16'1 Mollusks

1
16.1 Mollusks
2
Objectives

• Describe the characteristics of mollusks.
• Compare mollusks to other invertebrates.
• Describe the features of each class of mollusks.

3

• Snails and clams are invertebrates called
  mollusks MAHLuhsks).
• All mollusks have soft bodies, and many are
  covered by hard shells.

4
Characteristics of Mollusks

• Mollusks live mostly in the ocean, but some can
  be found in freshwater habitats.
• A number of mollusks have also adapted to life on
  land.
• Snails and slugs are common in damp places in
  your backyard.

5

• There are more than 100,000 species of mollusks.
• They range in size from tiny snails only a few
  millimeters across to the 20-m-Iong giant squid.

6

• No matter what their shape or size, all mollusks
  have the same basic body plan.
• Like the bodies of segmented worms, mollusks'
  bodies contain organs organized into systems.
• However, their bodies are not built of segments
  like the segmented worms.
• Instead, mollusks have most of their organs in
  one area called a visceral (VIS ur uhl) mass.
• The visceral mass contains the digestive,
  excretory, circulatory, respiratory, and
  reproductive organs.

7

• Another characteristic of the mollusk body plan
  is a muscular foot.
• The foot is used by most mollusks for movement.
• In addition, a mollusk's soft body parts are
  covered by a skinlike tissue called a mantle.
• In many mollusks, the mantle produces a hard,
  protective shell.

8
Evolution of Mollusks

• The oldest known mollusk fossil is a
  single-shelled sluglike organism that lived about
  570 million years ago.
• After this time, mollusks evolved a variety of
  body forms.
• Many were common in the ancient oceans.
• Then about 225 million years ago, most mollusk
  species became extinct.
• The survivors are the ancestors of the kinds of
  mollusks alive today.

9

• Many oceanic mollusks have larvae similar to the
  larvae of segmented worms, or annelids.
• For this reason, some scientists think mollusks
  evolved from segmented worms.
• However, because of mollusks' lack of
  segmentation, other scientists hypothesize that
  mollusks evolved from ancient worms more similar
  to flatworms.

10
Diversity of Mollusks

• You can tell that snails are very different from
  clams and that both differ from an octopus.
• Would you classify these three mollusks in
  different groups?
• That is what scientists do.
• Each of these kinds of mollusks is a member of a
  different group, or class.

11
Gastropods

• Mollusks that glide along on a foot underneath
  their bodies are called gastropods (GAS troh
  PAHDZ).
• Snails, limpets, slugs, and the sluglike
  nudibranchs are all gastropods.
• Most gastropods have one coiled shell.
• The shells of limpets, however, are flattened
  cones.
• Slugs and nudibranchs have no shell at all.

12

• When you watch a snail moving on the ground, you
  might think it is scooting on its stomach.
• Actually, a gastropod's muscular foot spreads out
  under its body and is separate from the stomach.
• A rippling motion of the foot muscle moves the
  snail along.

13

• Many gastropods eat plants and algae.
• Some eat other invertebrates.
• As an adaptation for getting their food,
  gastropods have a tonguelike organ covered with
  rows of teeth.
• It is called a radula (RAJ 00 luh).

14

• A gastropod moves its radula back, and forth to
  scrape off
• and scoop up food.
• Gastropods that live in water have respiratory
  organs called gills.
• As water flows over the gills, oxygen is
  extracted.
• Gastropods living on land have evolved a cavity
  inside their Bodies that serves as a simple lung.

15
Bivalves

• Mollusks with two shells hinged together make up
  a second class called the bivalves (BYvalvz).
• Clams, oysters, mussels, and scallops are all
  bivalves.
• Like gastropods, bivalves have a muscular foot.
• It is normally hidden inside the two shells.
• Some bivalves move by hooking the foot in sand,
  and then pulling themselves along.

16

• Bivalves have no head, and most move very little.
  
• Some remain attached to one place.
• Movement isn't important because they are filter
  feeders.
• Cilia move water across the gills.
• Tiny food particles, such as plankton, are
  trapped by mucus
• on the gills.
• Cilia push the food into the animal's mouth.

17

• The gills in bivalves have a large surface area
  and a rich supply of blood.
• Oxygen from the water passing over the gills
  diffuses into the blood.
• Carbon dioxide, a waste product of animals,
  diffuses from the blood into the water.

18

• In some kinds of bivalves, water flows over the
  gills when the shells are open.
• In clams, the water enters through a muscular
  tube called a siphon.
• After it flows over the gills, it leaves through
  another siphon.

19
Cephalopods

• The octopus and squid are the major members of a
  third class of mollusks called cephalopods (SEF
  uh loh PARDZ).
• In these mollusks, the foot is divided into
  tentacles.
• The tentacles are located at the head, away from
  the rest of
• the body.

20

• Cephalopods are the only mollusks to have a
  closed circulatory system.
• In a closed system, the blood stays in blood
  vessels.

21

• Modern cephalopods all evolved from now-extinct
  mollusks with external shells.
• In the octopus, the shell has disappeared during
  the process of evolutionary change.
• Squids have only small internal shells.
• Only cephalopods called nautiluses (NAWTuh luhs
  uhz) still have the external shells of their
  ancestors.

22

• Cephalopods live in the oceans and most move
  about freely.
• Unlike the filter-feeding bivalves, they are
  active predators.
• Their tentacles are covered with suction cups for
  grasping prey.
• The octopus lives mostly on the ocean floor,
  crawling around
• in search of prey.
• Squid swim in the open water.

23
End