Worms and Mollusks
CHAPTER 27
Flatworms
SECTION 27-1
What is a flatworm?
 Phylum Platyhelminthes
 No more than a few
millimeters thick
 Have tissues and internal
organ systems
 Have bilateral symmetry
and cephalization
 Known as acoelomates
(“without coelem,” which
is a fluid filled body
cavity)
Form and Function in Flatworms
 Feeding
 Digestive cavity with a
single opening through
which both food and
wastes pass
 Parasitic worms obtain
nutrients from foods that
have already been
digested by their hosts
Form and Function in Flatworms
 Respiration, Circulation, and Excretion
 Rely on diffusion to:
 Transport oxygen and nutrients to internal tissues
 Remove carbon dioxide and other wastes from their bodies
 Have no gills or respiratory organs, heart, blood
vessels, or blood
 Some have flame cells – remove excess water and
filter wastes from the body
Excretory System
Form and Function in Flatworms
 Response
 A head encloses several ganglia (groups of nerve
cells) that control the nervous system
 Have eyespots that look like eyes, but are groups of
cells that can detect changes in the amount of light in
their environment
Form and Function in Flatworms
 Movement
 Two means of movement:
 Cilia to help glide through water
 Muscle cells allow them to twist and turn
Form and Function in Flatworms
 Reproduction
 Most are hermaphrodites that reproduce sexually
 A hermaphrodite is an individual that has both male and
female reproductive organs
 Asexual reproduction takes place by fission, in
which an organism splits in two
Groups of Flatworms
 Three main groups of Flatworms
 Turbellarians
 Flukes
 Tapeworms
 Most turbellarians are free-living
 Most other flatworm species are parasites
Turbellarians
 Free-living flatworms
 Live in marine or freshwater
Flukes
 Class Trematoda
 Parasitic flatworms
 Infect the internal organs
of their host
Tapeworms
 Class Cestoda
 Long, flat, parasitic
worms
 Adapted to life inside the
intestines of their host


No digestive tract
Absorb already digested
nutrients from host
Roundworms
SECTION 2
What is a Roundworm?
 Phylum Nematoda
 Slender, unsegmented worms with tapering ends
 Range in size from microscopic to a meter
 Most are free-living, inhabiting soil and water
 Others are parasitic
What is a Roundworm?
 Have a pseudocoelom
(“false coelom”)
 Have a digestive tract
with two openings – a
mouth and an anus
Form and Function in Roundworms
 Have specialized tissues and organ systems
 Body systems of free-living roundworms are more
complex than parasitic ones
Form and Function in Roundworms
 Feeding
 Predators that use grasping mouthparts to catch and eat small
animals
 Respiration, Circulation, and Excretion
 Exchange gases and excrete metabolic waste through their
body walls
 Depend on diffusion to carry nutrients and waste through their
bodies
Form and Function in Roundworms
 Response
 Simple nervous systems
 Have several types of sense organs
 Movement
 Muscles extend the length of their bodies
 Reproduction
 Reproduce sexually, and most have separate sexes
Roundworms and Human Disease
 Parasitic roundworms include:
 Trichinosis-causing worms
 Filarial worms
 Ascarid worms
 Hookworms
Trichinosis-Causing Worms
 Trichinosis – terrible disease caused by the
roundworm Trichinella
 Adult worms live and mate in the intestines of their
hosts
 Humans usually get the disease from eating
undercooked pork
Filarial Worms
 Threadlike worms that live in blood of birds and
mammals
 Causes elephantiasis
Ascarid Worms
 Serious parasite of vertebrate animals
 Causes malnutrition in more than 1 billion people
worldwide
 Absorbs digested food from the host’s small intestine
Hookworms
 25% of people in the world are affected with
hookworms
 Live in host’s intestines
 Feed on blood, causing weakness and poor growth
Annelids
SECTION 27-3
What is an Annelid?
 Phylum Annelida
 Worms with segmented
bodies

Each segment is separated
by a septum
 Have a true coelom
Form and Function in Annelids
 Feeding and Digestion
 Many get their food using a pharynx


Food moves from the pharynx, into the esophagus, the crop, the
gizzard, and then to the intestine
Others obtain food by filter feeding
 Circulation
 Closed circulatory system – blood is contained within a
network of blood vessels
Form and Function in Annelids
 Respiration
 Aquatic annelids often breath through gills
 Land-dwelling annelids take in oxygen and give off carbon
dioxide through their moist skin
 Excretion
 Digestive wastes pass through the anus at the end of the
digestive tract
 Cellular waste is eliminated through nephridia (excretory
organs)
Form and Function in Annelids
Nephridia
Form and Function in Annelids
 Response
 Well developed nervous system consisting of a brain and
several nerve cords
Brain
Ganglia
Form and Function in Annelids
 Movement
 Two groups of muscles that work together as part of a
hydrostatic skeleton
 Reproduction
 Most reproduce sexually

Some have separate sexes, others are hermaphrodites
Groups of Annelids
 Three classes of Annelids
 Oligochaetes
 Leeches
 Polychaetes
Oligochaetes
 Class Oligochaeta
 Contains earthworms and their relatives
 Streamlined bodies
 Relatively few setae
 Most live in soil or freshwater
Leeches
 Class Hirudinea
 External parasites that suck the blood and body
fluids of their host
Polychaetes
 Class Polychaeta
 Contains sandworms, blood worms, and relatives
 Marine annelids that have paired, paddlelike
appendages tipped with setae (brushlike structures)
Ecology of Annelids
 Earthworms and many other annelids spend their
lives burrowing through soil, aerating and mixing it
 Earthworms help plant matter decompose
 Earthworm castings are rich in nitrogen,
phosphorus, potassium, micronutrients, and
beneficial bacteria
Mollusks
SECTION 4
What is a Mollusk?
 Soft-bodied animals
 Usually have an internal
or external shell
 Free-swimming larval
stage called a
trocophore
Form and Function in Mollusks
 True coeloms
 Complex, interrelated organ systems
Form and Function in Mollusks
Body Plan
 Variation on four main parts:




Foot – takes many forms
Mantle – layer of tissue that covers the
mollusk’s body
Shell – made by glands in the mantle
Visceral mass – consists of internal organs
Form and Function in Mollusks
Feeding
 Can be herbivores,
carnivores, filter feeders,
detritivores, or parasites
 Snails and slugs feed
using a tongue-shaped
structure called a radula
Form and Function in Mollusks
Feeding
 Clams, oysters, and
scallops use gills
 Food enters through a
siphon – tubelike
structure through which
water enters and leaves
the body
Excurrent
siphon
Incurrent
siphon
Form and Function in Mollusks
Respiration
 Aquatic mollusks breathe
using gills inside their
mantle cavity
 Land snails and slugs
respire through the moist
surface of their skin
Gills
Form and Function in Mollusks
Circulation
 Some have open circulatory systems – works well for
slow-moving mollusks (snails and clams)
 Others have closed circulatory systems – works best
for fast moving mollusks (octopi and squid)
Heart
Form and Function in Mollusks
Excretion
 Nephridia remove wastes from the blood and release
it outside the body
Nephridium
Form and Function in Mollusks
Response
 Two-shelled mollusks have simple nervous systems
 Octopi and relatives have the most highly developed
nervous systems of all invertebrates
 Octopus opening a jar
Form and Function in Mollusks
Movement
 Move in a variety of ways
 Snails secrete mucus and
move over surfaces using
the foot
 Octopi use a form a jet
propulsion
Reproduction
 Reproduce in a variety of
ways
 Snails and two-shelled
mollusks: external
fertilization (sexually)
 Tentacled mollusks and
some snails: internal
fertilization (sexually)
Groups of Mollusks
 Three major classes:
 Gastropods
 Bivalves
 Cephalopods
Gastropods
 Class Gastropoda
 Shell-less or single-shelled
 Move using muscular foot on ventral side
 Includes: pond snails, land slugs, sea butterflies, sea
hares, limpets, and nudibranchs
Bivalves
 Class Bivalvia
 Have two shells held together by one or two powerful
muscles
 Include: clams, oysters, mussels, and scallops
Cephalopods
 Class Cephalopoda
 Soft-bodied
 Head is attached to a single foot
 Foot is divided into tentacles or arms
 Includes: octopi, squids, cuttlefishes, and nautiluses
Ecology of Mollusks
 Mollusks play many different roles in living systems:
 Feed on plants
 Prey on animals
 Filter algae out of the water
 Eat detritus
 Some mollusks are hosts to symbiotic algae or to
parasites; others are themselves parasites
 Mollusks are food for many organisms