Transcript Chapter 27: Mollusks and Annelids Section 1: Mollusks

Chapter 27:
Mollusks and Annelids
Section 1:
Mollusks
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
Examples of Mollusks
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
Form and Function in Mollusks
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
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
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
Feeding
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
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
Respiration
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 mollusk’s 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
Excretion



Mollusks must eliminate waste products
Undigested food becomes solid waste that
leaves through the anus in the form of feces
Cellular metabolism produces nitrogencontaining 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
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
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
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
Snails, Slugs, and Their Relatives
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 motherof-pearl
 If a grain of sand or small pebble gets caught between
the mantle and the shell it forms a pearl
Two-Shelled Mollusks
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
Tentacled Mollusks
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
Chapter 27:
Mollusks and Annelids
Section 2:
Annelids
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
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
Form and Function in Annelids




The many segments of an annelid’s 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
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
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
Many polychaete annelids, such
as the sandworm Nereis, use
hook like jaws to capture prey or
nibble on algae
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.
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
The spaghetti worm uses its
long tentacles to pluck bits of
detritus from the ocean floor
In plume worms, a brushshaped structure on the head is
used in filter feeding and in
respiration.
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
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
Response






Many annelids are active animals with welldeveloped 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
Response


Sense organs are best developed in the freeliving 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
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.
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
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
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 worm’s body and
forms a cocoon that shelters the eggs
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
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.
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
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
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
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
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