiltrate the body and
carry oxygen directly to cells. The tracheal
system opens to the outside of the body
through spiracles, pores that can control air
flow and water loss by opening or closing .
''JI:~~
Nerve cords. The insect
nervous system
consists of a pair of
ventral nerve cords
with several
segmental ganglia.
Insect mouthparts are formed from
several pairs of modified appendages.
The mouthparts include mandibles,
which grasshoppers use for chewing.
In other insects, mouthparts are
specialized for lapping, piercing,
or sucking.
... Figure 33.35 Anatomy of a grasshopper, an insect.
Insects
Insects and their relatives (subphylum Hexapoda) are more
species-rich than all other forms of life combined. They live in
almost every terrestrial habitat and in fresh water, and flying insects fill the air. Insects are rare, though not absent, in marine
habitats, where crustaceans are the dominant arthropods. The
internal anatomy of an insect includes several complex organ
systems, which are highlighted in Figure 33.35.
The oldest insect fossils date from the Devonian period,
which began about 416 million years ago. However, when
flight evolved during the Carboniferous and Permian periods,
it spurred an explosion in insect diversity. Afossil record ofdiverse insect mouthparts indicates that specialized feeding on
gymnosperms and other Carboniferous plants also contributed to early adaptive radiations of insects. Later, a major
increase in insect diversity appears to have been stimulated by
the evolutionary expansion of flowering plants during the
mid-Cretaceous period (about 90 million years ago). Although
insect and plant diversity decreased during the Cretaceous
mass extinction, both groups rebounded over the next 65 million years. Studies indicate that rebounds of particular insect
groups often were associated with radiations of the flowering
plants on which they fed.
688
UNIT fiVE
The Evolutionary History of Biological Diversity
Flight is obviously one key to the great success of insects. An
animal that can fly can escape many predators, find food and
mates, and disperse to new habitats much faster than an animal
that must crawl about on the ground. Many insects have one or
two pairs of wings that emerge from the dorsal side of the thorax. Because the wings are extensions ofthe cuticle and not true
appendages, insects can fly without sacrificing any walking legs.
By contrast, the flying vertebrates-birds and bats-have one
of their two pairs of walking legs modified into wings, making
some of these species clumsy on the ground.
Inse<t wings may have first evolved as extensions of the cuticle that helped the inse<t body absorb heat, only later becoming organs for flight. Other hypotheses suggest that wings
allowed terrestrial insects to glide from vegetation to the
ground or that they served as gills in aquatic insects. Still another hypothesis is that insect wings functioned for swimming
before they functioned for flight.
Morphological and molecular data indicate that wings
evolved only once in insects. Dragonflies, which have two similar pairs of wings, were among the first insects to fly. Several
insect orders that evolved later than dragonflies have modified
flight equipment. The wings of bees and wasps, for instance,
are hooked together and move as a single pair. Butterfly wings
operate in a similar fashion because the anterior pair overlaps
the posterior wings. In beetles, the posterior wings function in
flight, while the anterior ones are modified as covers that prote<t the flight wings when the beetle is walking on the ground
or burrowing.
Many insects undergo metamorphosis during their development. In the incomplete metamorphosis of grasshoppers
and some other inse<t groups, the young (called nymphs) resemble adults but are smaller, have different body proportions,
and lack wings. The nymph undergoes a series of molts, each
time looking more like an adult. With the final molt, the insect
reaches full size, acquires wings, and becomes sexually mature.
Insects with complete metamorphosis have larval stages specialized for eating and growing that are known by such names
as caterpillar, maggot, or grub. The larval stage looks entirely
different from the adult stage, which is specialized for dispersal
and reproduction. Metamorphosis from the larval stage to the
adult occurs during a pupal stage (Figure 33.36).
Reproduction in insects is usually sexual, with separate male
and female individuals. Adults come together and re<ognize
each other as members of the same species by advertising with
bright colors (as in butterflies), sound (as in crickets), or odors
(as in moths). Fertilization is generally internal. In most species,
sperm are deposited directly into the female's vagina at the time
ofcopulation, though insome species the male deposits a sperm
packet outside the female, and the female picks it up. An internal structure in the female called the spermathe<a stores the
sperm, usually enough to fertilize more than one batch of eggs.
Many insects mate only once in a lifetime. After mating, a female often lays her eggs on an appropriate food source where
the next generation can begin eating as soon as it hatches.
Insects are classified in morethan 30 orders, IS ofwhich are
presented in Figure 33.37, on the next two pages.
Animals as numerous, diverse, and widespread as insects
are bound to affect the lives of most other terrestrial organisms, including humans. We depend on bees, flies, and many
other insects to pollinate our crops and orchards. On the other
hand, insects are carriers for many diseases, including African
sleeping sickness (spread by tsetse flies that carry the protist
Trypanosoma; see Figure 28.6) and malaria (spread by mosquitoes that carry the protist Plasmodium; see Figure 28.10).
Furthermore, inse<ts compete with humans for food. In parts
of Africa, for instance, insects claim about 75% of the crops. In
the United States, billions ofdollars are spent each year on pesticides, spraying crops with massive doses ofsome ofthe deadliest poisons ever invented. Try as they may, not even humans
have challenged the preeminence of insects and their arthropod kin. As Cornell University entomologist Thomas Eisner
puts it: "Bugs are not going to inherit the Earth. They own it
now. So we might as well make peace with the landlord."
(b) Pupa
(d) Emerging adult
... Figure 33.36 Metamorphosis of a butterfly. (a) The larva (caterpillar) spends its
time eating and growing. molting as it grows, (b) After several molts. the larva de~elops
into a pupa, (c) Within the pupa. the larval tissues are broken down. and the adult is built
by the di~ision and differentiation of cells that were quiescent in the larva, (d) Eventually,
the adult begins to emerge from the pupal cuticle (e) Hemolymph is pumped into veins of
the wings and then withdrawn. leaving the hardened ~eins as struts supporting the wings.
The insect will fly off and reproduce, deri~ing much of its nourishment from the food
reserves stored by the feeding lar~a.
(e) Adult
CHAPTER THIRTY·THREE
Invertebrates
689
Exploring Insect Diversity
,
Approximate
Order
Main Characteristics
Number of
Examples
Species
8lattodea
4,""
Cockroaches ha\'c a dorso\'entrally l1att~ body, with lqs
modified for rapid running. Forewings hen present, are leath·
ery, ",ilereas hind wings are fan6J.:e. F w than 40 cockroKh
§Ilt'cies moe in houses; the rest exploit habitUs r.lnging from
tro~
350.000
Coleoptera
German
cockroach
forest floors to caves and descn.s.
BcrtIcs comprise the most specks-rich Ofdcr of ins«u. Thqha\'e ''''-0 pairs of ",;ngs, one of.....hich is thick and stiff, the
other membranous. lbty h;l\'e an armomi nosktltton and
mouthPMU adapled for biting and ehe",;ng. BertIes undergo
complete metamorphosis.
1.200
DermaptcnII
151,000
Diptera
85,000
Hemiptera
Hymenopten
lsoptcra
690
UNIT
125,000
2,""
fiVE
Earwigs are generally nocturnal Ka\'mgers. Sotmo species are
'o\ingless, while others ha...e Iwo p,airs of"'ings, one of ....hich is
thick and koathery, the other membranous. EalVoigs have biting
mouthparts and large posterior pincer" They undergo incomplete metamorphosis.
Diptl"I'1UIS ha.\T one pair of ..... ings;!.he SCCOlld pair has be<:OITK'
modined into balancing organs called haltcrtS. Their mouthpam are adapll'd for sucking, piercing. or lapping. Dipll."rans
undergo complete metamorphosis. Flies and mosquitoes are
among the best·known dipterans, which live as scavengers.
predators. and parasites.
Hemipterans are so-called "true bugs; including bed bugs, u·
sassin bugs, and chinch bugs. ([nsects in other orders are some·
times erroneously called bugs.) Hemipterans have t..... o pairs of
wings, one pair partly leathery. the other membranous. They
have piercing or sucking mouthparts and undergo incomplete
metamorphosis.
Ants, bees, and wasps are generally highly social insects. They
have two pairs of membranous wings. a mobile head. and
chewing or sucking mouthparts. The females of many spe<ies
have a posterior stinging organ. Hymenopterans undergo complete metamorphosis.
Termites are Widespread social ins«ts that produce enormous
colonies. It has been estimated that there are 700 kg oftet'mites
for every per50Il on Earth! Some termites ha,'e t"'·o pairs of
membranous ....ings, .....hile others are ....ingless. They f«d on
.....ood. ....ith the aid of microbial symbionts carried in spe<ial.izcd
chambers in their hindgut.690
The Evolutionary History of Biological Diversity
Earwig
Horsefly
Leaffooted
bog
Cicad<l-killer wasp
Terrmte
Order
Approximate
Number of
Examples
Main Characteristics
Species
Lepidoptera
120.000
Butterflies and moths arc amOllg the best-known insects.
They have two J»irs of ""ings covered ....ith tiny SQlI('$. To
feed, they uncoil ill long proboscis. Most ft'rd on n«ur, but
$Orne species feed on other substan<:es. including animal
blood or tears.
Odonala
5.000
SwallowtaIl
butterfly
Dragonflies and damselflies han' two J»irs of large. mem~­
nous '>lings. They N'''' an dongakd abdomen. Large. compound ~. and cheo.ooing mouthparts. They undcrgo
incompktt metamorphosis and arc active predators.
Dragoofty
Orthopl~ra
13.000
Grasshoppers. crickcu. and their rdatin'$ alt' mostly herbr.'Orous. They have Large hind Iq;s adapted for jumping. '....0 pairs
of.... ings (one leathery, one membnnous), and biting orcbewing mouthparts. Males commonly make courtship sounds by
rubbing togelht-r body palU. such as ill ridge on their hind legs.
Orthopterans undergo incomplete mrtamorphosis.
Stick inserts and leaf insects are exquisite mimics of plants.
Phasmalodea
l"he eggs of some sjX'Cies e"en mimic seeds orthe plants on
....hich the insects live. Their body iscylindrkal or natlenro
dorsoventrally. They lack foreWings but have fanlike hind
\dngs. Their mouthparts arc adapted for biting or chewing.
Phthiraplcr<i
Siphonaplel'll
Katydid
2.400
2.400
450
Commonly called sucking lice, these insects spend their entire
life as an ectoparasite feeding on the hair or feathers of a single
host. Their legs, equipped with clawlike tarsi, are adapted for
clinging to their hosts. They lack wings and have reduced eyes.
Sucking lice undergo incomplete metamorphosis.
Stick insect
Human
body
louse
fleas are bloodsucking ectoparasites on birds and mammals.
Their body is Wingless and laterally compressed. Their legs are
modified for clinging to their hosts and for long-distance jumping. They undergo complete metamorphosis.
f~
,
~.
Flea
Silverfish arc small, Wingless insects with a nauenM body and
reduced eycs. They live inleaflittl'l" or under Inri:. Theyan
also infest buildings, whl'l"e they can become pests.
Silverfish
Trichapten
7.100
The larvae of caddistlies lin' in streams, ,,'here they make
house'S from sand gr;AiRS•..-ood fragments. or ot.hef material
held tosether by silk. Adults haw f',Hl pairs of hairy wings and
chOling or lapping mouthpalU. They undtrgo complete metamorphosis.
CH.~lU lHIUl·TllIIU
Caddisfty
Invertebrates
691
Crustaceans
\Vhjle arachnids and insects thrive on land, crustaceans, for the
most part. have remained in marine and freshwater environ-
ments. Crustlre.ms (subph~um Crustlce,) typically M" highly
specialized appendages. Lobsters and crayfIShes, for instance.
have a toolkit of 19 pairs of appendages (~Figure 33.29). The
anteriormost appendages are antennae; crustaceans are the
only arthropods with two pairs. Three or more pairs of appendages are modified as mouthparts. including the hard
mandibles. Walking legs are present on the thorax, and, unlike
insects, crustaceans also have appendages on their abdomen.
A lost appendage can be regenerated at next molt.
Small crustaceans exchange gases across thin areas of the
cuticle; larger species have gills. Nitrogenous ....'astes also diffuse through thin areas of the cuticle. but a pair of glands regulates the salt balance of the hemolymph.
Sexes are separate in most crustaceans. In the case of lobsters and crayfish. the male uses a specialized pair of abdominal appendages to transfer sperm to the reproductive pore of
the female during copulation. Most aquatic crustaceans go
through one or more swimming larval stages.
One ofthe largest groups ofcrustaceans (numbering about
IO,lXX') species) is the isopods, which include terrestrial. fresh·
water, and marine species. Some isopod species are abundant
in habitats at the bottom of the deep ocean. Among the ter·
restrial isopods are the pill bugs. or wood lice. common on the
undersides of moist logs and leaves.
Lobsters. crayfishes. crabs. and shrimpsare all relati\'t~ly large
crust:aceanscalleddecapods (Figure 33.38a). Thecuticleofdecapods is hardened by calcium carbonate; the portion that covers the dorsal side ofthe cephalothorax forms a shield called the
carapace. Most decapod species are marine. Crayfishes, however, live in fresh water, and some tropical crabs live on land.
Many small crustaceans are important members of marine
and freshwater plankton communities. Planktonic crustaceans
include many species ofcopcpods, which are among the most
numerous of all animals, as well as the shrimplike krill, which
grow to about 5 cm long (Figure 33.38b). A major food source
for baleen whales (indudingblue whales. humpbacks, and right
whales), krill are now being harvested in great numbers by humans for food and agricultural fertilizer. The larvae of many
larger-bodied crustaceans are also planktonic.
Barnacles are a group of mostly sessile austaceans ....t\ose cutide is hardened into a sheI.I containing ca.kium carbonate
(FtgUre 33.38c). Most bamades anchor tIlernseh'eS to rocks, boat
hulls, pilings, ,00 othe< sub"""ll"d...raa.. Thek nalur.l! _ .
is as strong as synthetic glues. To feed. these barnacles extend appendages from their shell to strain food from the wat:el: Bamades
were not rerogniz.ed as crustaceans until the I~ when naturoJ·
ists discovered that bamacIe larvae resemble the larvae of other
crustaceans. The remarkable mix of unique traits and crustarean
homologies fourKI. in barnacles was a major inspiration to Oiarles
Darwin as he developed his theory ofevolution.
692
UNIT fiVE
The
Evolutionary History of Biological Diversity
(a) Ghost crabs live on sandy ocean beaches worldwide. Pnmarily
nocturnal. they take shelter In burrows dunng the day_
(b) Planktonic crustaceans known
as I:::rin are consumed In vast
quantities by some whales.
(el The JOinted appendages
prOjectlng from the shells
of these barnacles capture
organISms and organic
partlCles suspended in
the water.
• Figure 33.38 Crustaceans.
CONCEPT
CHECK
33.4
I. How do nematode and annelid body plans differ?
2. In contrast to our jaws. which move up and down, the
mouthparts of arthropods move side to side. Explain
this feature ofarthropods in terms of the origin of
their mouthparts.
3. Describe two adaptations that have enabled insects to
thrive on land.
4.
i1iM Traditionally. annelids and arthropods
were thought to be closely related because both
groups show body segmentation. Yet molecular data
indicate that annelids belong to one clade
(Lophotrochozoa) and arthropods to another
(Ecdysozoa). Could traditional and molecular hypotheses be tested by studying the expression of Hox
genes that control body segmentation? Explain.
-lim
For suggested answers. see Appendix A.
-..J
Deuterostomia is defined primarily by DNA similarities-not
developmental similarities.
r;:~~::;e~~;~nd chordates are
Echinoderms
deuterostomes
C~IQlfe~ ~Md Sili(e~
!~P.;;;;;;;;! Cmdana
Lopholrochozoa
Ecdysozoa
Deuterostomia
Sea stars and most other echinoderms (from the Greek echin,
spiny, and derma, skin) are slow-moving or sessile marine animals. A thin epidermis covers an endoskeleton of hard calcareous plates. Most echinoderms are prickly from skeletal
bumps and spines. Unique to echinoderms is the water
vascular system, a network of hydraulic canals branching into
extensions called tube feet that function in locomotion, feeding, and gas exchange (Figure 33.39). Sexual reproduction of
echinoderms usually involves separate male and female individuals that release their gametes into the water.
The internal and external parts of most adult echinoderms
radiate from the center, often as five spokes. However, echinoderm larvae have bilateral symmetry. Furthermore, the symmetry of adult echinoderms is not truly radial. For example,
the opening (madreporite) ofa sea star's water vascular system
is not central but shifted to one side.
Sea stars, sea urchins, and other
e<hinoderms (phylum Echino~
dermata) may seem to have lit-
tle in common with phylum
Chordata, which includes the
vertebrates-animals that have a backbone. In fact, however,
echinoderms and chordates share features characteristic of a
deuterostome mode of development, such as radial cleavage
and formation of the mouth at the end of the embryo opposite
the blastopore (see Figure 32.9). Molecular systematics has reinforced Deuterostomia as a clade of bilaterian animals. But
molecular evidence also indicates that some animal phyla with
members that have deuterostome developmental features, including ectoprocts and brachiopods, are not in the deuterostome dade (see Chapter 32). Hence, despite its name, the clade
The surface of a sea star is
covered by spines that help
defend against predators, as
well as by small gills that
provide gas exchange.
Ashort digestive tract runs from the
mouth on the bottom of the central
disk to the anus on top of the disk.
~
Central disk. The central disk
has a nerve ring and nerve
cords radiating from the ring
into the arms.
r
Madreporite. Water
can flow in or out
of the water vascular
system into the
surrounding water
through the
madreporite.
Digestive glands secrete
digestive juices and aid in the
absorption and storage
of nutrients.
/
Radial canal. The water vascular
system consists of a ring canal in the
central disk and five radial canals,
each running in a groove down the
entire length of an arm. Branching
from each radial canal are hundreds
of hollow, muscular tube feet filled
with fluid.
Each tube foot consists of a bulb-like ampulla and a podium (foot
portion). When the ampulla squeezes. water is forced into the podium,
which expands and contacts the substrate. Adhesive chemicals are then
secreted from the base of the podium, attaching it to the substrate. To
detach the tube foot, de-adhesive chemicals are secreted and muscles
in the podium contract, forcing water back into the ampulla and
shortening the podium. As it moves, a sea star leaves an observable
Nfootprint of adhesive material on the substrate.
H
... Figure 33.39 Anatomy of a sea star, an echinoderm.
CHAPTER THIRTY·THREE
Invertebrates
693
''''33.6 Classes of Phylum Echinodermata
Class and Examples
Main Characteristics
Asteroidea (sea stars; see
Figure 33.39 and
Figure 33.4Oa)
Star.shaped body with multiple
arms; mouth directed to substrate
Ophiuroidea (brittle stars;
see Figure 33.4Ob)
Distinct central disk; long, flexible
arms; incomplete digestive system
Echinoidea (sea urchins,
sand dollars; see
Figure 33.4Oc)
Roughly spherical or disk-shaped;
no arms; five rows of tube feet,
enabling slow movement; mouth
ringed by complex, jaw-like
structure
Crinoidea (sea lilies,
feather stars; see
Figure 33.4Odl
Feathered arms surrounding
upward-pointing mouth;
suspension feeders
Holothuroidea (sea
cucumbers; see
Figure 33.4Oe)
Cucumber-shaped body; five rows
of tube feet; tube feet around the
mouth are modified as feeding
tentacles; reduced skeleton;
no spines
Concentricycloidea (sea
daisies; see Figure 33.4Of)
Armless, disk-shaped body ringed
with small spines: incomplete
digestive system; live on submerged wood
Living echinoderms are divided into six classes (Table 33.6;
Figure 33.40): Asteroidea (sea stars), Ophiuroidea (brittle
stars), Echinoidea (sea urchins and sand dollars), Crinoidea
(sea lilies and feather stars), Holothuroidea (sea cucumbers),
and Concentricycloidea (sea daisies).
Sea Stars
Sea stars have multiple arms radiating from a central disk,
the undersurfaces of which bear tube feet. By a combination
of muscular and chemical actions, the tube feet can attach to
or detach from a substrate. The sea star adheres firmly to
rocks or creeps along slowly as its tube feet extend, grip, release, extend, and grip again. Although the base of a sea star
tube foot has a flattened disk that resembles a suction cup, the
gripping action results from adhesive chemicals, not suction
(see Figure 33.39). Sea stars also use their tube feet to grasp
prey, such as clams and oysters. The arms of the sea star embrace the closed bivalve, clinging tightly with the tube feet.
The sea star then turns part of its stomach inside out, everting
it through its mouth and into the narrow opening between the
halves of the bivalve's shell. The digestive system of the sea
star secretes juices that begin digesting the soft body of the
mollusc within its own shell.
Sea stars and some other echinoderms have considerable
powers of regeneration. Sea stars can regrow lost arms, and
694
UNIT fiVE
The Evolutionary History of Biological Diversity
members of one genus can even regrow an entire body from a
single arm if part of the central disk remains attached.
Brittle Stars
Brittle stars have a distinct central disk and long, flexible arms.
They move primarily by lashing their arms in serpentine
movements. The base of a brittle star tube foot lacks the flattened disk found in sea stars but does secrete adhesive chemicals. Hence, like sea stars and other echinoderms, brittle stars
can use their tube feet to grip substrates. Some species are
suspension feeders; others are predators or scavengers.
Sea Urchins and Sand Dollars
Sea urchins and sand dollars have no arms, but they do have
five rows of tube feet that function in slow movement. Sea
urchins also have muscles that pivot their long spines, which
aid in locomotion as well as protection. The mouth of a sea
urchin is ringed by highly complex, jaw-like structures that
are well adapted to eating seaweed. Sea urchins are roughly
spherical, whereas sand donars are flat disks.
Sea Lilies and Feather Stars
Sea lilies live attached to the substrate by a stalk: feather stars
crawl about by using their long, flexible arms. Both use their
arms in suspension feeding. The arms encircle the mouth,
which is directed upward, away from the substrate. Crinoidea
is an ancient class whose evolution has been very conservative;
fossilized sea lilies some 500 million years old are extremely
similar to present-day members of the class.
Sea Cucumbers
On casual inspection, sea cucumbers do not look much like
other echinoderms. They lack spines, and their endoskeleton
is much reduced. They are also elongated in their oral-aboral
axis, giving them the shape for which they are named and further disguising their relationship to sea stars and sea urchins.
Closer examination, however, reveals that sea cucumbers
have five rows of hlbe feet. Some of the tube feet around the
mouth are developed as feeding tentacles.
Sea Daisies
Sea daisies were discovered in 1986, and only three species are
known (one near New Zealand, a second in the Bahamas, and
a third in the North Pacific). All live on submerged wood. A
sea daisy's body is armless and is typically disk-shaped; it has
a five-sided organization and measures less than a centimeter
in diameter. The edge of the body is ringed with small spines.
Sea daisies absorb nutrients through the membrane surrounding their body. The relationship of sea daisies to other
echinoderms remains unclear; some systematists consider sea
daisies the sister group of sea stars.
(a) Asea star (class Asteroidea)
(b) Abrittle star (class Ophiuroidea)
«) Asea urchin (class E(hinoidea)
(d) Afeather star (class Crinoidea)
(e) Asea cucumber (class Holothuroidea)
(f) A sea daisy (class Con(entricycloidea)
.. Figure 33.40 Echinoderms.
Chordales
Phylum Chordata consists of m'o subphyla of invertebrates as
well as the hagfishes and the vertebrates. Chordates are bilaterally symmetrical coelomates with segmented bodies. The
dose relationship between echinoderms and chordates does
not mean that one phylum evolved from the other. In fact,
echinoderms and chordates have evolved independently of
one another for at least 500 million years. We will trace the
phylogeny ofchordates in Chapter 34, focusing on the history
of vertebrates.
CONCEPT
CHECK
33.5
1. Describe how sea star tube feet attach to substrates.
2. Are characteristics shared by a sea star and a sea daisy
homologous or analogous? Explain.
3. •i,il:O'I,. The insect Drosophila me/anogasler and
the nematode Caenurhabditis e/egans are prominent
model organisms. Are these species the most appropriate invertebrates for making inferences about humans and other vertebrates in Deuterostomia? Explain.
For suggested answers. see AppendiK A.
(HAPTER THIRTY·THREE
Invertebrates
695
-N·if.• Go to the Study Area at www.masteringbio.com for BioFli~
SUMMARY OF KEY CONCEPTS
3·D Animations, MP3 Tutors, Videos, Practice Tests, an eBook, and more.
The table below summarizes the groups of animals surveyed in this chapter.
Selected Animal Phyla
Key Concept
Phylum
Description
Concept 33.1
Calcarea, Silicea
(sponges)
Lack true tissues; have choanocytes
(collar cells-flagellated cens that ingest
bacteria and tiny food particles)
Sponges are basal animals
that lack true tissues
(pp.670-671)
Concept 33.2
Cnidarians are an ancient
phylum of eumetaloans
Cnidaria (hydras. jellies,
sea anemones, corals)
(pp.671-673)
Concept 33.3
LQphotrochOloans, a clade
identified by molecular
data, have the widest range
of animal body forms
Unique stinging structures (nematocysts)
housed in specialized cells (cnidocytes);
diploblastic; radially symmetrical;
gastrovascular cavity (digestive
compartment with a single opening)
Platyhelminthes
(flatworms)
Dorsoventrally flattened, unsegmented
acoelomates; gastrovascular cavity or no
digestive tract
Rotifera (rotifers)
Pseudocoelomates with alimentary canal
(digestive tube with mouth and anus);
jaws (trophi) in pharynx; head with
ciliated crown
Lophophorates:
Ectoprocta, Brachiopoda
Coelomates with lophophores (feeding
structures bearing ciliated tentacles)
Mollusca (clams, snails,
squids)
Coelomates with three main body parts
(muscular foot, visceral mass, mantle);
coelom reduced; most have hard shell
made of calcium carbonate
Annelida (segmented
worms)
Coelomates with segmented body wall
and internal organs (except digestive
tract, which is unsegmented)
Nematoda (roundworms)
Cylindrical. unsegmented
pseudocoelomates with tapered ends;
no circulatory system; undergo ecdysis
Arthropoda (crustaceans,
insects, spiders)
Coelomates with segmented body,
jointed appendages, and exoskeleton
made of protein and chitin
Echinodermata (sea stars,
sea urchins)
Coelomates with bilaterally symmetrical
larvae and five-part body organization as
adults; unique water vascular system;
endoskeleton
Chordata (lancelets,
tunicates, vertebrates)
Coelomates with notochord; dorsal,
hollow nerve cord; pharyngeal slits;
post-anal tail (see Chapter 34)
(pp.674-682)
Concept 33.4
-"
Ec:dysoloans are the most
species.rich animal group
(pp.683-692)
In\'estigation How Ar~ Insect
Specic-< Identified?
Concept 33.5
Ec:hinoderms and chordates are deutcrostomes
(pp.693-695)
,\etl\ity Oar.cter;stics of
Invertcbrat~s
6%
UNIT fiVE
The Evolutionary History of Biological Diversity
TESTING YOUR KNOWLEDGE
SELF-QUIZ
I. \Vhich two main clades branch from the most recent common
ancestor of the eumetazoans?
a. Calcarea and $ilicea
b. Lophotrochozoa and Ecdysozoa
c. Cnidaria and Bilateria
d. Rotifera and Deuterostomia
e. Deuterostomia and Bilateria
2. A land snail, a clam. and an octopus all share
a. a mantle.
d. embryonic torsion,
b. a radula.
e. distinct cephalization.
c. gills.
3. \Vhich phylum is characterized by animals that have a segmented body?
d. Arthropoda
a. Cnidaria
b. Platyhelminthes
e. Mollusca
c. Silicea
4. Which of the following characteristics is probably most responsible for the great diversification of insects on land?
a. segmentation
d. bilateral symmetry
b. antennae
e. exoskeleton
c. eyes
5. The water vascular system of echinoderms
a. functions as a circulatory system that distributes nutrients
to body cells.
b. functions in locomotion, feeding, and gas exchange.
c. is bilateral in organization, even though the adult animal is
not bilaterally symmetrical.
d. moves water through the animal's body during suspension
feeding.
e. is analogous to the gastrovascular cavity of flatworms.
EVOLUTION CONNECTION
7. ••I;f.W"1 Draw a phylogenetic tree ofBilateria that includes
the ten phyla ofbilaterians discussed in detail in this chapter.
Label each br,lllch that leads to a phylum with a C, P, or A, depending on whether members of the phylum are coelomates
(C), pseudocoelomates (P), or acoelomates (A). Use your labeled tree to answer the following questions:
(a) For each of the three major clades of bilaterians, what (if
anything) can be inferred about whether the common
ancestor of the clade had a true coelom?
(b) To what extent has the presence of a true coelom in
animals changed over the course of evolution?
SCIENTIFIC INQUIRY
8. A marine biologist has dredged up an unknown animal from
the seafloor. Describe some of the characteristics she should
look at to determine the ph}~um to which the animal should
be assigned.
SCIENCE, TECHNOLOGY, AND SOCIETY
9. Construction of a dam and irrigation canals in an African
country has enabled fJrmers to increase the amount of food
they can grow. In the past, crops were planted only after spring
floods; the fields were too dry the rest of the year. Now fields
can be watered year-round. But improved crop yields have had
an unexpected cost-a tremendous increase in the incidence of
schistosomiasis. Imagine that your Peace Corps assignment is
to help local health officials control the disease. Reviewing the
blood fluke life cycle in Figure 33.11, why do you think the irrigation project increased the incidence of schistosomiasis? It is
difficult and expensive to control this disease with drugs. Suggest three other methods that could be tried to prevent people
from becoming infected.
6. Which of the following combinations of phylum and description is incorrect?
a. Echinodermata-bilateral symmetry as a larva, coelom
present
b. Nematoda-roundworms, pseudocoelomate
c. Cnidaria-radial symmetry, polyp and medusa body forms
d. Platyhelminthes-flatworms, gastrovascular cavity,
acoelomate
e. Calcarea-gastrovascular cavity, coelom present
For Self-Quiz tlnSWer.l, sec Appendix A.
-$14",,- Visit the Study Area at www.masteringbio.com for a
Practice Test.
CHAPTER THIRTY·THREE
Invertebrates
697