Chapter 39: Annelids, molluscs,
nematodes and arthropods
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Body cavities
•
Internal body cavity surrounding organs
• Coelom
– body cavity entirely lined with mesoderm
– example: 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 Euclitellata (worms with clitellum)
– subclass Oligochaeta (earthworms)
– subclass Hirudinea (leeches)
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Anatomy
•
•
Body divided into three parts
Prostomium
– presegmental
– contains brain
•
Soma
– segmented body
•
Pygidium
– postsegmental
– contains anus
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Fig. 39.2: Structure of earthworm
(a) General body morphology
(b) Three-dimensional structure of individual segments
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(cont.)
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Fig. 39.2: 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
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Classification
•
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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Anatomy
•
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
(cont.)
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Anatomy (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. 39.10a-e: Molluscan anatomy
(a)
(d)
(b)
(e)
(c)
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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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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 1800
brings posterior structures to anterior
secondary detorsion in some species
torsion not the same as coiling in shells
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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)
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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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Anatomy
•
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
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Anatomy
•
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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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 (see shrimps)
• Class Malacostraca (crabs, crayfish, prawns)
• Other classes
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Anatomy
•
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 Insecta
•
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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Anatomy
•
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. 39.33: External features of 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
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