Trilobites, Chelicerates,
and Myriapods
Chapter 19
Phylum Arthropoda
 Two out of every
three known
species of animals
are arthropods.
 Members of the
phylum
Arthropoda are
found in nearly all
habitats of the
biosphere.
Phylum Arthropoda
 Arthropods are:





Multicellular
Bilaterally symmetrical
Triploblastic
Have a true coelom (protostomes)
Segmented
General Characteristics of
Arthropods
 The diversity and success of arthropods are largely
related to their segmentation, hard exoskeleton
(made of chitin), and jointed appendages.
General Characteristics of
Arthropods
 Segments have combined into functional groups called
tagmata.
 Tagmata have specialized purposes.
General Characteristics of
Arthropods
 As arthropods evolved, the segments fused, and the
appendages became more specialized.
 The appendages of some living arthropods are
modified for many different functions.
General Characteristics of
Arthropods
 Arthropods have an open circulatory system in which
fluid called hemolymph is circulated into the spaces
surrounding the tissues and organs.
 A variety of organs specialized for gas exchange have
evolved in arthropods.
A Versatile Exoskeleton
 The exoskeleton of
arthropods is very
protective, but still
flexible.
 The exoskeleton is
made of chitin.
 Prevents desiccation.
 Provides places for muscle attachment.
 Does not allow for growth, the outer covering must
be molted – ecdysis.
More Efficient Locomotion
 Usually, each segment bears a pair of jointed
appendages.
 The appendages have sensory hairs and may be
modified for sensory functions, food handling, or
walking & swimming.
Air Piped Directly to Cells
 Most terrestrial arthropods have an efficient tracheal
system of air tubes, which delivers oxygen directly to
the tissues and cells.
 Limits body size.
 Aquatic arthropods breathe using internal or external gills.
Highly Developed Sensory Organs
 Arthropods have a
variety of sensory
organs.
Complex Behavior Patterns
 Arthropods show
complex behavior
patterns.
 Mostly innate
behaviors.
 Some learned.
Metamorphosis
 Intraspecific
competition
(between members
of one species) is
reduced because of
metamorphosis.
 Larval forms may
be quite different
from adults.
Relationships Among Arthropod
Subgroups
 Clade Panarthropoda, Phylum Arthropoda
 Divided into subphyla based on relationships
between subgroups based on molecular data.
Relationships Among Arthropod
Subgroups
 Centipedes,
millipedes,
pauropods, and
symphylans are
placed in subphylum
Myriapoda.
 Insects are placed in
subphylum
Hexapoda.
 Spiders, ticks, horseshoe crabs and their relatives form
subphylum Chelicerata.
 Lobsters, crabs, barnacles, and others form
subphylum Crustacea.
Relationships Among Arthropod
Subgroups
 Formerly, insects and
myriapods were placed
together in uniramia.
 Uniramous
appendages.
 The “mandibulate
hypothesis” suggests all
arthropods with
mandibles are more
closely related to each
other than to arthropods
with chelicerae.
 Mandibles in each group may or may not be
homologous.
 mtDNA data support this hypothesis.
Subphylum Trilobita
 Early arthropods, such as trilobites showed
little variation from segment to segment.
Subphylum Trilobita
 Trilobites arose
during the Cambrian
– maybe earlier and
lasted for 300 million
years.
Subphylum Trilobita
 Trilobites had a trilobed shape.
 Three tagmata:
 Head (cephalon) with a mouth, compound eyes,
antennae, and 4 pairs of leglike appendages.
 Trunk with a variable number of segments each with a
pair of biramous appendages.
 One of the branches of biramous appendage was fringed
and may have been a gill.
 Pygidium – segments fused into a plate.
Subphylum Trilobita
 Most could roll up like pill bugs.
 Probably benthic scavengers.
 Many (especially later species) had large, complex,
many-faceted eyes.
Subphylum Chelicerata
 Chelicerate arthropods include eurypterids, horseshoe
crabs, spiders, ticks, mites, scorpions, & sea spiders.
Subphylum Chelicerata
 They have 6
pairs of
cephalothoracic
appendages:
 Chelicerae
(mouthparts)
 Pedipalps
 4 pairs of
walking legs
 Lack mandibles
and antennae.
Class Merostomata
 Class Merostomata
includes the eurypterids
and horseshoe crabs.
 Eurypterids were giant
water scorpions up to 3 m
in length.
 Cambrian through
Permian.
 Predators, some with large
crushing claws.
Class Merostomata
 Three genera of
horseshoe crabs
live today.
 Limulus, found in
North America,
has existed on
earth almost
unchanged since
the Triassic
period.
Class Merostomata
 Horseshoe crabs have an unsegmented
carapace (hard dorsal shield), a broad
abdomen, and a long telson (tail piece).
 Cephalothorax
 Chelicerae
 Pedipalps
 4 pairs walking legs
 Abdomen
 6 pairs of thin appendages
 Book gills found on 5.
Class Merostomata
 Horseshoe crabs have simple and compound eyes.
 Feed at night on worms and small molluscs.
 Come to shore in large numbers to mate at high tide.
 Trilobite larvae resemble trilobites.
Class Pycnogonida
 Sea spiders, class
Pycnogonida, have
small, thin bodies and
usually 4 pairs of
walking legs.
 Found in all oceans,
most common in polar
seas.
 Some have chelicerae
and pedipalps.
Class Arachnida
 Class Arachnida includes spiders, scorpions, mites,
and ticks.
50 µm
Class Arachnida
 Two tagmata:
 Cephalothorax
 Chelicerae
 Pedipalps
 4 pairs walking legs
 Abdomen
Class Arachnida - Order Araneae
 Most spiders – order Araneae
– have 8 simple eyes that can
detect light and motion.
 Some hunting & jumping
spiders may form images.
 Sensory setae detect air
currents, web vibrations, and
other stimuli.
 Spider’s vision usually poor,
so awareness of environment
depends largely on cuticular
mechanoreceptors such as
sensory setae.
Class Arachnida - Order Araneae
 All are predaceous,
mostly on insects.
 Many spin a web used
for prey capture.
 Some chase & catch
prey.
 Injected venom
liquefies and digests
the tissues which is
sucked into spider’s
stomach.
Class Arachnida - Order Araneae
 Two or three pairs of
spinnerets contain
microscopic tubes that run
to silk glands.
 Liquid scleroprotein
secretion hardens as it is
extruded from spinnerets.
 Silk threads are very
strong and will stretch
considerably before
breaking.
 Silk is used for orb webs,
lining burrows, forming
egg sacs, and wrapping
prey.
Class Arachnida - Order Araneae
 Breathe by book lungs and/or tracheae.
 Book lungs unique to spiders - parallel air pockets extend into
blood-filled chamber.
 Air enters chamber through a slit in body wall.
 Tracheae system is less extensive than in insects.
 Transports air directly to tissues.
 Tracheal systems of arthropods represent a case of evolutionary
convergence.
Class Arachnida - Order Araneae
 In spiders and insects, Malpighian tubules
serve as excretory structures.
 Potassium, other solutes, and waste are secreted
into tubules.
 Rectal glands reabsorb the potassium and water,
leaving wastes and uric acid for excretion.
 Conserves water and allows the organisms to live in
dry environments.
 Many spiders have coxal glands, modified
nephridia, at the base of legs.
Class Arachnida - Order Araneae
 Reproduction - before mating, male stores
sperm in pedipalps.
 A courtship ritual is often required before the female
will allow mating.
 Eggs may develop in a cocoon in the web or may be
carried by female.
 Young hatch in about two weeks and may molt
before leaving the egg cocoon.
Class Arachnida - Order Araneae
 Are spiders really dangerous?
 Spiders are allies of humans in our battle with insects.
 American tarantulas rarely bite, and bite is not dangerous.
 Species of black widow spiders are dangerous.
 Venom is neurotoxic.
 Brown recluse spider has hemolytic venom that destroys tissue
around the bite.
 Some Australian and South American spiders are the most
dangerous and aggressive.
Class Arachnida - Order Scorpiones
 Scorpions – order
Scorpiones – feed on
insects & spiders which
they seize with their
pedipalps.
 The last segment
contains a bulbous base
and a curved barb that
injects venom.
 Scorpions are viviparous
or ovoviviparous –
females brood young
within their reproductive
tract.
Class Arachnida - Order Opiliones
 Harvestmen – order
Opiliones – differ from
spiders in that the
abdomen and
cephalothorax are
broadly joined rather
than constricted.
 Only two eyes
 Abdomen shows
segmentation
 Long legs end in tiny
claws.
Class Arachnida - Order Acari
 Mites and ticks – order Acari – have a fused
cephalothorax & abdomen.
 Mites are tiny – 1mm or less.
 Some feed on plant juices and can be major pests.
 Several species of ticks carry diseases such as
Lyme disease.
Class Arachnida - Order Acari
 House dust mites free-living and often
cause allergies.
 Spider mites - one of
many important
agricultural pest
mites that suck out
plant nutrients.
Class Arachnida - Order Acari
 Hair follicle mite
Demodex - harmless
but other species
cause mange in
domestic animals.
 Human itch mite
causes intense
itching.
Class Arachnida - Order Acari
 Tick species of
Ixodes carry Lyme
disease.
 Tick species of
Dermacentor
transmit Rocky
Mountain spotted
fever.
 Cattle tick transmits
Texas cattle fever.
Subphylum Myriapoda
 Subphylum
Myriapoda includes
these classes:
 Chilopoda
(centipedes)
 Diplopoda (millipedes)
 Pauropoda
(pauropods)
 Symphyla
(symphylans)
 Use trachea to transport air.
 Excretion usually by Malpighian tubules.
Class Chilopoda
 Centipedes – class
Chilopoda – contain
a few or many
segments each
(except the first
behind the head and
the last two) with a
pair of jointed legs.
 Last pair of legs has
a sensory function.
Class Chilopoda
 Head appendages:
 One pair antennae
 One pair mandibles
 One or two pairs of maxillae.
 Dorsoventrally flattened.
Class Chilopoda
 Sexes are separate.
 Some lay eggs (oviparous).
 Some have live young (viviparous).
 Young like little adults – no metamorphosis.
Class Chilopoda
 Centipedes live in moist environments.
 They are carnivores, feeding on insects &
worms.
 Prey is killed with poison claws on the first
segment.
Class Diplopoda
 Millipedes (Class
Diplopoda) have two
pairs of legs on each
segment.
 Head has one pair
each of antennae,
mandibles, &
maxillae.
 Body is more
cylindrical.
Class Diplopoda
 Millipedes live in dark, moist places – under
rocks or logs.
 Most are herbivores, feeding on decayed plant
matter or occasionally living plants.
 Slow moving, coil up when disturbed.
 Toxic or repellent fluids secreted when
disturbed.
Class Diplopoda
 After copulation,
female lays eggs in
a nest and guards
them.
 Larvae have only
one pair of legs per
segment.
Class Pauropoda
 Live in moist soil, leaf litter, decaying vegetation, or
under bark and debris.
 Least well known of myriapods.
 Soft-bodied, small (2 mm or less).
 Approximately 500 species.
 Head lacks true eyes, has branched antennae, and a
pair of sense organs.
Class Pauropoda
 12 trunks segments
bear 9 pairs of legs
but none on the first
or last 2 segments.
 One tergal plate
covers two segments.
 Lack tracheae,
spiracles, and
circulatory system.
 Probably most closely
related to diplopods.
Class Symphyla
 Live in humus, leaf mold,
and debris.
 Male Scutigerella places a
spermatophore at end of a
stalk.
 Female stores the sperm
in special pouches.
 Removes and smears
eggs with sperm before
attaching them to moss or
lichen.
 Young hatch with only 6 or
7 pairs of legs.
Class Symphyla
 Small (2–10 mm) with centipede-like bodies.
 Soft-bodied with 14 segments - 12 segments bear
legs and one bears a pair of spinnerets.
 Antennae are long and unbranched.
 About 160 species are known.
 Eyeless with sensory pits at base of antennae.
Phylogeny
 Relationships between subphyla are debated.
 Taxon of Pancrustacea, which includes
hexapods and crustaceans, is well-supported.
 Phylogenies using molecular data rarely
support grouping Myriapoda with
Pancrustacea.
 There is support for placement of Myriapoda as
the sister taxon for Cheliceratae.
Phylogeny
 Biologists assume that the ancestral arthropod
had a segmented body with one pair of legs per
segment.
 Evolution caused adjacent segments to fuse
and to make body regions.
 Hox gene studies indicate that the first five
segments fused to form the head tagma in all
four extant subphyla.
 In spiders, Hox gene studies indicate that the
entire prosoma corresponds to the head of
other arthropods.
Phylogeny
 Genetic studies have been helpful in
understanding the evolution of uniramous and
biramous appendages.
 Molecular evidence repeatedly places hexapods with
crustaceans even though hexapods have uniramous
appendages and crustaceans have biramous
appendages.
 Leads to the question: Did uniramous
appendage development evolve more than
once?
Adaptive Diversification
 In contrast to annelids, arthropods have
pronounced tagmatization by fusion of somites.
 Those with primitive characters have appendages
on each somite.
 Derived forms are specialized.
 Modification of exoskeleton and appendages
allowed variation in feeding and movement.
 Adaptations made possible by cuticular
exoskeleton and small size fostered high diversity.
Phylogeny
 Subphylum Trilobita
 Subphylum Chelicerata
 Class Merostomata
 Class Pycnogonida
 Class Arachnida
 Subphylum Myriapoda
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
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Class Diplopoda
Class Chilopoda
Class Pauropoda
Class Symphyla
 Subphylum Crustacea
 Subphylum Hexapoda