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Coelomate Invertebrates
Chapter 33
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Coelomates
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Coelomates have a new body design that
allows for the development of complex
tissues and organs.
– allows wider array of body architectures
and increased body size
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Mollusks
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Mollusks (Mollusca)
– extremely diverse
– characterized by a coelom
 great economic significance
 pearls
 mother of pearl
 economic / environmental costs
 zebra mussel invasion
 intermediate hosts for parasites
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Body Plan of the Mollusks
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Distinct bilateral symmetry
Digestive, excretory, and reproductive organs
are all concentrated in a visceral mass and a
muscular foot.
May have differentiated head
Folds constituting a mantle
– gills - increased surface area for gas
exchange
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Body Plan of the Mollusks
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Shells serve primarily for protection
Radula - rasping tongue-like organ used for
feeding
Circulatory system (except cephalopods)
consists of a heart and an open circulatory
system.
Nitrogenous wasted removed by nephridia
– nephrostome lined with cilia
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Mollusk Body Plans
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Body Plan of the Mollusks
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Reproduction in mollusks
– most have distinct male and female
individuals
– most engage in external fertilization
– many have free-swimming larvae
(trochophores) which closely resemble
larval stage of many marine annelids
 veliger stage follows trochophore stage
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Classes of Mollusks
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Polyplacophora: chitons
– oval bodies with eight overlapping
calcareous plates
Gastropoda: snails and slugs
– heads of most have pair of tentacles with
eyes at the ends
– undergo torsion during
embryological development.
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Classes of Mollusks
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Bivalvia: bivalves
– clams, scallops, mussels and oysters
– two lateral shells hinged together dorsally
– mantle secretes shell and ligaments
– most are sessile filter-feeders
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Classes of Mollusks
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Cephalopoda: octopuses, squids, nautilus
– most intelligent of the invertebrates
– active marine predators
– foot evolved into a series of tentacles
equipped with structures to capture prey
– highly developed nervous systems
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Segmented Animals
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Building of body from series of similar
segments
– small change in existing segment can
produce new kind of segment with
different function
Annelids
– three characteristics:
 repeated segments
 specialized segments
 connections
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Segmented Animals
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Body plan of the annelid
– tube within a tube
– internal digestive tract within the coelom
 specialized for different functions
– hydrostatic skeleton for locomotion
– each segment typically possesses setae,
that help anchor during locomotion
– most have closed circulatory system
– nephridia collect and transport wastes
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Classes of Annelids
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Polychaeta: polychaetes
– well developed head with specialized
sense organs
– parapodia on most segments
– usually lack permanent gonads
Oligochaeta: earthworms
– hermaphroditic
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Classes of Annelids
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Hirudinea: leeches
– occur mostly in fresh water
– hermaphroditic
– develop clitellum during breeding season
– unable to self-fertilize
– secrete anticoagulant into wounds
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Lophophorates
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Lophophore - circular or U-shaped ridge
around the mouth bearing one or two rows of
ciliated, hollow tentacles
– functions as surface for gas exchange and
as food-collection organs
– use cilia to capture food
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Jointed Appendages and Exoskeleton
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All arthropods have jointed appendages.
Rigid external skeleton (exoskeleton)
– protects animal and provides sites for
muscle attachment
 brittle, thus arthropod body size limited
due to exoskeleton thickness
– estimates of a quintillion insects alive at
any one time
 1,000,000 species
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Arthropod Body Plan
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Exoskeleton
– tough outer covering that also serves to
anchor muscles
Molting (ecdysis)
– shedding of outer cuticular layer
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Arthropod Body Plan
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Compound eye
– composed of many ommatidia
 each covered with a lens and linked to a
complex of eight retinal cells and a light
sensitive core rhabdom
– Simple eyes (ocelli) have single lenses.
 function in distinguishing light from
darkness
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The Compound Eye
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General Characteristics of Arthropods
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Circulatory system
– greatly reduced coelom
– open circulatory system
Nervous system
– double chain of segmented ganglia
running along the animal’s ventral surface
 brain appears to be inhibitor rather than
stimulator
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General Characteristics of Arthropods
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Respiratory system
– no single major respiratory organ
– small branched air ducts - tracheae
 branch into tracheoles
 air passes into trachea through
spiracles
Excretory system
– Malpighian tubules
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Crustaceans
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Most crustaceans have two pairs of
antennae, three types of chewing
appendages, and various numbers of leg
pairs.
– all pass through nauplius larval stage
– mandibles likely originated from a pair of
limbs that took on chewing function during
course of evolution
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Crustaceans
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Decapod crustaceans
– “ten footed”
 exoskeleton usually reinforced with
calcium carbonate
 most body segments are fused into
cephalothorax covered by carapace
 Lobsters and crayfish have
swimmerets and uropods to aid in
swimming, and may have a telson
(tail spine).
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Decapod Crustacean
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Crustaceans
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Terrestrial and freshwater crustaceans
– about half of estimated 4,500 species are
terrestrial and live in moist habitats
 pillbugs
 sowbugs
 isopods
Sessile crustaceans
– barnacles
 free-swimming larvae
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Class Arachnida
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Largest of three classes
– have a pair of chelicerae, a pair of
pedipalps, and four pairs of walking legs
– most are carnivorous, except for mites
– many spiders have book lungs
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Class Arachnida
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Order Araneae: spiders
– about 35,000 named species of spiders
 many do not spin webs, but actively
hunt
 have poison glands leading through
their chelicerae
 used to bite and paralyze prey
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Class Arachnida
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Order Acari: mites and ticks
– largest in terms of number of species and
most diverse of arachnids
 about 30,000 named species
 diverse in structure and habitat
 found in virtually every habitat known
 ticks can carry many diseases
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Class Chilopoda and Diplopoda
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Centipedes and millipedes
– both have bodies that consist of a head
region followed by numerous segments
 centipedes have 30+ legs
 carnivorous
 millipedes have 60+ legs
 herbivorous
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Class Insecta
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Largest group of organisms on earth
– More than half of all named species on
earth are insects.
 hectare of lowland tropical rainforest is
estimated to be inhabited by as many as
41,000 insect species
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Class Insecta
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External features
– three body segments
 head, thorax, and abdomen
– three pairs of legs
– one pair of antennae
– modified mouthparts
– solid wings
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Class Insecta
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Internal organization
– tubular digestive system
 dilute digestive enzymes
– trachea extends throughout body
– fat body for food storage
Sense receptors
– sensory hairs - linked to nerve cells
– tympanum - found with tracheal air sacs
– pheromones – communication signals
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Insect Life Histories
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Metamorphosis
– simple
 immature stages
– complete
 larvae
 pupa (chrysalis)
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Deuterostome Development
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Echinoderms
– ancient group of marine animals
consisting of about 6,000 living species
 name refers to hard, calcium-rich
endoskeleton beneath the skin
 unique water-vascular system is a fluidfilled system used to aid in movement
and feeding
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Echinoderm Body Plan
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Secondary radial symmetry
– bilaterally symmetrical during larval
development, but become radially
symmetrical as adults.
Five part body plan
Nervous system - nerve ring
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Echinoderm Body Plan
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Endoskeleton
– delicate epidermis containing thousands of
neurosensory cells
– continuous growth
– body plates often pierced to allow tube
foot extension
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Echinoderm Body Plan
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Water vascular system
– radiated from a ring canal that encircles
esophagus
 five radial canals extend into each of the
five body parts
 water enters through madreporite
 radial canals extend into the hollow
tube feet
 ampulla located at base
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Echinoderm Water-Vascular System
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Echinoderm Body Plan
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Body cavity
– coelom connects with tubular systems and
helps provide circulation and respiration
Reproduction
– many echinoderms have the ability to
regenerate
– most reproduction is sexual and external
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Class Asteroidea
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Sea stars
– abundant in intertidal zone
– important marine predators
– body composed of central disc that
merges gradually with the arms
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