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Chapter 39: Annelids, molluscs, nematodes and arthropods

Fig. 39.1: Phylogeny of the protostomes

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Body cavities• Internal body cavity surrounding organs• Coelom

– body cavity entirely lined with mesoderm– examples: annelids, molluscs, arthropods

• Pseudocoel– body cavity not entirely lined with mesoderm– example: nematodes

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Phylum Annelida• Segmented worms

– marine worms, earthworms, leeches

• Metameric segmentation– repetition of functional units in body

• Characteristics– closed vascular (circulatory) system– through-gut– epidermal setae/chaetae (bristles)– hydrostatic skeleton– septa

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Classification • Class Polychaeta (marine worms, bristle worms)

• Class Clitellata (worms with clitellum)

– subclass Oligochaeta (earthworms)– subclass Hirudinea (leeches)

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The annelid body• Body divided into three parts• Prostomium

– presegmental– contains brain

• Soma– segmented body

• Pygidium – postsegmental– contains anus

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Fig. 39.3: Structure of earthworm(a) General body morphology

(b) Three-dimensional structure of individual segments

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Fig. 39.3: Structure of earthworm (cont.)(c) Dorsal view inside the body

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Locomotion• Transverse septa divide body cavity into separate

units– allow hydrostatic pressure to vary between segments

• Body wall can change shape– elongation– contraction

• Movement by peristaltic locomotion– crawling– burrowing

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Respiration and circulation• Transverse septa prevent free circulation of body

fluid– dissolved gases, nutrients etc. cannot circulate

• Requires system for transport– closed circulatory system– passes through septa

• Respiratory pigments– erythrocruorin– chlorocruorin

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Excretion• Excretory products in body fluid

– paired excretory organs (nephridia) in each segment– protonephridia

blind tubes with flagellated cells embryos some polychaetes

– metanephridia funnel-shaped oligochaetes, leeches, many polychaetes

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Phylum Mollusca• Molluscs

– clams, mussels, snails, slugs, octopus, squid, chitons

• Diverse group• Characteristics

– mantle– visceral mass– foot– radula– shell– veliger

Fig. 39.10: Phylogeny of major mollusc groups

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Classification• Mollusca is the second-largest phylum• Class Polyplacophora (chitons)

• Class Bivalvia (clams, mussels, oysters)

• Class Gastropoda (snails, slugs, sea slugs)

• Class Cephalopoda (squid, octopus, cuttlefish)

• Other classes (Neopilina, tusk shells)

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Molluscan body plan• Mantle

– fold of body wall enclosing mantle cavity– secretes shell

• Visceral mass– contains body organs– organs open into mantle cavity

• Foot– locomotion– may be highly modified

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Molluscan body plan (cont.)• Radula

– feeding structure– rasping teeth on flexible strip

• Shell– one, two or eight elements– may be absent

• Veliger– second larval stage

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Fig. 39.11: Body structure of molluscs(a)

(b)

(c)

(d)

(e)

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Bivalvia (bivalves)• Adaptations to sessile existence as adults• Locomotion

– reduced, laterally-compressed foot burrowing

• Feeding– most species are filter-feeders– lack radula– gill surfaces expanded for food gathering and sorting – cilia sort and transport particles to mouth

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Gastropoda (gastropods)• Most diverse group of molluscs

– only group with terrestrial species

• Single coiled shell – reduced or lost in some species

• Visceral mass undergoes torsion– rotation through 180°– brings posterior structures to anterior– secondary detorsion in some species– torsion not the same as coiling in shells

Fig. 39.19b: Land snail

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Cephalopoda (cephalopods)• Octopus, squid, cuttlefish, nautilus• Arms and tentacles for manipulating prey• Complex brains and eyes

– colour vision– visual communication– chromatophores under nervous and hormonal control

• High metabolic rate– closed circulatory system– accessory branchial heart at base of each gill

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Phylum Nematoda• Roundworms• Free-living or endoparasitic• Characteristics

– not segmented– fixed number of cells (eutely)– lack cilia– lack circular muscle– covered in cuticle

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Phylum Arthropoda• Most diverse phylum• Spiders, crabs, millipedes, insects• Characteristics

– jointed limbs– chitinous exoskeleton, sometimes with calcium– metameric segmentation– tagmatisation (segments clustered into functional regions

or tagmata)

Fig. 39.24: Arthropod phylogeny

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Classification• Chelicerate arthropods (arthropods with chelicerae)

– subphylum Chelicerata (spiders, horseshoe crabs)

• Mandibulate arthropods (arthropods with mandibles)

– subphylum Myriapoda (millipedes, centipedes)

– subphylum Crustacea (prawns, crabs, yabbies)

– subphylum Insecta (insects)

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Features of the arthropod body• Exoskeleton

– continuous integument– thickened and rigid in regions (sclerites)– thin and flexible between sclerites

• Does not expand as animal grows– must be shed (moulted) periodically to allow growth

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Subphylum Chelicerata• Class Pycnogonida (sea spiders)

• Class Merostomata (horseshoe crabs)

• Class Arachnida (spiders and allies)

– spiders– scorpions– pseudoscorpions– ticks and mites– others

Fig. 39.29a: Spider anatomy

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The chelicerate body• Body divided into

– prosoma (cephalothorax)– opisthosoma (abdomen)

• Appendages– 1st pair = chelicerae (claw-like for feeding)– 2nd pair = pedipalps (manipulation)– 3rd to 6th pair = legs

• Book lungs or book gills for respiration• Excretion by coxal glands and Malpighian tubules

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Mandibulate arthropods (Mandibulata)• Mouthparts with mandibles not chelicerae• One or two pairs of antennae• Compound eyes

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Subphylum Myriapoda• Class Chilopoda (centipedes)

– typically dorso-ventrally flattened– one pair of legs per segment– poison claws at anterior– carnivorous

• Class Diplopoda (millipedes)

– typically rounded in cross-section– two pairs of legs per segment– may produce fluid from repugnatorial glands– herbivorous

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Subphylum Crustacea• Class Branchiopoda (water fleas)

• Class Copepoda (copepods)

• Class Ostracoda (seed shrimps)

• Class Malacostraca (crabs, crayfish, prawns)

• Other classes

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The crustacean body• Multiple pairs of biramous (branched) appendages

– each appendage with endopodite and exopodite– 2 pairs antennae, 1 pair mandibles, 2 pairs maxillae

• Compound eyes, simple ocelli (clustered as median eye)

• Body surface and gills for gas exchange• Antennal and maxillary glands for excretion

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Reproduction• Sexes usually separate

– some hermaphroditic (e.g. barnacles)– some parthenogenetic (e.g. branchiopods)

• Egg-brooding common• Nauplius larva

– one eye, three pairs of appendages– complex larvae in decapods and barnacles

• Larval stage sometimes suppressed

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Subphylum Hexapoda (insects and their relatives)• Class Apterygota (wingless insects)

– springtails, silverfish, others

• Class Pterygota (winged insects)

– exopterygotes cockroaches, mantids, grasshoppers, others

– endopterygotes beetles, flies, butterflies, moths, ants, others

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Insect body structure• Three pairs of uniramous (unbranched)

appendages• One or two pairs of wings• Compound eyes, simple ocelli• Tracheal system for gas exchange• Malpighian tubules excreting insoluble uric acid

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Fig. 39.34: External features of a grasshopper

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Metamorphosis• Some insect groups undergo a change in form

between the larval and adult stages• Exopterygotes change form gradually, developing

adult characteristics with each moult– incomplete metamorphosis

• Endopterygotes undergo a major change between the last larval stage and the adult stage– complete metamorphosis

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Complete metamorphosis

Larva (caterpillar, maggot, grub)

Pupa (chrysalis)

Imago (adult butterfly, fly, beetle)

Larva and adult occupy different niches

Fig. 39.38: Butterfly life stages

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Question 1:

Which characteristic is probably the most important for allowing insects to diversify on land?

a) Antennae

b) Wings

c) Eyes

d) Exoskeleton

e) Bilateral symmetry

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Summary• Annelid and arthropod bodies show metameric

segmentation with serial repetition on functional units

• Molluscs are a diverse group of aquatic and terrestrial animals with complex and varied body plans

• Arthropods are the most abundant phylum and can be found in all habitats

• Arthropods include two major lines of evolution: the chelicerata and mandibulata

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