Chapter 27: Mollusks and Annelids

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Chapter 27Mollusks and Annelids

• Section 1
• Mollusks

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Mollusks

• Phylum Mollusca
• Evolved in the sea more than 600 million years
  ago
• More than 100,000 mollusk species
• Divided into 7 classes
• Live everywhere
• Range in size from snails as small as a grain of
  sand to giant squids that may grow more than 20
  meters long
• Wide range of forms and colors

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Examples of Mollusks
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What is a Mollusk?

• Very different but all share similar
  developmental patterns
• Their different forms are the results of
  variations on the same basic body plan
• Mollusks are defined as soft-bodied animals that
  have an internal or external shell
• Although some present day mollusks lack shells,
  they are thought to have evolved from shelled
  ancestors
• Most mollusks have a special kind of larva called
  a trochophore
• Swim in open water and feed on tiny floating
  plants
• Also seen in segmented worms
• Evolved from a common ancestor

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Form and Function in Mollusks
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Form and Function in Mollusks

• The body plan of almost all mollusks consists of
  four basic parts
• Foot
• Usually contains the mouth and other structures
  associated with feeding
• Many different shapes
• Mantle
• Thin, delicate tissue layer that covers most of
  the body
• Shell
• Made by glands in the mantle that secrete calcium
  carbonate
• Visceral mass
• Contains the internal organs

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Form and Function in Mollusks

• Basic body parts have taken on different forms as
  mollusks evolved adaptations to different
  habitats
• Type of foot and shell mollusks have are used to
  group them into classes

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Feeding

• Every mode of feeding is seen in this phylum
• Many mollusks feed with a tongue-shaped structure
  called a radula
• Layer of flexible skin that carries hundreds of
  tiny teeth
• Inside is a rod of cartilage
• When the mollusk feeds, it places the tip of the
  radula on the food and pulls the skin back and
  forth over the cartilage

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Feeding
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Feeding

• Although they may have a radula, carnivorous
  mollusks such as octopi and certain sea slugs
  typically use sharp jaws to eat their prey
• Produce poisons
• Mollusks such as clams, oysters, and scallops are
  filter feeders
• Use feathery gills to sift food from the water

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Respiration

• Gills serve as organs of respiration as well as
  filters for food
• Aquatic mollusks breathe by using gills located
  inside their mantle cavities
• Land snails and slugs breathe by using a
  specially adapted mantle cavity that is lined
  with many blood vessels
• The surface is constantly kept moist so that
  oxygen can enter the cells
• Because the mantle loses water in dry air, most
  land snails and slugs must live in moist places
• Prefer to move around at night, during
  rainstorms, and times when humidity is high

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Respiration
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Internal Transport

• Oxygen that is taken in by the respiratory system
  and nutrients that are the products of digestion
  are carried by the blood to all parts of a
  mollusks body
• The blood is pumped by a simple heart through an
  open circulatory system
• Blood does not always travel inside blood vessels
• Instead, blood works its way through body tissues
  in open spaces called sinuses
• The flow of blood through sinuses is not
  efficient enough for fast-moving octopi and
  squids
• Closed circulatory system
• Blood always moves inside vessels

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Excretion

• Mollusks must eliminate waste products
• Undigested food becomes solid waste that leaves
  through the anus in the form of feces
• Cellular metabolism produces nitrogen-containing
  waste in the form of ammonia
• Must be removed from body fluids
• Simple tube-shaped organ called nephridia
• Remove ammonia from the blood and release it to
  the outside

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Response

• Vary greatly in the complexity of their nervous
  systems
• Clams and other two-shelled mollusks
• Simple nervous system
• Several small ganglia near mouth, a few nerve
  cords, simple sense organs
• Octopi and other tentacled mollusks
• Highly developed nervous systems
• Well-developed brain
• Complex sense organs
• Can be trained to perform different tasks in
  order to obtain a reward or avoid punishment
• Often studied by psychologists interested in the
  way animals learn

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Reproduction

• Sexes are separate and fertilization is external
• Most mollusks release eggs and sperm into water
• Find each other by chance
• Free-swimming larvae develop
• Tentacled mollusks fertilization takes place
  inside the body of the female
• Some snails are hermaphrodites

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Snails, Slugs, and Their Relatives

• Class Gastropoda
• Gastropods stomach-foot
• Many have a one piece shell that protects their
  soft bodies
• Some gastropods, such as slugs, have no shell
• Protected by their behavior
• Some also contain toxins or chemicals that are
  secreted to warn predators
• Bright colors warn predators to stay away

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Snails, Slugs, and Their Relatives
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Two-Shelled Mollusks

• Class Bivalvia
• Shells with a hinge
• Common bivalves include clams, oysters, and
  scallops
• Although larvae are free-swimming, they soon
  settle down to the relatively quiet life on the
  bottom of a body of water
• Most are sessile
• Mantle glands make the shells
• Mantle glands also keep the shells inside
  surfaces smooth and comfortable by secreting
  layers of mother-of-pearl
• If a grain of sand or small pebble gets caught
  between the mantle and the shell it forms a pearl

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Two-Shelled Mollusks
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Tentacled Mollusks

• Cephalopods members of the class Cephalopoda
  are among the most active and interesting
  mollusks
• Includes octopi, squids, cuttlefish, and
  nautiluses
• head-foot
• Most have 8 flexible tentacles equipped with a
  number of round sucking disks that are used to
  grab their prey
• Move by a siphon and jet propulsion
• Can secrete large amounts of dark colored, foul
  tasting ink
• Can change color to to match their surroundings
• Most modern cephalopods have an internal shell or
  no shell at all
• Contains gases that allow them to float

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Tentacled Mollusks
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How Mollusks Fit into the World

• Many different roles in living systems
• Important source of food
• Environmental monitors
• Biological research
• Can cause harm to crops
• Cause sickness on occasion

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Chapter 27Mollusks and Annelids

• Section 2
• Annelids

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Annelids

• The soft-bodied earthworm is the most common
  terrestrial, or land-dwelling, segmented worm
• There are approximately 9000 species of segmented
  worms that live in moist soil, in fresh water,
  and in the sea
• Segmented worms, or annelids, live just about
  everywhere in the world

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What Is an Annelid?

• Phylum Annelida
• An annelid is a round, wormlike animal that has a
  long, segmented body
• Annelids range in size from tiny aquatic worms
  less than half a millimeter long to giant
  earthworms more than 3 meters long
• Annelids also vary greatly in color, patterning,
  number of bristles, and other superficial features

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Form and Function in Annelids

• The many segments of an annelids body are
  separated by internal walls called septa
• Most of the body segments are virtually identical
  to one another
• However, some segments are modified to perform
  special functions
• For example, the first few segments may carry one
  or more pairs of eyes, several pairs of antennae,
  and other sense organs

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Feeding

• The digestive tract extends from the mouth to the
  anus
• Food enters through the mouth and travels through
  the gut, where it is digested
• Like mollusks, annelids have evolved structures
  and behaviors that allow them to use a wide
  variety of foods
• One feeding organ that has evolved many different
  forms in different groups of annelids is the
  pharynx, or the muscular front end of the
  digestive tube

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Feeding

• Many annelids can extend the pharynx through the
  mouth
• In carnivorous annelids, this type of pharynx
  usually has two or more sharp jaws attached to it
• When a suitable animal approaches, the worm
  lunges forward, rapidly extends the pharynx, and
  grabs the prey with its jaws
• When the pharynx returns to its normal position,
  it carries the food back to the gut

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Many polychaete annelids, such as the sandworm
Nereis, use hook like jaws to capture prey or
nibble on algae
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The digestive system of an earthworm is shown
here. The pharynx pumps a mixture of food and
soil into a tube called the esophagus. The food
then moves through the crop, where it can be
stored, and through the gizzard, where it is
ground into smaller pieces. The food is digested
in the intestine. Undigested materials pass
through the intestine and are eliminated through
the anus.
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Respiration

• Aquatic annelids often breathe through gills
• Terrestrial annelids take in oxygen and give off
  carbon dioxide through their skin
• Because the skin must stay moist to make gas
  exchange possible, the worms die if the skin
  dries out
• To help guard against this, terrestrial annelids,
  such as earthworms, secrete a thin protective
  coating called a cuticle to hold moisture around
  them

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The spaghetti worm uses its long tentacles to
pluck bits of detritus from the ocean
floor In plume worms, a brush-shaped
structure on the head is used in filter feeding
and in respiration.
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Internal Transport

• Annelids typically have closed circulatory
  systems organized around two blood vessels that
  run the length of their bodies
• In each body segment is a pair of smaller vessels
  called ring vessels that connect the two main
  blood vessels and supply blood to the internal
  organs
• In annelids such as earthworms, several of the
  ring vessels near the anterior end of the worm
  are larger than the other ring vessels and have
  muscle tissue in their walls
• These vessels are often called hearts because
  they contract rhythmically and help pump blood
  through the system

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Excretion

• Annelids produce two kinds of wastes
• Solid wastes pass out through the anus at the end
  of the gut
• Wastes resulting from cellular metabolism are
  eliminated by nephridia
• A pair of nephridia in each body segment removes
  waste products from the body fluids and carries
  them to the outside

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Response

• Many annelids are active animals with
  well-developed nervous systems
• The brain sits on top of the gut at the front end
  of the body
• Two large nerves pass around the gut and connect
  the brain with a pair of ganglia below
• From these ganglia, a ventral nerve cord runs the
  entire length of the worm
• Nerves from each segment of the worm enter and
  leave the nerve cord at a pair of small ganglia
• These nerves help carry messages from sense
  organs and coordinate the movements of muscles

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Response

• Sense organs are best developed in the
  free-living marine species of annelids
• Have sensory tentacles, statocysts, chemical
  receptors, and two or more pairs of eyes
• Many other annelids have much simpler sensory
  systems
• Earthworms have no specialized sense organs
• They rely on simple sensory cells in the skin

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Sense organs are best developed in free-swimming
annelids such as the paddleworm, which has a pair
of beady eyes and a number of sensory tentacles
on its head.
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Movement

• Annelids have two major groups of muscles in
  their body walls
• Longitudinal muscles
• Runs from the front of the worm to the rear
• When contracted, they make the worm shorter
• Circular muscles
• Runs in circles around the body of the worm
• When contracted, they make the worm skinnier

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Reproduction

• Most annelids reproduce sexually
• In some annelids, the sexes are separate
• However, annelids such as earthworms and leeches
  are hermaphrodites that undergo internal
  fertilization
• Although an individual worm produces both sperm
  and eggs, it rarely fertilizes its own eggs

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Reproduction

• Instead, worms pair up, attach themselves to each
  other, and exchange sperm
• Each worm stores the sperm it has received in
  special sacs
• When eggs are ready for fertilization, a band of
  thickened, specialized segments called the
  clitellum secretes a mucus ring into which eggs
  and sperm are released
• The ring then slips off the worms body and forms
  a cocoon that shelters the eggs

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Sandworms, Bloodworms, and Their Relatives

• Class Polychaeta
• Common and important marine worms
• Polychaetes are characterized by paired paddle
  like appendages on their body segments
• These appendages are tipped with bristles
• Polychaetes live in cracks and crevices in coral
  reefs, in sand, mud, and poles of rocks, and even
  out in the open water

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Although they look very different from each
other, both the fanworm and the fireworm are
polychaetes. The fanworm is a filter feeder that
retreats into its tube when threatened. The
fireworm defends itself with poisonous bristles
that break off and penetrate skin at the
slightest touch. The pain caused by these
bristles gives the fireworm its name.
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Earthworms and Their Relatives

• Class Oligochaeta
• Contains earthworms and related species
• Oligochaetes are annelid worms that live in soil
  and open water
• Most oligochaetes live in soil or freshwater
• Oligochaetes have fewer bristles than polychaetes

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Leeches

• Class Hirudinea
• Contains the leeches, most of which live in
  tropical countries
• Freshwater organisms that exist as external
  parasites, drinking blood and body fluids from
  their host
• All leeches have powerful suckers at both ends of
  their bodies
• These suckers are used to attach a leech to its
  host

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Leeches

• Leeches penetrate the skin of their host in one
  of two ways
• Use a muscular proboscis
• Tubular organ that they force into the tissue of
  their host
• Use razor sharp jaws
• Once the wound has been made, the leech uses its
  muscular pharynx to suck blood from the area

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Leeches

• Both types of leeches release a special secretion
  from their salivary glands to prevent the blood
  from clotting as they drink it
• Some leeches also produce a substance that
  anesthetizes the wound thus keeping the host
  from knowing it has been bitten
• During feeding, a leech can swallow as much as 10
  times its weight in blood
• Can take up to 200 days to digest
• A leech can live for a year before it must feed
  again

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How Annelids Fit into the World

• Important in many habitats
• Aquatic species are food for many fish, crab, and
  lobster
• Earthworms perform an essential task in
  conditioning soil
• By constantly burrowing through the ground, they
  help aerate the soil
• Without the efforts of these annelids, the
  structure and fertility of farm soils would
  degenerate quickly, lowering crop yields