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Chapter 29
Mollusks and Annelids
Section 1: Mollusks
Section 2: Annelids
Section 1
Mollusks
Objectives:
•Summarize the evolutionary relationship between
mollusks and annelids.
•Describe the key characteristics of mollusks.
•Describe excretion, circulation, respiration, and
reproduction in mollusks.
•Compare the body plans and feeding adaptations of
gastropods, bivalves, and cephalopods.
Section 1
Mollusks
A True Coelom
•Coelom and Trochophore Mollusks and annelids
have a true body cavity and many mollusks and
annelids have a larval form called a trochophore.
Section 1
Mollusks
Key Characteristics of Mollusks
•Key Traits The mollusk body has three distinct
parts: a visceral mass, a mantle, and foot. All mollusks
except bivalves have a rasping tonguelike radula.
Mollusks have a true coelom and well-developed
organs.
•Organ Systems Most mollusks respire with gills but
some respire with a primitive lung. Nephridia enable
mollusks to recover the useful substances from their
bodily wastes.
Section 1
Mollusks
Body Plans of Mollusks
•Gastropods Gastropods (snails and slugs) live in
oceans, in fresh water, and on land. Gastropods have
an open circulatory system.
•Bivalves Bivalves (clams, oysters, and their kin) are
aquatic and have hard shells called valves that protect
their soft bodies. Bivalves have an open circulatory
system.
•Cephalopods Cephalopods (octopuses, squids, and
their kin) have a well-developed head region, many
tentacles, and a closed circulatory system. Most
cephalopods have no external shell.
Section 2
Annelids
Objectives:
•Identify the major change in body plan that
distinguishes annelids from mollusks.
•Describe the basic annelid body plan.
•Describe the annelid digestive system.
•Compare the three classes of annelids.
Section 2
Annelids
The First Segmented Animals
•Characteristics of Annelids Annelids are coelomate
worms that have segmented bodies and complex
nervous systems. Annelids are classified according to
the presence or absence of setae and parapodia.
Annelids respire through their skin, and they have a
closed circulatory system.
Section 2
Annelids
Annelid Groups
•Marine Worms Marine polychaetes have parapodia
and setae. Some are active predators and others are
filter feeders.
•Earthworms Earthworms burrow through the soil,
ingesting it as they crawl.
•Hydrostatic Skeleton The fluids within the coelom
of each body segment creates a hydrostatic skeleton
that supports the segment.
•Leeches Leeches lack parapodia and setae, and their
segments are not separated internally. They may be
aquatic or terrestrial, and some are parasites.