ch. 35 – mollusks & annelids mollusks and annelids are grouped together because they were the...
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Ch. 35 – Mollusks & Annelids Mollusks and annelids are grouped
together because they were the 1st organisms to develop a true coelom.– This suggests that these two groups of
animals may have evolved from a common ancestor.
The coelom provides space for a circulatory system to function without interference from other organs. They can contract muscles without hindering the movement of food through the gut.
Both have the same 1st stage in larva development which is called a trochophore.
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Phylum: Mollusca–Snails, clams, octopuses,
oysters, squids–Video–Mollusk is Latin for “soft”–Most have a shell to protect
them and are soft inside the shell.
–Develop from a trochophore larva which is a free-swimmer. Cilia helps them swim.
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Mollusk CharacteristicsMost are filter feeders (100,000
+ species).True coelom3 main body parts: muscular
foot, head, & visceral mass.Circulation, respiration,
digestion, excretion, reproduction, nerve impulses.
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Body Arrangement1. Muscular foot – used for locomotion.2. Head – has a cerebral ganglia &
mouth.3. Visceral Mass – contains many
organs.4. Mantle – protects the visceral mass.
It secretes a shell in most mollusks.Shell is made of calcium carbonate.
–How does the mollusk breathe with a shell?
Gills are used for respiration.
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4 Classes of Mollusks1. Class: Gastropoda – snails &
slugs–Largest class of mollusks–Gastropoda means “stomach
foot”–Slugs have no shell–Snails have a single shell–The body mass can turn 180
degrees this is called torsion
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SnailsGills for respirationSurvive in moist environmentsOpen Circulatory SystemHemocoel – open body cavity for
blood circulation.Radula – a saw-like tongue
made of chitin.Use foot to move with wavelike
contractions.
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2. Class: Polyplacophora – chitons–Polyplacophora means
“many plates”–Their shells consist of 8
overlapping plates.
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3. Class: Cephalopoda – squids & octopus–Cephalopod means “head
foot”–They have tentacles and a
sharp radula to kill prey.–Closed Circulatory System –
blood circulates in vessels.
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SquidsLargest invertebrateGiant squid are up to 3 meters
long.Squid have the ability to squirt ink.They have the ability to change
colors.– Video
Squids have separate sexesSquid & octopus use jet propulsion
to move.Video
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4. Class: Bivalvia – clams & oysters–Bivalvia means “two
halves”–They are sessile & filter
food from the water.–They have a muscular foot–They don’t have a distinct
head
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–If a piece of sand grain gets into its shell the mantle coats the sand grain with secretion to form a pearl.
Clams–Filter feeders–Have incurrent siphon for water
to enter and an excurrent siphon for water to exit.
–Have separate sexes like most mollusks.
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Body parts of a clamDissectionClam Video
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Annelida Phylum: Annelida – earthworms &
leeches– Video
Characteristics– Body is divided into segments
metamerism– True coelom– Well developed organ system– Most have setae external bristles– Some have parapodia fleshy
appendages– Video
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3 Classes1. Class: Polychaeta –
marine annelids–Have setae & parapodia–Have antannae (the only class)
–Some are free-swimmers
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2. Class: Hirudinea – leeches–No setae–Live in calm bodies of freshwater
–Sucker at each end–Smallest class of annelids
•Video
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3. Class: Oligochaeta – earthworm–Have setae – used to anchor their segments to soil as they burrow through the soil.
–Contract and expand in order to move•Video
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Main Systems of an EarthwormDigestive systemWorms feed on organic matter in
the soil. Worms loosen & aerate soil. Travels in the following pathway:
prostomium (lip) mouth pharynx esophagus crop gizzard intestine
Crop stores food, gizzard grinds food
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Circulatory systemClosed system – have
blood vesselsAortic arches – serve as a
heart–5 pair
Have a dorsal & ventral blood vessel
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Respiratory & Excretory systems
No lungs or gill directly through the skin
When it rains why do worms migrate to the surface?
Nephridia is the excretory organ that eliminates waste.
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Reproductive systemHermaphroditesHowever, they can’t
fertilize their own eggs.They attach at the
clitellum & secrete a mucus layer in which the egg & sperm are placed in.
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Sperm is injected from the seminal receptacle.
Eggs are released from the oviducts.