Download Worms and Mollusks

Worms and
Mollusks
Characteristics of Worms
• All worms are invertebrates that have long, narrow bodies
without legs
• Body structure:
– Bilateral symmetry; they have head and tail ends, all tissues,
organs and body systems.
• Nervous System:
– Simplest organisms with a brain; brain controls most of its body
functions
– Can detect objects, food, mates and predators quickly
• Reproduction:
– Some reproduce asexually by dividing into two separate
organisms (genetically identical)
– most reproduce sexually
• Some are hermaphrodites, but still exchange sperm
to fertilize egg
Flatworms
• Flatworms are flat and soft as jelly
• Some are a parasite: an organism that lives inside or on another
organism
– robs food from its host and makes host weak
– may injure the host’s tissues or organs, but rarely kill
• Others (Planarians) are free living flatworms
– scavengers: they feed on dead or
decaying material
– predators; feeds like a vacuum
cleaner
• Tapeworms: one kind of
parasitic flatworm
– Absorbs food from the host’s digestive
system
– Most live in more than one host in their lifetime
(some even live in human hosts-see figure 15)
types of flatworms
tapeworms
planarian
Roundworms
• Can live in nearly any moist environment; most abundant animals on
Earth; cylindrical bodies (unlike flatworms)
• Have a digestive system that is like a tube, open at both ends: food
enters at mouth, and exits through an opening (anus)
• One-way digestive system is efficient; like an assembly line 
enables the animal’s body to absorb a large amount of food
Segmented Worms
• Earthworms, leeches, and some sea-floor
worms are segmented worms
• Body structure
– Segmented worms have bodies
made up of many linked sections
called segments
– Have well-developed organ systems;
ex. Nervous system that includes a
brain and a nerve cord that runs the
length of the worm’s body
– One-way digestive system, like
roundworms (see figure 17)
• Closed circulatory system: blood moves
only within a connected network of tubes
called blood vessel; can move quickly
Characteristics of Mollusks
•
•
•
•
•
Mollusks: invertebrates with soft, unsegmented bodies that are often protected by a hard
outer shell
In addition to a soft body often covered by a shell, a mollusk has a thin layer of
tissue called a mantle that covers its internal organs, and an organ called a foot
Body structure
– have bilateral symmetry
– unlike segmented worms, body parts aren’t usually repeated
– digestive system with 2 openings
Open circulatory system
– blood is not always in blood
vessels; sloshes around inside body
Obtaining oxygen
– gills: organs that
remove oxygen from the water
and carry carbon dioxide out of blood
Diversity of Mollusks
The three major groups of mollusks are…
Gastropods
-include snails and slugs
-single shell or no shell
-herbivores, scavengers,
or carnivores
-all use a radula to obtain
food (flexible organ with
tiny teeth)
Bivalves
-mollusks that have two
shells held together by
hinges and strong
muscles
-do not have radulas: use
filter feeders
-most are herbivores
-don’t move quickly
Cephalopods
-ocean-dwelling mollusk
whose foot forms tentacles
around its mouth
-only mollusks with a
closed circulatory system
-Carnivores: use muscular
tentacles to capture prey
-large brains