Download 23.3 What Are the Major Animal Phyla?

Chapter 23
Animal Diversity I:
Invertebrates
Lecture Outlines by Gregory Ahearn,
University of North Florida
Copyright © 2011 Pearson Education Inc.
Chapter 23 At a Glance
 23.1 What Are the Key Features of Animals?
 23.2 Which Anatomical Features Mark Branch
Points on the Animal Evolutionary Tree?
 23.3 What Are the Major Animal Phyla?
Biology: Life on Earth, 9e
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23.1 What Are the Key Features of Animals?
 Animals possess all of the following
characteristics
– Multicellularity
– Their cells lack a cell wall
– They obtain energy by consuming other
organisms
– Most reproduce sexually
– They are motile at some point in the life cycle
– They are able to respond rapidly to external
stimuli
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23.2 Which Anatomical Features Mark Branch
Points on the Animal Evolutionary Tree?
 Most animal phyla that currently populate Earth
were present by the Cambrian period (544
million years ago)
– The scarcity of pre-Cambrian fossils led
systematists to search for clues about the
evolutionary history of animals by examining
features of:
–Anatomy
–Embryological development
–DNA sequences
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23.2 Which Anatomical Features Mark Branch
Points on the Animal Evolutionary Tree?
 Certain features represent evolutionary
milestones, and mark major branching points on
the animal evolutionary tree
– The appearance of tissues
– The appearance of body symmetry
– Protostome and deuterostome development
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An Evolutionary Tree of Some Major Animal Phyla
Fig. 23-1
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23.2 Which Anatomical Features Mark Branch
Points on the Animal Evolutionary Tree?
 Lack of tissues separates sponges from all other
animals
– Tissues are groups of similar cells that carry out a
specific function (e.g., muscle)
– Sponges are the only modern-day animals that lack
tissues
– Individual cells in sponges may be specialized, but
they act independently and are not organized into true
tissues
– Sponges and all remaining tissue-containing phyla arose
from an ancient common ancestor without tissues
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23.2 Which Anatomical Features Mark Branch
Points on the Animal Evolutionary Tree?
 Animals with tissues exhibit either radial or
bilateral symmetry
– Symmetrical animals have an upper (dorsal)
surface and a lower (ventral) surface
– Symmetrical animals are divided into two groups:
–Animals that exhibit radial symmetry
–Animals that exhibit bilateral symmetry
Biology: Life on Earth, 9e
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23.2 Which Anatomical Features Mark Branch
Points on the Animal Evolutionary Tree?
 Animals with radial symmetry can be divided into
roughly equal halves by any plane that passes through
the central axis
– Animals with radial symmetry have two embryonic
tissue (germ) layers
– Ectoderm, which is an outer layer that covers the
body, lines its inner cavities, and forms the nervous
system
– Endoderm, which is an inner layer that lines most
hollow organs
– Bilaterally symmetrical animals have three embryonic
tissue (germ) layers
– A layer of mesoderm between the ectoderm and
endoderm forms muscles, and the circulatory and
skeletal systems
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23.2 Which Anatomical Features Mark Branch
Points on the Animal Evolutionary Tree?
 Bilaterally symmetrical animals have heads
– Animals with bilateral symmetry can be divided
into mirror-image halves only along one plane
that runs down the midline
– These animals exhibit cephalization, the
concentration of sensory organs and a brain in a
well-defined head, with definite anterior (head)
and posterior (which may feature a tail) regions
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Body Symmetry and Cephalization
central axis
anterior
plane of
symmetry
plane of
symmetry
posterior
(a) Radial symmetry
(b) Bilateral symmetry
Fig. 23-2
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23.2 Which Anatomical Features Mark Branch
Points on the Animal Evolutionary Tree?
 Most bilateral animals have body cavities
– Body cavities are fluid-filled cavities between the
digestive tube and the outer body wall
– Body cavities have a variety of functions
–They can act as a skeleton, providing support
for the body and a framework against which
muscles can act
–They can form a protective buffer between the
internal organs and the outside world
–They can allow organs to move independently
of the body wall
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23.2 Which Anatomical Features Mark Branch
Points on the Animal Evolutionary Tree?
 Body cavity structure varies among phyla
– The most common body cavity is a coelom, a
fluid-filled body cavity that is completely lined
with mesoderm
–Phyla with animals that have this type of body
cavity are called coelomates, and include
annelids, arthropods, mollusks, echinoderms,
and chordates (which include humans)
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23.2 Which Anatomical Features Mark Branch
Points on the Animal Evolutionary Tree?
 Body cavity structure varies among phyla
(continued)
– A body cavity that is not completely surrounded
by mesoderm is known as a pseudocoelom
–Phyla with animals that have this type of body
cavity are called pseudocoelomates and
include roundworms, such as nematodes
– Some bilateral animals have no body cavity and
are known as acoelomates; they include the
flatworms
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Body Cavities
The body cavity is
completely lined with tissue
derived from mesoderm
(a) “True” coelom
(annelids, chordates)
The body cavity is partially,
but not completely, lined with
tissue derived from mesoderm
There is no cavity
between the body wall
and digestive tract
body wall
body wall
coelom
pseudocoelom
digestive
tract
digestive
tract
digestive
tract
digestive
cavity
digestive
cavity
digestive
cavity
(b) “False” or pseudocoelom
(roundworms)
body wall
(c) No coelom
(cnidarians, flatworms)
Fig. 23-3
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23.2 Which Anatomical Features Mark Branch
Points on the Animal Evolutionary Tree?
 Bilateral organisms develop in one of two ways based
on embryological development
– Protostome development
– In these animals, the body cavity forms within the
space between the body wall and the digestive cavity
– These animals include nematodes, arthropods,
annelids, and mollusks
– Deuterostome development
– In these animals, the body cavity forms as an
outgrowth of the digestive cavity
– Echinoderms and chordates are deuterostomes
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Author Animation: The Architecture of Animals
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23.2 Which Anatomical Features Mark Branch
Points on the Animal Evolutionary Tree?
 Protostomes include two distinct evolutionary lines
– Ecdysozoans
– Bodies are covered by an outer layer that is
periodically shed
– Examples include the arthropods and roundworms
– Lophotrochozoans
– This group has a lophophore (a special feeding
structure) as well as a phyla that passes through a
trochophore larva developmental stage
– Examples include mollusks, annelids, and flatworms
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23.3 What Are the Major Animal Phyla?
 Animals probably originated from ancestral
colonial protists
 Present day biologists recognize about 27 phyla
of animals
 Most animals are invertebrates (lack a
vertebral column)
 Less than 3% of all known animals are
vertebrates (possess a vertebral column)
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Table 23-1, 1 of 2
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Table 23-1, 2 of 2
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23.3 What Are the Major Animal Phyla?
 Sponges have a simple body plan
– Sponges belong to the phylum Porifera and are
found in most marine and aquatic environments
–Sponges do not move, but occur in a variety of
sizes and shapes
–They may reproduce asexually by budding,
where the adult produces miniature versions
of itself that drop off and assume an
independent existence
–They may reproduce sexually through fusion
of sperm and eggs
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The Diversity of Sponges
Fig. 23-4
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23.3 What Are the Major Animal Phyla?
 Sponges lack true tissues and organs
– The sponge body is perforated by tiny pores
through which water passes, and by fewer, large
openings through which water is expelled
– As water passes through the sponge, oxygen is
extracted, and microorganisms are filtered out
and digested by individual cells
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23.3 What Are the Major Animal Phyla?
 Sponge cells are specialized for different
functions
– Epithelial cells are flat cells that cover the outer
body surface
– Pore cells are modified epithelial cells that
regulate the flow of water through pores
– Collar cells are flagellated cells that maintain
water flow through the sponge
– Amoeboid cells are motile cells that digest and
distribute nutrients, produce reproductive cells,
and secrete skeletal projections called spicules
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The Body Plan of Sponges
(water flow out
of the sponge)
epithelial
cell
spicules
pore
(water flow into
the sponge)
amoeboid
cell
pore cell
collar cell
(water flow)
Fig. 23-5
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23.3 What Are the Major Animal Phyla?
 Some sponges contain chemicals useful to
humans
– A number of chemicals within sponges have
proved to be valuable medicines
–The drug spongistatin is an emerging
treatment for the fungal infections that sicken
AIDS patients
–Some medicines derived from sponges
include some promising new cancer drugs
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23.3 What Are the Major Animal Phyla?
 Cnidarians are well-armed predators
– Sea jellies, sea anemones, corals, and
hydrozoans belong to the phylum Cnidaria
– These animals are mostly marine and are all
carnivorous predators
– The cells of cnidarians are arranged into distinct
tissues, including a contractile muscle-like tissue
and an organized nerve net
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23.3 What Are the Major Animal Phyla?
 Cnidarians are well-armed predators (continued)
– The nerve net of cnidarians branches throughout
the body and controls the contractile tissue to
bring about movement and feeding behavior
– Most cnidarians lack true organs and have no
brain
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Cnidarian Diversity
Fig. 23-6
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23.3 What Are the Major Animal Phyla?
 Cnidarians have two body plans: the polyp and
the medusa
– Many species have life cycles that include both
body plans
– Polyps and medusae develop from two germ
layers—the endoderm and the ectoderm—with a
jelly-like substance between the layers
– The polyp is attached to rocks, while the medusa
floats in the water; both have tentacles with
cnidocytes, specialized cells that function in
defense and the capture of prey
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Polyp and Medusa
mouth
lining of
gastrovascular
cavity
lining of
gastrovascular
cavity
gastrovascular
cavity
tentacle
body wall
body wall
tentacle
gastrovascular
cavity
foot
(a) Polyp
mouth
(b) Medusa
Fig. 23-7
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23.3 What Are the Major Animal Phyla?
 Cnidarians have stinging cells called cnidocytes
that are used to capture prey and for defense
– Cnidocytes contain a finely coiled filament that is
explosively expelled when the trigger is touched
–Some filaments inject poison into the prey
–Others either stick to or entangle small prey
–The venom of some can cause extreme pain
or death in humans
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Cnidarian Weaponry: The Cnidocyte
trigger
filament
trigger
nuclei
(b) Cnidocytes
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Fig. 23-8
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23.3 What Are the Major Animal Phyla?
 Cnidarians have a gastrovascular cavity, a saclike digestive chamber with a single opening that
serves as both a mouth and an anus
– Tentacles force the prey through the opening into
the gastrovascular cavity
– Digestive enzymes secreted into this cavity
break down some of the food, and further
digestion occurs within the cells lining the cavity
– Undigested wastes are expelled from the
opening when digestion is completed
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23.3 What Are the Major Animal Phyla?
 Many corals secrete hard skeletons
– In many coral species, polyps form colonies
–Colonial polyps secrete a hard external
skeleton of calcium carbonate
–The skeleton remains after the polyp dies
–New polyps build on the skeletal remnants of
earlier generations
– Coral reefs are found in both cold and warm
oceans, and provide undersea habitats that
support a wealth of diversity
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23.3 What Are the Major Animal Phyla?
 Comb jellies belong to the phylum Ctenophora
– These animals resemble some cnidarians, but
form, instead, a distinct evolutionary lineage
– The comb jellies use cilia to move
– All comb jellies are carnivorous, eating tiny
invertebrate animals that they capture with sticky
tentacles
– Most comb jellies are hermaphroditic and can
release both eggs and sperm into seawater;
fertilized eggs gradually develop into larvae and
then adults
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23.3 What Are the Major Animal Phyla?
 Flatworms may be parasitic or free living
– Flatworms are bilaterally symmetrical and belong
to the phylum Platyhelminthes
– Many species are parasites, organisms that live
in or on the body of another organism
– Non-parasitic, free-living flatworms inhabit
aquatic, marine, and moist terrestrial habitats
– Flatworms can reproduce both sexually and
asexually; most are hermaphroditic, having
both male and female sexual organs
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23.3 What Are the Major Animal Phyla?
 Flatworms have organs but lack respiratory and
circulatory systems
– They possess a distinct head, along with sensory organs
– The eyespots of freshwater planarians detect light and
dark
– Their nervous system consist of clusters of nerve cells
called ganglia (singular: ganglion) in the head, forming a
simple brain
– They have paired nerve cords that extend the length
of the body and conduct nerve signals to and from the
ganglia
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23.3 What Are the Major Animal Phyla?
 Flatworms have organs but lack respiratory and
circulatory systems (continued)
– In the absence of a respiratory system, gas
exchange is accomplished by diffusion between
body cells and the environment
– In the absence of a circulatory system, the
digestive cavity has a branching structure that
reaches all parts of the body and allows digested
nutrients to diffuse into nearby cells
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23.3 What Are the Major Animal Phyla?
 Flatworms are bilaterally symmetrical, rather
than radially symmetrical
– This body plan is accompanied by cephalization,
where sense organs are concentrated in the
anterior portion of the body
– This enhances an animal’s ability to respond
appropriately to any stimuli that it encounters
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23.3 What Are the Major Animal Phyla?
 Some flatworms are harmful to humans
– Tapeworms can infect people who eat improperly
cooked beef, pork, or fish that has been infected
by the worms
–Tapeworm larvae form encapsulated resting
structures, called cysts, in the muscles of
these animals
–The cysts hatch in the human digestive tract,
and the young tapeworms attach themselves
to the lining of the intestine
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23.3 What Are the Major Animal Phyla?
 Some flatworms are harmful to humans
(continued)
–Tapeworms may grow to a length of more than
20 feet (7 meters), absorbing nutrients across
their outer surface
–They release eggs that are shed in the hosts
feces, continuing the infective cycle
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The Life Cycle of the Human Pork Tapeworm
1 A human eats
poorly cooked
pork with
live cysts
2 A larval
tapeworm
is liberated by
digestion and
attaches to
the human’s
intestine
adult tapeworm
6 inches
head (attachment site)
3 The tapeworm matures in a human
intestine, producing a series of
reproductive segments; each segment
contains both male and female sex organs
8 The larvae form
cysts in pig muscle
4 Eggs are shed from
the posterior end of the
worm and are passed
with human feces
5 A pig eats food
contaminated by
infected feces
7 The larvae migrate through
blood vessels to pig muscle
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6 Larvae hatch
in the pig’s intestine
Fig. 23-10
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23.3 What Are the Major Animal Phyla?
 Some flatworms are harmful to humans
(continued)
– Another group of parasitic flatworms is the flukes
–Flukes have complex life cycles that include
an intermediate host, such as a snail
–Blood flukes cause schistosomiasis, which
causes symptoms such as diarrhea, anemia,
and possible brain damage
–As many as 200 million people worldwide may
be infected with flukes
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Flatworm Diversity
Fig. 23-9
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23.3 What Are the Major Animal Phyla?
 Annelids are segmented worms and belong to
the phylum Annelida
– The annelid body is divided into a series of
repeating units (segmentation)
–The segments contain identical copies of
nerves, excretory structures, and muscles that
allows for complex movement
– Annelids have a fluid-filled coelom
–The coelom functions as a hydrostatic
skeleton, where pressurized fluid provides a
framework against which muscles can act
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23.3 What Are the Major Animal Phyla?
 Many annelids reproduce sexually
– Some species are hermaphroditic; others have separate
sexes
– During copulation, sperm are transferred from one
individual to the other
– In hermaphroditic species, sperm transfer may be
mutual, with each partner donating and receiving sperm
– Some annelids reproduce asexually by fragmentation
– The body breaks into two pieces, each of which
regenerates the missing part
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23.3 What Are the Major Animal Phyla?
 Annelids have a closed circulatory system
(blood is confined to the heart and blood
vessels)
– The circulatory system distributes gases and
nutrients throughout body
– The blood is filtered and wastes are removed by
excretory organs called nephridia
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23.3 What Are the Major Animal Phyla?
 The annelid nervous system consists of:
– A simple brain in the head
– A series of repeating paired segmental ganglia
joined by a pair of ventral nerve cords extending
the length of the body
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23.3 What Are the Major Animal Phyla?
 The annelid digestive system consists of a
tubular gut with two openings—a mouth and an
anus
 Digestion occurs in a series of compartments
–Pharynx: draws in food
–Esophagus: conducts food to crop
–Crop: stores food
–Gizzard: grinds food
–Intestine: absorbs digested nutrients
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An Annelid, The Earthworm
coelom
nephridia
intestine
excretory
pore
ventral
nerve cord
anus
coelom
brain
mouth
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pharynx
ventral
hearts esophagus crop gizzard intestine
vessel
ventral nerve cord
Fig. 23-11
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23.3 What Are the Major Animal Phyla?
 The annelid species fall into three main
subgroups
– Oligochaetes
– Polychaetes
– Leeches
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23.3 What Are the Major Animal Phyla?
 Oligochaetes
– These worms live in moist terrestrial habitats
– This group includes earthworms and its relatives
–As a result of their extensive tunneling through
soils, earthworms have major roles in aerating
the soil and mixing its organic matter, which
are favorable activities for plant growth
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23.3 What Are the Major Animal Phyla?
 Polychaetes
– Most polychaetes live in the ocean
–Some have tubes from which they project
feathery gills used for gas exchange and to
filter the water for microscopic food
–Others have segmental, paired, fleshy paddles
that are used for locomotion
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23.3 What Are the Major Animal Phyla?
 Leeches
– Leeches live in fresh water or moist terrestrial
habitats
– They are either carnivorous (prey on smaller
invertebrates) or parasitic (suck the blood of
larger animals)
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Diverse Annelids
Fig. 23-12
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23.3 What Are the Major Animal Phyla?
 Most mollusks have shells
– The phylum Mollusca consists of clams, snails, and
cephalopods
– Most mollusks have open circulatory systems (blood is
not confined to the heart and blood vessels)
– Blood percolates through a hemocoel (or blood
cavity), bathing the internal organs directly
– Mollusks have an extension of the body wall, called a
mantle, that forms a chamber for the gills and, in shelled
species, secretes the shell
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23.3 What Are the Major Animal Phyla?
 The mollusk nervous system is similar to that of
annelids (ganglia connected by ventral nerves);
however, more of the ganglia are concentrated
in the head
 Mollusks reproduce sexually
–Some species have separate sexes
–Others are hermaphroditic
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A Bivalve Mollusk
shell
heart
kidney
hinge
adductor
muscle
stomach
anus
mouth
gills
foot
gonad
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mantle
intestine
Fig. 23-13
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23.3 What Are the Major Animal Phyla?
 The three classes of mollusks are:
– Gastropods
– Bivalves
– Cephalopods
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23.3 What Are the Major Animal Phyla?
 Gastropods are one-footed crawlers
– The snails and slugs are collectively known as
gastropods
–They have a muscular foot for locomotion
–They may possess a shell, but not all
gastropods are shelled
–They feed using a radula, a flexible ribbon
studded with spines that scrape algae from
rocks or grasp larger plants or prey
–Most use their skin and gills for respiration, but
terrestrial mollusks have a simple lung
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The Diversity of Gastropod Mollusks
Fig. 23-14
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23.3 What Are the Major Animal Phyla?
 Bivalves are filter feeders
– Bivalves include scallops, oysters, mussels, and
clams
–They live in fresh water and marine habitats
–They possess two shells that can be clamped
shut by a strong muscle
–They are filter feeders and use gills for both
feeding and respiration
–Most have a muscular foot used for burrowing
or for attaching to rocks
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The Diversity of Bivalve Mollusks
Fig. 23-15
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23.3 What Are the Major Animal Phyla?
 Cephalopods are marine predators
– The cephalopods include octopuses, nautiluses,
cuttlefish, and squids
–All cephalopods are predatory carnivores, and
all are marine
–They have large, complex brains and are
capable of learning
–They possess highly developed sensory
systems
–Some may have a shell (e.g., nautiluses)
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23.3 What Are the Major Animal Phyla?
 Cephalopods are marine predators (continued)
– Cephalopods have tentacles with chemosensory
abilities and suction disks
–The tentacles are used for locomotion and to
capture prey
– These animals are able to move rapidly by
forcefully expelling water from the mantle cavity
– They possess closed circulatory systems
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The Diversity of Cephalopod Mollusks
Fig. 23-16
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23.3 What Are the Major Animal Phyla?
 Arthropods are the most diverse and abundant
animals
– The phylum Arthropoda includes insects,
arachnids, myriopods, and crustaceans
– Arthropods have appendages and an
exoskeleton (external skeleton)
–The exoskeleton is secreted by the epidermis
(the outer layer of skin)
–It is composed primarily of protein and chitin
(a polysaccharide)
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23.3 What Are the Major Animal Phyla?
 Arthropods have appendages and an
exoskeleton (continued)
– The exoskeleton:
–Protects against predators
–Provides rigid attachment sites for muscles,
but are thin and flexible at joints to increase
the range of movement of the appendages
–Provides a watertight covering
– The exoskeleton must be molted (shed)
periodically so that the animal can grow in size
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The Exoskeleton Allows Precise Movements
Fig. 23-17
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The Exoskeleton Must Be Molted
Fig. 23-18
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23.3 What Are the Major Animal Phyla?
 Arthropods have specialized segments and
adaptations for active lifestyles
 The insect body is divided into three distinct
regions, each specialized for different functions
–Head (front region): for feeding and sensing
the environment
–Thorax (middle region): the segment where
structures used for locomotion (wings, legs)
are attached
–Abdomen (rear region): contains digestive
structures
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Segments Are Fused and Specialized in Insects
antennae
head
thorax
abdomen
compound eye
mouth parts
wing
Fig. 23-19
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23.3 What Are the Major Animal Phyla?
 Arthropods have specialized segments and adaptations
for active lifestyles (continued)
 Arthropods have efficient gas exchange
– Aquatic arthropods (crustaceans) have gills (thin,
external respiratory membranes)
– Terrestrial arthropods have either:
– Lungs, such as in arachnids
– Tracheae (singular, trachea): a network of narrow,
branching respiratory tubes
 Most arthropods have an open circulatory system
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23.3 What Are the Major Animal Phyla?
 Arthropods have complex sensory and nervous
systems, responsible for finely coordinated
movement and complex behaviors
– The nervous system consists of the brain, which
is composed of fused ganglia, and a series of
additional ganglia along the length of the body
that are linked by a ventral nerve cord
– Arthropods possess well-developed sensory
structures, including compound eyes, and
chemical and tactile receptors
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Arthropods Possess Compound Eyes
Fig. 23-20
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23.3 What Are the Major Animal Phyla?
 Insects are the only flying invertebrates
– They are the most abundant and diverse class of
arthropods
– They are the only invertebrates that are capable
of flight
– The ability to fly has contributed to the enormous
success of insects
–It helps insects escape predators
–It allows insects to find widely dispersed food
– Insects use tracheae for gas exchange
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The Diversity of Insects
Fig. 23-21
Biology: Life on Earth, 9e
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23.3 What Are the Major Animal Phyla?
 Insects undergo metamorphosis, a radical
change from a juvenile body form to an adult
body form
– In many insects, the juvenile is a larva, and has
a worm-shaped body form
–Examples include maggots (housefly larva)
and caterpillars (moth or butterfly larva)
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23.3 What Are the Major Animal Phyla?
 In insects with complete metamorphosis,
developmental stages include egg  larva
(feeding stage)  pupa (nonfeeding stage) 
adult
– Houseflies, moths, and butterflies undergo
complete metamorphosis
 In insects with incomplete metamorphosis,
developmental stages include egg  nymph
(feeding stage that resembles the adult)  adult
– Grasshoppers and crickets undergo incomplete
metamorphosis
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23.3 What Are the Major Animal Phyla?
 Insects are divided into several dozen groups;
three of these groups include:
– Butterflies and moths
– Bees, ants, and wasps
– Beetles
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23.3 What Are the Major Animal Phyla?
 Butterflies and moths
– These are the most conspicuous and beststudied group of insects
– Butterflies fly during the day; moths fly at night
– The evolution of butterflies and moths has been
closely tied to the evolution of flowering plants,
which depend on these flying insects for
pollination
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23.3 What Are the Major Animal Phyla?
 Bees, ants, and wasps
– This group of insects is equipped with a stinger
that extends from the abdomen and can be used
to inject venom into the victim of a sting
– Representatives of this group (e.g., bees and
ants) have complex social behavior in which
individuals specialize in particular tasks such as
foraging, defense, reproduction, or rearing larvae
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23.3 What Are the Major Animal Phyla?
 Beetles
– One-third of all known insect species are beetles
– They have a hard, protective, exoskeletal
structure that covers their wings
– Representatives may be destructive agricultural
pests or predators that are used to control other
insect pests
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23.3 What Are the Major Animal Phyla?
 Most arachnids are predatory meat eaters
– The arachnids include spiders, mites, ticks, and
scorpions
– Arachnids lack antennae
– Arachnids have eight walking legs
– Most arachnids are carnivorous (feed on blood
or predigested prey)
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23.3 What Are the Major Animal Phyla?
 Most arachnids are predatory meat eaters (continued)
– Some arachnids, such as spiders and scorpions, inject
paralyzing venom into prey
– Arachnids breathe by using tracheae, lungs, or both
– Some have simple image-forming eyes, each with a
single lens
– In arachnids, abdominal glands produce protein threads
(silk) that is used to weave webs
– Spiders have sensory hairs that are sensitive to touch,
smell, or taste, and aid in detecting predators or prey
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The Diversity of Arachnids
Fig. 23-22
Biology: Life on Earth, 9e
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23.3 What Are the Major Animal Phyla?
 Myriapods have many legs
– The myriapods include the centipedes and
millipedes
–They have one pair of antennae
–They have simple light-detecting eyes
–They respire by means of tracheae
– Centipedes have one pair of legs per body
segment
– Millipedes have two pairs of legs per body
segment
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23.3 What Are the Major Animal Phyla?
 Myriapods (continued)
– Myriapods inhabit terrestrial environments, living
in the soil or in leaf litter
– Centipedes are usually carnivorous
–They capture prey with their frontmost legs,
and inject poison into the prey
– Millipedes feed on decaying vegetation
–They secrete a foul-smelling, distasteful liquid
when attacked
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The Diversity of Myriapods
Fig. 23-23
Biology: Life on Earth, 9e
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23.3 What Are the Major Animal Phyla?
 Most crustaceans are aquatic
– Crustaceans include crabs, crayfish, lobsters,
shrimp, and barnacles
–They possess two pairs of antennae
–Most have compound eyes
–Most respire by means of gills
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The Diversity of Crustaceans
Fig. 23-24
Biology: Life on Earth, 9e
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23.3 What Are the Major Animal Phyla?
 Roundworms are abundant and mostly tiny
– Most roundworms are microscopic, but some
parasitic forms reach a meter in length
– Most have an elongate body that is protected by
a cuticle that must be molted periodically
– They have a tubular gut with a separate mouth
and anus
– They have sensory organs that transmit
information to a simple brain
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A Freshwater Nematode
posterior
end
intestine
anterior
end
ovary
vagina
eggs
mouth
cuticle
Fig. 23-25
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23.3 What Are the Major Animal Phyla?
 Roundworms are Pseudocoelomates and have
a simplified body plan
– They lack circulatory and respiratory systems
–Gas exchange occurs by diffusion between
body cells and the environment
– They mostly reproduce sexually
–A male places sperm in the body of a female
– Most roundworms are free-living and break down
organic matter
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23.3 What Are the Major Animal Phyla?
 A few roundworms are harmful to humans
– Some roundworms are parasitic
–Hookworm larvae bore into human feet and
travel to the intestine, where they cause
continuous bleeding
–Trichinella worms infect people who eat
improperly cooked infected pork; their larvae
invade blood vessels and muscles, causing
bleeding and muscle damage
–Heartworms can be transmitted to dogs by the
bite of an infected mosquito
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Some Parasitic Nematodes
Fig. 23-26
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23.3 What Are the Major Animal Phyla?
 Echinoderms have a calcium carbonate
skeleton
 The phylum Echinodermata includes sand
dollars, sea urchins, sea stars, sea cucumbers,
and sea lilies
– Echinoderm larvae exhibit bilateral symmetry;
adults show radial symmetry
– They exhibit deuterostome development
– They possess an endoskeleton (internal
skeleton) that sends projections through the skin
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The Diversity of Echinoderms
Fig. 23-27
Biology: Life on Earth, 9e
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23.3 What Are the Major Animal Phyla?
 Echinoderms have a unique water-vascular
system
– It consists of the sieve plate, a circular central
canal, several radial canals, and numerous tube
feet
– The water-vascular system functions in
locomotion, respiration, and food capture
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The Water-Vascular System of Echinoderms
sieve plate
stomach
canals
ampulla
tube feet
(a) Starfish body plan
(b) Starfish consuming a mussel
Fig. 23-28
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23.3 What Are the Major Animal Phyla?
 Echinoderms have a primitive nervous system
with no distinct brain
– The nervous system consists of a nerve ring,
radial nerves, and a nerve network running
through the epidermis
–Sea stars have simple light and chemical
receptors
–Some brittle star species have lens-containing
light receptors
 Echinoderms lack a circulatory system
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23.3 What Are the Major Animal Phyla?
 Most echinoderms reproduce sexually
– They shed eggs and sperm into water
– Fertilization is external
 Many echinoderms are able to regenerate lost
body parts
– A sea star arm with part of the central body
attached is able to form a whole animal
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23.3 What Are the Major Animal Phyla?
 The phylum Chordata includes two invertebrate
groups (the sea squirts and the lancelets) plus
the vertebrates
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