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Coelomate Invertebrates
Chapter 34
Introduction
Coelomates have a body design that:
1. Repositions the body’s fluid
2. Allows complex tissues/organs to develop
3. Allows for a larger body size
Coeloms evolved multiple times during animal
evolution
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Phylum Mollusca
Mollusks are second in diversity only to
arthropods
-Exhibit a wide variety of sizes and body
forms
-Live in many different environments
-Include snails, slugs, clams, octopuses and
others
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Phylum Mollusca
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Phylum Mollusca (Cont.)
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Phylum Mollusca
Mollusks evolved in the oceans and most
groups have remained there
They are an important source of human food
They are economically significant in other ways
-Pearls are produced in oysters
-Mother-of-pearl is produced in the shells of
abalone
Mollusks can also be pests
-Zebra mussel (Dreissena polymorpha) 6
Mollusk Body Plan
Mollusks are bilaterally symmetrical
-Characterized by a reduced coelom
surrounding the heart and excretory organs
The digestive, excretory and reproductive
organs are concentrated in a visceral mass
Mollusks may have a differentiated head at the
anterior end of the body
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Mollusk Body Plan
Mantle = A thick epidermis that covers the
dorsal side of the body
-Forms a cavity which houses the respiratory
organs (ctenidia, or gills) and the openings
of excretory, reproductive & digestive organs
The muscular foot of a mollusk is adapted for
locomotion, attachment, food capture
-Or a combination of the above
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Mollusk Body Plan
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Mollusk Body Plan (Cont.)
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Mollusk Body Plan
Most mollusks produce an external calcium
carbonate-rich shell
-Used for protection
-Some species have internalized or
reduced shells
Most mollusks have a rasping tongue-like
organ called the radula
-Used for feeding
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Mollusk Body Plan
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Mollusk Body Plan
Nephridia = Special excretory structures that
remove nitrogenous wastes
-Consist of cilia-lined openings called
nephrostomes
Except for cephalopods, all mollusks have an
open circulatory system
Cephalopods have a closed circulatory
system
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Mollusk Reproduction
Most mollusks have distinct male and female
individuals
Most engage in external fertilization
In marine mollusks, embryos develop through
spiral cleavage
-Trochophores: Free-swimming larval stage
-Second free-swimming larval stage
-Only in bivalves and snails
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Mollusk Reproduction
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Classes of Mollusks
There are eight recognized classes
-Four are representatives of the phylum
1. Polyplacophora
2. Gastropoda
3. Bivalvia
4. Cephalopoda
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Classes of Mollusks
Class Polyplacophora (Chitons)
-Marine mollusks that have oval bodies with
eight overlapping dorsal calcareous plates
-Body is not
segmented under
the plates
-Most chitons are
grazing herbivores
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Classes of Mollusks
Class Gastropoda (Snails and slugs)
-A primarily marine group
-Heads typically have pairs of tentacles with
eyes at the ends
-During embryological development,
gastropods undergo
1. Torsion – Mantle cavity and anus are
moved from the posterior to the front
2. Coiling – Spiral winding of the shell
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Classes of Mollusks
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Classes of Mollusks
Class Bivalvia (Bivalves)
-Includes clams, scallops, mussels, oysters
and others
-Have two lateral (right and left) shells
(valves) hinged together dorsally
-Most are sessile filter-feeders
-Water circulation is mediated by
siphons and rhythmic beating of cilia on
gills
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Classes of Mollusks
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Classes of Mollusks
Class Cephalopoda
-Active marine predators
-Foot has evolved into a series of arms
equipped with suction cups
-Squids have 10; octopuses, 8; and
nautiluses, 80 to 90
-Have highly developed nervous systems
-Exhibit complex patterns of behavior
and a high level of intelligence
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Classes of Mollusks
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Classes of Mollusks
Class Cephalopoda
-Many have an ink sac and are capable of
expelling ink to confuse predators
-Octopuses and
squids can change
color using pouches
of pigment called
chromatophores
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Phylum Annelida
Annelid worms exhibit segmentation
(building of body from repeated units)
-Allows for specialization
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Phylum Annelida
An annelid consists of a series of ring-like
elements running the length of the body
-Divided internally by septa
The anterior (front) segments contain
specialized sensory organs
A ventral nerve cord connects the ganglia in
each segment with each other and the brain
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Phylum Annelida
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Phylum Annelida
Annelids move using their hydrostatic skeleton
-Each segment contains chitin bristles
(chaetae) that help anchor the worms
Most have a closed circulatory system
They exchange O2 and CO2 through their
body surfaces
Excretory system consists of a pair of ciliated,
funnel-shaped nephridia per segment
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Phylum Annelida
Annelids have traditionally been classified into
three classes:
1. Class Polychaeta
2. Class Oligochaeta
3. Class Hirudinea
Hirudinea is now considered a sub-class of
Oligochaeta
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Phylum Annelida
Class Polychaeta (Polychaetes)
-Mostly marine worms, such as tubeworms
-Unusual forms and iridescent colors
-Have a differentiated head
-Have paired parapodia on most segments
-Used in swimming, burrowing, crawling
-Sexes are usually separate
-Typically lack permanent gonads
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Phylum Annelida
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Phylum Annelida
Class Oligochaeta (Earthworms and leeches)
-Mostly terrestrial
Earthworms
-Consist of 100-175 segments, with a mouth
on the first and an anus on the last
-Lack eyes, parapodia and head
-Have fewer setae than polychaetes
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Phylum Annelida
Earthworms are hermaphroditic
-Clitellum secretes mucus that holds the
worms together during copulation
-Also secretes a
mucus cocoon, in
which the fertilized
egg develops
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Phylum Annelida
Leeches
-Occur mostly in freshwater
-Usually flattened dorsoventrally
-Cross-fertilization is obligatory
-Have no chaetae (except for one species)
-Medicinal leech,
Hirudo medicinalis
-Secretes
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anticoagulant
The Lophophorates
Include two phyla of mostly marine animals:
Bryozoa and Brachiopoda
Both convergently evolved a lophophore
-A circular or U-shaped ridge around the
mouth with 1-2 rows of ciliated tentacles
-Functions in gas exchange and feeding
Most members undergo radial cleavage
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Phylum Bryozoa
Bryozoans are small and live in colonies
-Their anus opens near their mouth
-Asexual reproduction occurs frequently by
budding
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Phylum Bryozoa
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Phylum Bryozoa (Cont.)
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Phylum Brachiopoda
Brachiopods have two calcified shells
-Valves are dorsal and ventral (not lateral as
in bivalves)
-Solitary lophophorates
-Each individual secretes a chitinous tube
and lives out its life within it
-Develop as protostomes (unlike
brachiopods)
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Phylum Arthropoda
Arthropods are the most successful animals
-2/3rd of all species are insects
-For each human, 200 million insects
are alive at any one time
Are divided into four major classes:
-Arachnids, myriapods, crustaceans and
insects
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Arthropod Morphology
All arthropods have jointed appendages
-Modifications: antennae, mouthparts, legs
Arthropods also have an exoskeleton made
of secreted chitin and protein
-Functions: protection & muscle attachment
-Its thickness limits arthropod body size
Arthropod bodies are segmented
-Some segments are specialized into
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functional groups, or tagmata
Arthropod Morphology
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Arthropod Morphology
Arthropods have an open circulatory system
Compound eyes are found in many
arthropods
-Composed of independent visual units
called ommatidia
Other arthropods have simple eyes, or ocelli
-Have single lenses
-Distinguish light from darkness
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Arthropod Morphology
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Arthropod Morphology
Nervous system consists of a double chain
of segmented ganglia on ventral surface
-Brain seems to be an inhibitor, rather than
as a stimulator, as it is in vertebrates
Respiratory system consists of tracheae
and tracheoles
-Connected to the exterior by spiracles
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Arthropod Morphology
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Arthropod Morphology
Arthropods have a unique excretory system
consisting of Malpighian tubules
-Eliminates nitrogenous wastes as
concentrated uric acid or guanine
Arthropods periodically undergo molting
-Shedding the outer cuticular layer
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Arthropod Morphology
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Class Arachnida
Arachnids are largely terrestrial organisms
-Spiders, ticks, mites, scorpions, and others
The most anterior appendages, chelicerae,
often function as fangs or pincers
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Class Arachnida
Pedipalps (palps) are posterior to chelicerae
-Resemble legs, but have one less segment
-Used as copulatory organs, pincers, or
sensors
Most arachnids are carnivorous
-Mites are largely herbivorous
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Class Arachnida
Order Araneae (spiders)
-About 35,000 species
-Many spiders catch their prey in silk webs
-Silk protein forced out of spinnerets
found on the posterior of the abdomen
-Other spiders actively hunt their prey
-All spiders have poison glands leading
through their chelicerae
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Class Arachnida
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Class Arachnida
Order Acari (mites and ticks)
-Largest and most diverse arachnid order
-Most mites are small
-Cephalothorax and abdomen are fused
into an unsegmented ovoid body
-Ticks are larger than mites
-Are blood-eating parasites
-Can carry many diseases (spotted
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fevers, Lyme disease)
Centipedes and Millipedes
Centipedes (class Chilopoda) and
Millipedes (class Diplopoda) have bodies
with a head followed by numerous segments
Centipedes are all carnivores (eat insects)
Millipedes are largely herbivores
In both fertilization is internal
-The sexes are separate
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Centipedes and Millipedes
Centipedes have fewer legs than millipedes
-Centipedes: one leg pair on each segment
-Millipedes: two on some or all segments
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Class Crustacea
Crustaceans are primarily aquatic organisms
-Crabs, shrimps, lobsters, barnacles, others
Have three tagmata
-The two most anterior fuse to form a
cephalothorax
Have two pairs of antennae, three pairs of
appendages, and various pairs of legs
Most appendages are biramous
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Class Crustacea
Mandibles (biting jaws) likely evolved from a
pair of limbs that took on a chewing function
Most crustaceans have
separate sexes
Majority develop through
a nauplius stage
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Class Crustacea
Decapod crustaceans include shrimps,
lobsters, crabs and crayfish
-Have ten feet
-Exoskeleton usually enforced with CaCO3
-Most body segments are fused into a
cephalothorax
-Lobsters and crayfish have appendages
that aid in swimming
-Swimmerets and uropods
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Class Crustacea
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Class Crustacea
Order Cirripedia
-Barnacles are crustaceans that are sessile
as adults
-Free-swimming
larvae
-Are hermaphroditic
-Some have stalks
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Class Hexapoda
Insects are by far the largest group of animals
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Class Hexapoda
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Class Hexapoda (Cont.)
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Class Hexapoda (Cont.)
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Class Hexapoda
External features
-Three body regions
1. Head = Has pair of antennae and
modified mouthparts
2. Thorax = Has three segments, each with
a pair of legs
-May have one or two pairs of wings
3. Abdomen
Most insects have compound eyes
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Class Hexapoda
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Class Hexapoda
Internal organization
-The digestive tract is a coiled tube
-Lined with cuticle on anterior and
posterior regions
-Digestion takes place in stomach (midgut)
-Excretion tales place through Malpighian
tubules
-Tracheae permeate all tissues
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Class Hexapoda
Sensory receptors
-Sensory setae are hair-like structures
-Detect chemical and mechanical signals
-Tympanal organs are composed of a thin
membrane, the tympanum
-Detect sound
-In addition, insects can communicate by
means of pheromones
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Class Hexapoda
Insect life histories
-Many insects undergo metamorphosis
-Simple metamorphosis (grasshopers)
-Immature stages similar to adults
-Complete metamorphosis (butterflies)
-Immature larva are wormlike
-A resting stage, pupa or chrysalis,
precedes the final molt into adult form
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Phylum Echinodermata
Echinoderms are an ancient group of marine
animals
-Characterized by deuterostome
development and an endoskeleton
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Echinoderm Body Plan
The echinoderm body plan undergoes a
fundamental shift during development
-Larvae = bilateral symmetry
-Adults = pentaradial symmetry
Body structure is discussed in reference to
their mouths, which define the oral surface
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Echinoderm Body Plan
Endoskeleton
-Found internal to a delicate epidermis which
contains thousands of neurosensory cells
-Composed of either movable or fixed
calcium-rich (calcite) plates called ossicles
-Perforated by pores to allow extension
of tube feet
-Contains mutable collagenous tissue
-Provides ability to autotomize body parts
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Echinoderm Body Plan
Water-vascular system
-A hydraulic system that aids in movement
and feeding
-Composed of a central ring canal from
which five radial canals extend into each of
the body’s five parts
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Echinoderm Body Plan
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Echinoderm Body Plan
Echinoderms have a large coelom which
connects with a complicated tub system
-Helps provide circulation and respiration
through extensions called papulae
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Echinoderm Body Plan
Reproduction
-In some echinoderms, asexual reproduction
takes place by splitting
-Broken parts can regenerate the whole
animal
-Most reproduction in the phylum is sexual
and external
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Classes of Echinoderms
There are more than 20 extinct classes
In addition, there are five extant classes
1. Asteroidea (sea stars and sea daisies)
2. Crinoidea (sea lilies and feather stars)
3. Echinoidea (sea urchins & sand dollars)
4. Holothuroidea (sea cucumbers)
5. Ophiuroidea (brittle stars)
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Classes of Echinoderms
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Classes of Echinoderms (Cont.)
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Classes of Echinoderms (Cont.)
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