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Chapter 27
Mollusks and Segmented Worms
Chapter 27
• 27.1 Mollusks
– A. What is a Mollusk?
• 1. Slugs, snails, squids, and animals that once lived
in shells in the ocean
a. Most live in ocean, but some live in fresh water and moist
terrestrial habitats
• 2. Phylum - Mollusca
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• A. What are Mollusks? ( cont )
– 3. Classes Shell-less, hinged shells, and single
shell
– 4. Have bilateral symmetry, a coelom, two
body openings, a muscular foot, and a mantle
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• B. Benefits of coelom:
• 1 Separates muscles of gut from those of body
wall allowing food to move through the body
independent of locomotion
• 2. Allows space for circulatory system to function
without interference from other organs
• 3. In some species, the fluids in the coelom act as
a hydrostatic skeleton for the muscles to contract
against
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• C. How mollusks obtain food
– 1. Have a radula – tongue-like organ with rows
of teeth that drill, scrape, grate or cut food
– 2. Then pull food into mouth
– 3. Bivalves obtain food by filtering it out of the
water
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• D. Reproduction in Mollusks
– 1. Separate sexes and reproduce sexually
• A. Egg and sperm released at the same time into the
water where external fertilization takes place
• B. Gastropods are hermaphrodites
• C. Form larva called trochophore - pear shaped
larva with cilia protruding out each end
– 1 Cilia propel larva and push food into mouth
– 2 Free swimming larva aid in dispersal
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• E. Systems of body
– 1. Nervous - simple system of brain and nerves
that coordinate their movement and behavior
• Paired eyes
– 2. Circulation
• Well-developed, three chambered heart, which
pumps blood into an open circulatory system
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• E. Systems ( cont )
– 3. Respiration
• A have gills for respiration
• B gases can diffuse
• C some land snails had their mantle cavity evolved
into primitive lung
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• E. Systems ( cont )
– 4. Excretion
• A. Oldest known creatures with excretion structures
• B. Nephridia remove metabolic wastes from animal
– 1 One or two and they collect waste from the coelom and
discharge the waste by pumping the gills
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• F. Diversity of Mollusks
– 1. Gastropoda largest and most diverse class
• A. Have one shell
• B. Body plan based on “Torsion” twisting of the
body, allowing it to withdraw it’s head
• C. Movement is by wave-like action
• D. Two eyes at end of tentacles
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• F. Diversity ( cont )
• 2. Bivalvia
– A. Most are sessile and filter feeders
– B. Two valves or shells
• 1 1st layer thin outer that protects shell from acid
• 2 2nd layer thick layer of calcium carbonate for strength
• 3 3 rd layer smooth protects the animals soft body ( pearl may
develop )
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• F. Diversity ( cont )
– 3. Cephalopoda octopus
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A.
B.
C.
D.
E.
Name means Head-Foot
Use tentacles to capture food
Closed circulatory system
All live in oceans
Propelled by mantle pushing water though excurrent siphon
– 1 Squids can attain speeds of 20 m / sec
• F. Squids use ink-like substance for defense
• G. Have separate sexes and internal fertilization