WORMS AND MOLLUSKS
Biology 112
FLATWORMS
Phylum Platyhelminthes
Soft, flattened worms with tissues and
internal organ systems
The simplest animals with three germ layers,
bilateral symmetry and cephalization
Acoelomates
No coelom between the tissues of flatworms
A coelom is a fluid-filled body cavity that is
lined with tissue in the mesoderm
BODY FUNCTIONS OF FLATWORMS
All use diffusion for distribution of
nutrients and gases
Some have specialized flame cells used
to remove excess water from the body
Contain a digestive cavity with one
opening – a mouth
A muscular tube, called a pharnyx,
pumps food into the digestive cavity
CLASSIFICATION OF FLATWORMS
1. Tubellarians
-
Free-living flatworms,
can live in fresh and
salt water
Ganglia control the
nervous system
Some contain an
eyespot to detect light
Reproduce asexually
and sexually
Planarians are the
most familiar species
CLASSIFICATION OF FLATWORMS
2. Flukes
- Parasitic
- Infect the internal
organs of their host
- Reproduce sexually
in the primary host
and asexually in the
intermediate host
CLASSIFICATION OF FLATWORMS
3. Tapeworms
- long, flat parasitic
flatworms
- Live in the intestines
of their hosts
ROUNDWORMS
Phylum Nematoda
Unsegmented worms that have
pseudocoeloms
A body cavity that lies between the
endoderm and the mesoderm
tissues
Also have a digestive system with a
mouth and an anus
ROUNDWORMS AND BODY FUNCTIONS
Rely on diffusion for respiration,
circulation and excretion
Muscles and fluid in the
pseudocoelom act as a hydrostatic
skeleton
Reproduce sexually by internal
fertilization
CHARACTERISTICS OF ROUNDWORMS
Most are free-living
but some are
parasitic
Parasitic include:
 Trichinosis-causing
worms
 Filarial worms
 Ascarid worms
 Hookworms
ANNELIDS
 Phylum Annelida
 Earthworms
 Segmented bodies
 Have a true coelom that is lined with tissue from the
mesoderm
 Internal walls called septa separate the segments
that divide the body
 Most segments are similar to one another
 A few are specialized with eyes or antennae
 Many have bristles called setae attached to each
segment
INTERNAL STRUCTURE OF ANNELIDS
Complex organ structures
Many contain a pharynx that is used to get
their food
Food moves through the crop, where it is
stored
Then moves to the gizzard, an organ that
grinds it into smaller pieces
Have a closed circulatory system in which
blood is contained in vessels
EARTHWORMS
Class Oligochaeta
Hermaphroditic
Has both male and female reproductive
organs
When eggs are ready to be fertilized, a
clitellum, a band of thickened segments,
secretes a mucous ring in which fertilization
takes place
Streamlined body with few setae
Can live in fresh water or soil
MOLLUSKS
Phylum Mollusca
Soft-bodied animals that often have
an internal or external shell
Many have a larval stage called a
trochophore
Can be herbivores, carnivores,
detritophores or parasites
BODY PARTS OF A MOLLUSK
 Foot
 Used for crawling,
burrowing and capturing
prey
 Mantle
 A thin layer of tissue that
covers most of the body
 Visceral mass
 Makes up the internal
organs
 Shell
 Glands secrete calcium
carbonate
BODY SYSTEMS OF THE MOLLUSK
Either closed or open circulatory systems
When it is closed, blood is pumped
through vessels into a large saclike
structures called sinuses
When it is open, blood is contained in
blood vessels throughout the body
Nervous systems varies from simple to
complex
CLASSIFICATION OF MOLLUSKS
 Gastropods
 Includes pond snails and land slugs
 Single-shelled or shell-less
 Move by using a muscular foot on their left ventral side
 Use a flexible tongue called a radula to eat
 Bivalves
 Clams, oysters, mussels and scallops
 Two-shells held together by muscles
 Cephalopods
 Includes octopus and squid
 Soft-bodied mollusks in which the head is attached to a single
foot
 Foot is divided into tentacles
 Small internal shells or no shell at all
 Have complex sense organs