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Ecdysozoans:
The Molting Animals
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Ecdysozoans: The Molting Animals
• Cuticles: Flexible, Unsegmented Exoskeletons
• Arthropods and Their Relatives: Segmented
External Skeletons
• Crustaceans: Diverse and Abundant
• Insects: Terrestrial Descendants of Marine
Crustaceans
• Arthropods with Two Body Regions
• Themes in the Evolution of Protostomes
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Introduction
• An exoskeleton is a firm, nonliving covering that
is difficult to penetrate and provides an animal
with both protection and support.
• Exoskeletons pose a huge problem: They cannot
grow as the animal body inside them grows.
• A solution evolved in the ancestors of the
ecdysozoans.
• They shed, or molt, the outgrown exoskeleton and
expand and harden a new, larger one.
• The presence of the exoskeleton required the
evolution of new forms of movement and
respiration.
Figure 33.1 A Current Phylogeny of the Ecdysozoans
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Cuticles: Flexible, Unsegmented Exoskeletons
• Some ecdysozoans have wormlike bodies
covered by thick and flexible exoskeletons called
cuticles.
• The three phyla with thin cuticles include the
Priapulida, Kinorhyncha, and Chaetognatha.
These phyla contain very few species.
• A thin cuticle allows the exchange of gases,
minerals, and water across the body surface, but
restricts the animal to moist habitats.
• Their bodies are supported primarily by
hydrostatic skeletons, not by their thin cuticles.
Figure 33.2 A Priapulid
Figure 33.3 An Arrow Worm
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Cuticles: Flexible, Unsegmented Exoskeletons
• Two phyla represent a lineage of ecdysozoans
that developed tough external cuticles:
Nematomorpha and Nematoda.
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Cuticles: Flexible, Unsegmented Exoskeletons
• The roundworms (phylum Nematoda) know as
the pseudocoelomates.
• They have a thick, multilayered cuticle that gives
their body its shape. A roundworm sheds its
cuticle four times as it grows.
• About 25,000 species of roundworms have been
described, but the actual number of living species
may be more than a million.
• Roundworms exchange oxygen and nutrients with
the environment through their cuticle and
intestine.
• Rhythmic contraction of the pharynx at the worm’s
anterior moves materials through the gut.
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Cuticles: Flexible, Unsegmented Exoskeletons
• The roundworms are one of the most abundant
and universally distributed animal groups.
• There are predatory and parasitic species of
roundworms.
• Some parasitic roundworms cause serious
diseases such as trichinosis.
• The life cycles of many parasitic species have
special stages that facilitate their transfer among
hosts.
Figure 33.5 Roundworms
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Arthropods and Their Relatives:
Segmented External Skeletons
• During the Precambrian era, some wormlike
ecdysozoans developed a layer of protective
material called chitin, a strong, flexible,
waterproof polysaccharide.
• This change gave their rigid body covering both
support and locomotory functions.
• A rigid body does not allow wormlike movement;
such animals require appendages that can be
manipulated by muscles.
• Appendages evolved several times late in the
Precambrian era, leading to the phyla collectively
called the arthropods.
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Arthropods and Their Relatives:
Segmented External Skeletons
• The trilobites (phylum Trilobita) were once the
dominant line of arthropods, flourishing in the
Cambrian and Ordovician seas before becoming
extinct by the Paleozoic era.
• They were heavily armored and had jointed
appendages that showed the beginnings of
specialization.
Figure 33.7 A Trilobite
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Arthropods and Their Relatives:
Segmented External Skeletons
• Arthropod appendages have evolved an amazing
variety of forms and functions, including walking
and swimming, gas exchange, food capture,
copulation, and sensory perception.
• The similarities in segmentation patterns among
arthropods arise from the actions of common
developmental genes.
• The arthropod body plan is characterized by a
rigid exoskeleton with jointed appendages.
Figure 33.8 Arthropod Exoskeletons Are Rigid and Jointed
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Arthropods and Their Relatives:
Segmented External Skeletons
• This exoskeleton made aquatic arthropods
excellent candidates for invading terrestrial
environments, which they did several times.
• We will be looking at the following classes of
arthropods: Merostomata (Horseshoe crabs);
Crustacea (shrimp, crab, lobster); Arachnida
(spiders, ticks, mites, scorpions); Insecta
(insects); Chilopoda (centipedes); Diplopoda
(millipedes)
• The arthropods are the dominant animals on
Earth, with about 1.5 million described species
and an estimated 1018 individuals.
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Class Merostomata
• Class Merostomata - Horseshoe crabs
• Notable Features:
 Common on the Atlantic coast in shallow water
on sandy or muddy shores.
 Feed chiefly on marine worms.
 Characterized by oval shell and long spinelike
tail.
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Figure 33.16 Minor Chelicerate Phyla (Part 2)
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Crustaceans: Diverse and Abundant
• The crustaceans (phylum Crustacea) are the
dominant marine arthropods.
• Members of the crustacean class Copepoda may
be the most abundant of all animals.
• Nearly all members have a body divided into three
regions:
 A head (cephalothorax) whose segments are
fused together and which bears five pairs of
appendages
 A thorax with multiple segments that usually
bear one pair of appendages each
 An abdomen with multiple segments that also
usually bear one pair of appendages each
Figure 33.10 Crustacean Structure
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Crustaceans: Diverse and Abundant
• Many species have a fold of exoskeleton, called
the carapace, that extends dorsally and laterally
back from the head to protect other segments.
• In most species, fertilized eggs are attached
outside of the female’s body.
• Some species release the young as larvae, while
others are released as juveniles similar in form to
adults.
• The crustacean lineage may have been ancestral
to all present-day arthropods.
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Class Crustacea
• Order Branchiopoda - fairy shrimp, tadpole
shrimp, clam shrimp, water fleas
• Notable Features:
 Occur in fresh water; males are uncommon in
many species and parthenogenesis is a
common mode of reproduction.
 Ferry shrimp - body elongate and distinctly
segmented without carapace and 11 pair of
swimming legs; eyes are stalked; abundant in
temporary pools
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Class Crustacea
• Class Cirripedia - Barnacles
• Notable Features:
 Adults live attached to rocks, pilings, seaweed,
boats, or marine animals.
 Usually enclosed in calcarious shells; most are
hemaphrodidic.
 Most sessile and attach to object usually by a
stalk from the shell.
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Class Crustacea
• Class malacostraca - lobsters, crayfish, shrimp,
crab, sowbug
• Notable Features:
 Most have swimmarettes on the abdomen;
usually 19 pair of appendages.
 First 13 being cephalothoracic and last 6 being
abdominal; appendages on the cephalothorax
are often chelate.
Figure 33.9 Crustacean Diversity (Part 1)
Figure 33.9 Crustacean Diversity (Part 2)
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Insects: Terrestrial Descendants
of Marine Crustaceans
• Arthropods made the leap from aquatic to
terrestrial environments during the Devonian,
more than 400 million years ago.
• The insects (phylum Hexapoda)
• We will be looking at the Class Insecta.
• Are the most prominent of the several groups that
successfully colonized the terrestrial habitat.
Insects are found in almost every freshwater and
terrestrial environment.
• About 1.4 million species of insects have been
described.
• These are believed to be only a small fraction of
the total number living on Earth.
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Insects: Terrestrial Descendants
of Marine Crustaceans
• Insects have three basic body parts:
 A head with a single pair of antennae attached
 A thorax with three pairs of legs attached
 An abdomen
• To exchange gases with the environment, insects
use air sacs and tubular channels called
tracheae.
Figure 33.11 Structure of an Insect
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Order Thysanura
• Order Thysanura - silverfish
• Notable Features: Three caudal filaments; scales
on body; very fast
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Insects: Terrestrial Descendants
of Marine Crustaceans
• Insects that exhibit gradual changes between their
instars are said to undergo incomplete
metamorphosis.
• Insects that exhibit dramatic changes between
their instars are said to undergo complete
metamorphosis.
• The process in which a caterpillar changes into a
butterfly is an example of complete
metamorphosis.
• In this process, a wormlike larva transforms itself
during a specialized phase called the pupa.
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Insects: Terrestrial Descendants
of Marine Crustaceans
• The winged insects are divided into about 29
orders, with three major recognizable lineages:
 Winged insects that cannot fold their wings
back against the body
 Winged insects that can fold their wings and
that undergo incomplete metamorphosis
 Winged insects that can fold their wings and
that undergo complete metamorphosis
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Insects: Terrestrial Descendants
of Marine Crustaceans
• The orders Odonata (dragonflies and damselflies)
and Ephemeroptera (mayflies) are the only
surviving groups of the first lineage.
• Members of these two orders have aquatic larvae
that metamorphose into adults.
• Dragonflies and damselflies are active predators
as adults.
• Adult mayflies lack functional digestive tracts and
only live long enough to mate and lay eggs.
Figure 33.13 The Diversity of Insects (Part 1)
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Order Odonata
• Order Odonata - dragonflies and damselflies
• Notable Features:
 Two pair of similar wings.
 Well developed eyes.
 Long slender abdomen.
 Damselflies are fluttery fliers, and hold wings
back when at rest; Dragonflies are strong fliers
and hold their wings purpendicular to the body
at rest
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Insects: Terrestrial Descendants
of Marine Crustaceans
• Members of the second lineage have hatchlings
that are similar in form to adults and acquire adult
organ systems gradually through their instars.
 Orthoptera (grasshoppers, crickets, roaches,
mantids)
 Isoptera (termites)
 Plecoptera (stone flies)
 Dermaptera (earwigs)
 Thysanoptera (thrips)
 Hemiptera (true bugs)
 Homoptera (aphids, cicadas, leafhoppers)
Figure 33.13 The Diversity of Insects (Part 2)
Figure 33.13 The Diversity of Insects (Part 3)
Figure 33.13 The Diversity of Insects (Part 4)
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Insects: Terrestrial Descendants
of Marine Crustaceans
• Insects in the third lineage have life stages specialized
for living in different environments and using different
food sources.
• Many species have larval stages that are specialized
for feeding and growing, and adult stages specialized
for reproduction and dispersal.
 Neuroptera (lacewings)
 Coleoptera (beetles)
 Trichoptera (caddisflies)
 Lepidoptera (butterflies and moths)
 Diptera (flies)
 Hymenoptera (sawflies, bees, wasps, ants)
Figure 33.13 The Diversity of Insects (Part 5)
Figure 33.13 The Diversity of Insects (Part 6)
Figure 33.13 The Diversity of Insects (Part 7)
Figure 33.13 The Diversity of Insects (Part 8)
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Insects: Terrestrial Descendants
of Marine Crustaceans
• “Winged” insects that do not belong in the above
three lineages include two parasitic orders: the
Phthiraptera (lice) and Siphonaptera (fleas).
• Insects in these orders are descended from flying
ancestors but have lost the ability to fly.
• Molecular data suggest that the lineage leading to
insects separated from the lineage leading to
modern crustaceans about 450 million years ago.
• The remarkable success of insects is due in part
to their wings, which arose only once during
insect evolution.
Figure 33.14 Origin of Insect Wings
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Arthropods with Two Body Regions
• Whereas insects and most crustaceans have
tripartite body plans, two arthropod lineages
evolved a body plan with two regions, a head and
a trunk.
• The phylum Myriapoda includes the centipedes
(Chilopoda) and millipedes (Diplopoda).
• Centipedes have one pair of legs per trunk
segment; millipedes have two pairs of legs per
segment.
• More than 3,000 centipede species and 10,000
millipede species have been described.
Figure 33.15 Myriapods
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Arthropods with Two Body Regions
• There are 63,000 described species in the phylum
Chelicerata, collectively referred to as the
chelicerates.
• Their bodies are divided into two major regions
• The anterior region bears two pairs of
appendages, modified to form mouthparts.
• Many chelicerates also have four pairs of walking
legs.
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Arthropods with Two Body Regions
• Chelicerate species are usually placed in three
classes: Pycnogonida, Merostomata, and
Arachnida.
Figure 33.16 Minor Chelicerate Phyla (Part 1)
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Class Arachnida
• Class Arachnida - spiders and their allies
• Notable Features: Cephalothorax and abdomen;
no antennae; 10 pair of appendages, 4 pair of
legs, one pair of pedipalpi, one pair of chelicerae;
book lungs or trachea
• Orders within the Class Arachnida
• Order Scorpiones - scorpions
• Notable Features: posterior terminates in a sting;
pedipalps are long and chelate; usually nocturnal
and remain concealed during the day; feed on
insects and spiders
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Class Arachnida
• Order Araneae - true spiders
• Notable Features: abdomen is unsegmented and
attached to the cephalothorax by a slender
pedicel; eyes are simple; spiders have poison
glands, and ducts from these glands open near
the tips of the chelicerae; pedipalps, located
behind the chelicerae in front of the legs, are
leglike or palplike; posterior end of the abdomen
on the ventral side, are six finger-like structures
called spinnerets
• Order Opiliones - Harvestman spiders
• Notable Features: body rounded or oval; generally
two eyes; second pair of legs longest
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Class Arachnida
• Order Acari - mites and ticks
• Notable Features: body oval and little
differentiation between the two body regions;
many are parasitic on vertebrates, invertebrates,
and plants; many are scavengers
• Order Pseudoscorpiones - pseeudoscorpions
• Notable Features: seldom more than 5 mm in
length; resemble true scorpions having large
chelate pedipalps, but their is no sting; body is
quite flat; feed on small insects which are caught
with pedipalps
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Class Arachnida
• Order Solfugae - windscorpions, sunscorpions
• Notable Features: arid or desert regions of the
west; body slightly constricted in the middle; Very
large chelicerae, but no venom glands; fast
running and predacious, can even capture small
lizards.
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Arthropods with Two Body Regions
• The spiders, scorpions, mites, and ticks are the
most species-rich and abundant members of the
class Arachnida.
• The 30,000 described species of mites and ticks
live in a variety of environments, including as
parasites of plants, vertebrates, and invertebrates.
Figure 33.17 Arachnid Diversity (Part 1)
Figure 33.17 Arachnid Diversity (Part 2)
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Arthropods with Two Body Regions
• Spiders are important terrestrial predators.
• Spiders produce protein threads from which they
spin webs to snare prey and which they use to
construct homes, safety lines for climbing, for
mating structures, protection of developing young,
and for dispersal.
Figure 33.17 Arachnid Diversity (Part 3)
Figure 33.17 Arachnid Diversity (Part 4)
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Themes in the Evolution of Protostomes
• Most protostome evolution took place in the
oceans.
• Some protostome lineages gradually evolved the
ability to change their shape in complex ways and
to move with greater speed as a result of
subdivided body cavities that allowed better
control of movement.
• Many different lineages of animals evolved
feeding structures designed to extract small prey
from the water, most likely because this was the
only type of food available during much of animal
evolution.
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Themes in the Evolution of Protostomes
• Flowing water brings food with it. Thus, the
sessile lifestyle evolved repeatedly during
lophotrochozoan and ecdysozoan evolution.
• The competition for space is a consequence of
the sessile lifestyle, and many sessile animals
have evolved mechanisms for overgrowing one
another and engaging in toxic warfare when they
come into contact.
• Members of sessile colonies that are directly
connected can share resources, an ability that
enables some individuals to specialize for
particular functions.
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Themes in the Evolution of Protostomes
• External body coverings probably evolved as a
result of the selective pressures resulting from
predation.
• Body coverings evolved independently in many
lophotrochozoan and ecdysozoan lineages.
• The evolution of animals is, in one sense, a
complex arms race among predators and prey,
with each side developing structures and
behaviors to deal with the selective pressures
applied by the other.
• Although greater complexity has evolved in many
animal lineages, several lineages that have
remained simple have been very successful.
Table 33.1 Anatomical Characteristics of the Major Protostomate Phyla (Part 1)
Table 33.1 Anatomical Characteristics of the Major Protostomate Phyla (Part 2)