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Chapter 29
Section 1 Mollusks
Phylum Mollusca
• Snails, slugs, oysters, clams, scallops, octopuses,
and squids are all mollusks.
• Mollusks and annelids share a feature during the
larval stage called a trochophore which develops
from the fertilized egg.
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Chapter 29
Section 1 Mollusks
• In some species, the
trochophore is freeswimming and propels
itself through the water
by movement of cilia on
its surface.
• The presence of a
trochophore larva in
mollusks and annelids
suggests that they share
a common ancestor.
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Chapter 29
Section 1 Mollusks
Key Characteristics of Mollusks
1. The body cavity in mollusks is a true coelom,
although in most species it is reduced to a small
area immediately surrounding the heart.
2. Most mollusks exhibit bilateral symmetry.
3. Mollusks have organ systems for excretion,
circulation, respiration, digestion, and reproduction.
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Chapter 29
Section 1 Mollusks
Key Characteristics of Mollusks
4. The body of every mollusk has three distinct parts:
the visceral mass, the mantle, and the muscular foot.
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Chapter 29
Section 1 Mollusks
Key Characteristics of Mollusks
•
The visceral mass is a central section that
contains the mollusk’s organs.
•
The mantle is a heavy fold of tissue that forms
the outer layer of the body.
•
Finally, every mollusk has a muscular region
called a foot, which is used primarily for
locomotion.
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Chapter 29
Section 1 Mollusks
Key Characteristics of Mollusks
5. Many mollusks have either one or two shells that
serve as an exoskeleton, protecting their soft body.
6. All mollusks except bivalves have a radula, a
tongue-like organ located in their mouth. The radula
has thousands of pointed, backward-curving teeth
arranged in rows.
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Chapter 29
Section 1 Mollusks
Key Characteristics of Mollusks
Organ Systems: Excretion
•
A mollusk’s coelom is a collecting place for wasteladen body fluids.
•
The beating of cilia pulls the fluid from the coelom
into tiny tubular structures called nephridia.
•
The nephridia recover useful molecules (sugars,
salts, and water) from the coelomic fluid.
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Chapter 29
Section 1 Mollusks
Key Characteristics of Mollusks
Organ Systems: Circulation
•
In a circulatory system, blood carries nutrients and
oxygen to tissues and removes waste and carbon
dioxide.
•
Most mollusks have a three-chambered heart and
an open circulatory system.
•
Octopuses and squids are exceptions because
they each have a closed circulatory system.
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Chapter 29
Section 1 Mollusks
Key Characteristics of Mollusks
Organ Systems: Respiration
•
Most mollusks respire with gills, which are located in
the mantle cavity.
•
Most terrestrial snails have no gills. Instead, the thin
membrane that lines their empty mantle cavity
functions like a primitive lung.
•
Sea snails also lack gills, and gas exchange takes
place directly through their skin.
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Chapter 29
Section 1 Mollusks
Key Characteristics of Mollusks
Organ Systems: Reproduction
•
Most species of mollusks have distinct male and
female individuals, although some snails and slugs
are hermaphrodites.
•
Certain species of oysters and sea slugs are able to
change from one sex to the other and back again.
•
Many marine mollusks are moved from place to
place as their trochophore larvae drift in the ocean
currents.
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Chapter 29
Section 1 Mollusks
Body Plans of Mollusks
Gastropods- Stomach footed
•
Gastropods—snails and slugs—are primarily a
marine group that has successfully invaded
freshwater and terrestrial habitats.
•
Most gastropods have a pair of tentacles on their
head with eyes often located at the tips.
•
Gastropods display varied feeding habits. Many are
herbivores that scrape algae off rocks using their
radula.
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Chapter 29
Section 1 Mollusks
Characteristics of Gastropods
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Chapter 29
Section 1 Mollusks
Body Plans of Mollusks
Bivalves
•
All bivalves have a two-part hinged shell. The
valves, or shells, of a bivalve are secreted by the
mantle.
•
Two thick muscles, the adductor muscles,
connect the valves. When these muscles are
contracted, they cause the valves to close tightly.
•
Bivalves are unique among the mollusks because
they do not have a distinct head region or a
radula.
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Chapter 29
Section 1 Mollusks
Body Plans of Mollusks
Bivalves
• Many bivalves use their
muscular foot to dig
down into the sand.
• Sexual Reproduction
occurs by gametes
being released into
open water. (external
fertilization)
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Chapter 29
Section 1 Mollusks
Characteristics of Bivalves
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Chapter 29
Section 1 Mollusks
Body Plans of Mollusks
Cephalopods - Head footed
•
Squids, octopuses, cuttlefish, and nautiluses (only
one to have an external shell) are all cephalopods.
Most of their body is made up of a large head
attached to tentacles.
•
Cephalopods are the most intelligent of all
invertebrates. They have a complex nervous system
that includes a well-developed brain.
•
The structure of a cephalopod eye is similar in many
ways to that of a vertebrate eye, and some species
have color vision.
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Chapter 29
Section 1 Mollusks
Characteristics of Cephalopods
•Most have a small
internal shell or no shell
at all
•The Nautilus has only
one external shell.
•Move by jet propulsion
•Most have a darkcolored foul tasting ink
they can release when
frightened.
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Chapter 29
Section 2 Annelids
The First Segmented Animals
• Annelids are easily recognized by their segments,
which are visible as a series of ringlike structures
along the length of their body.
• Some of the segments are modified for specific
functions, such as reproduction, feeding, or
sensation. A well-developed cerebral ganglion, or
primitive brain, is located in one anterior segment.
• Internal body walls, called septa, separate the
segments of most annelids.
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Chapter 29
Section 2 Annelids
The First Segmented Animals
Characteristics of Annelids
1. The fluid-filled coelom is large and is located entirely
within the mesoderm.
2. The organ systems of annelids show a high degree
of specialization and include a closed circulatory
system and excretory structures called nephridia.
3. Most annelids have external bristles called setae.
Some annelids also have fleshy appendages called
parapodia.
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Chapter 29
Section 2 Annelids
Annelid Groups
Marine Worms
• Marine segmented worms are members of class
Polychaeta, the largest group of annelids.
Polychaetes live in virtually all ocean habitats.
• A distinctive characteristic of polychaetes is the
pair of fleshy, paddle-like parapodia that occur on
most of their segments.
• The parapodia, which usually have setae, are
used to swim, burrow, or crawl.
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Chapter 29
Section 2 Annelids
Annelid Groups
Marine Worms
• Nereis, a polychaete worm, grasps its prey in its
jaws, which open when it thrusts out its pharynx.
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Chapter 29
Section 2 Annelids
Annelid Groups
Earthworms
• Earthworms and some related freshwater worms are
members of the class Oligochaeta. Oligochaetes have
no parapodia and only a few setae on each segment.
• Earthworms lack the distinctive head region and have
no eyes.
• Their clitellum is involved in sexual reproduction.
• Earthworms are highly specialized scavengers. They
literally eat their way through the soil, consuming their
own weight in soil every day.
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Benefits of Earthworms
• Their castings contain nutrients which are
returned to the soil.
• They aerate the soil as they move and burrow
through it.
• They break up the soil in which they live
allowing plant roots to penetrate the soil.
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Chapter 29
Section 2 Annelids
Anatomy of an Earthworm
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Chapter 29
Section 2 Annelids
Annelid Groups
Hydrostatic Skeleton
• The fluid within the coelom of each body segment
creates a hydrostatic skeleton that supports the
segment.
• Each segment contains muscles that pull against this
hydrostatic skeleton.
• Circular muscles wrap around the segment, while
longitudinal muscles span its length.
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Chapter 29
Section 2 Annelids
Annelid Groups
Class Hirudinea Leeches
• Leeches are the only members of class Hirudinea.
Leeches lack both setae and parapodia.
• Most live in moist tropical countries
• Usually no more than 6cm long and are somewhat
flattened.
• Most are freshwater that exist as external parasites
few are marine or terrestrial
• ¼ are carnivores not parasites and feed on soft-bodied
invertebrates such as snails, worms, and insect larva
• Drink blood and body fluids of their host
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Hirudinea characteristics
• The body of a leech is flattened, and unlike other
annelids, its segments are not separated internally.
• A leech has suckers at both ends of its body.
• Suckers at the anterior end attach the leech to the
host. Suckers at the posterior end attach the leech
to an object as it waits for a host to come by
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Leech Attachment
• Leeches penetrate the skin of their host in one of two
ways
• Some have a muscular proboscis, a tubular organ
that they force into the tissue of the host.
• Other leeches slice the tissue with razor-sharp jaws.
The use a muscular pharynx to suck blood out of the
area.
• Some can anesthetize the wound so the host does
not know they have been bitten
• Leeches release an anti-clotting agent to prevent the
hosts blood from clotting
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Feeding
• A meal can be as much as 10 times the
weight of the leech
• It can take the leech up to 200 days to digest
its food.
• A leech may need to feed only once a year
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