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Mollusks and Annelids
Chapter 45
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Coelomates
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Coelomate
 Body cavity lined with mesoderm cells
 Allows the development of complex tissues
and organs.
 Allows wider array of body architectures
and increased body size.
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Coelomates
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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 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.
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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: The 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: The Bivalves
 Clams, scallops, mussels and oysters.
 Have 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.
 Closed circulatory system
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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: The
Polychaetes
 Well developed head
with specialized sense
organs.
 Parapodia on most
segments.
 Usually lack permanent
gonads.
Oligochaeta: The
Earthworms
 Hermaphroditic
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Classes of Annelids
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Hirudinea: The 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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Phylum Phoronida: The Phoronids
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Lophophorates
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Phylum Ectoprocta: The Bryozoans
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Lophophorates
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Phylum Brachiopoda: The Brachiopods
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Review
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Coelomates
Body Plan of the Mollusks
Classes of Mollusks
Segmented Animals
Classes of Annelids
Lophophorates
Raven - Johnson - Biology: 6th Ed. - All Rights Reserved - McGraw Hill Companies
Copyright © McGraw-Hill Companies Permission required for reproduction or display
Raven - Johnson - Biology: 6th Ed. - All Rights Reserved - McGraw Hill Companies