Ch 27: Molluscs and Annelids
There are some things on the
review sheet and test that you
should add to this presentation –
eg – mussels have special glue
Phylum Mollusca
Molluscs
– Characteristics that Unify
Have true coelom
Are protostomes
Have trochophore
larvae (like annelids)
Have foot, mantle,
visceral mass, and
possibly a shell
Protostomes, Coelomates
Trochophore larvae
Molluscs
and
Annelids –
a common
ancestor?
Internal Structures of Molluscs
Foot – structure
involved in feeding,
usually contains the
head as well
Visceral mass – contains
the internal organs
Mantle – secretes the
shell
Shell - protection
Molluscs – classified by the
modifications of the Foot
Gastropods –
“stomach foot”
Foot is broad and
muscular
found along the
ventral surface
used for moving,
eating
Examples: Snails,
slugs, nudibranchs
Class
Gastropoda
- Snails
Snail Diagram
Gastropods - Sea
slugs/nudibranchs
Cephalopod –
the tentacled molluscs
“Head –foot”
Foot is modified into
head with tentacles
coming out of it
Have very little shell if
any
Ex. Octopi, squid,
nautilus, cuttlefish,
Cephalopod –
the tentacled molluscs
Cephalopods have
a ‘head’
Cephalopods include:
Octopi
Squid
Cuttlefish
Nautiluses
Have internal shells
Swim with jet propulsion
Bivalves
Have 2
shells –
foot can
extend out
of shell
Clams,
mussels,
oysters
Class Bivalvia
2 shells
Clams
Oysters
Mussels
Typical Shell/Valve
Feeding - Molluscs
Many molluscs have
a structure that is
like a very rough
tongue – has
hundreds of tiny
teeth for grinding
through shells or
scraping algae off of
rocks
Molluscs – Filter feeders
filter
feeders
use their
GILLS to
catch the
tiny food
particles
They also
use the
gills to
BREATHE
Filter Feeders – Environmental
“Watchdogs”
Filter feeders remove
the pollutants from
the water and
concentrate them in
their bodies
Environmentalists can
use these animals to
learn information
about levels of
pollution in the water.
Don’t eat bivalves during a
RED TIDE
Red Tide
Clam – How it eats
Incurrent siphon
pulls water across
gills
Food particles stick
to gills
Coordinated cilia
move food to mouth
Food is
stored/digested in
stomach and
intestines
Solid wastes exit via
the anus
Respiration - Molluscs
Aquatic molluscs use gills
Land snails and slugs use a moist sac inside
the mantle
Sac’s lining is convoluted (to increase surface
area) and lined with blood vessels
Molluscs Circulatory Systems
Marine snails, slugs, clams:
Slow-moving molluscs have “open” circulatory
systems (like some insects)
Blood vessels connect to sinuses where oxygen
transfer takes place
Octopi, squid:
Fast-moving molluscs have “closed” circulatory
systems – blood is completely enclosed in
blood vessels (like ours)
Circulatory System
Comparisons
Excretion – Molluscs and
Annelids
Molluscs and annelids
are coelomates and
protostomes
– that means they
definitely have an
anus for solid wastes!
- they use NEPHRIDIA
for metabolic wastes
(like the nephrons in
our kidneys
Nervous Systems
-Molluscs
Slow moving
molluscs have
simple systems with
simple sensory cells
like the cnidarians
and the worms –
statocysts, ocelli,
chemoreceptors,
touch receptors, etc.
Scallop 
Nervous Systems - Molluscs
Fast-moving
molluscs have very
complex nervous
systems
Octopi:
Well-developed
brain (with
memory)
Eyes that are
similar to ours –
form images
Reproduction - Molluscs
MOST have separate sexes that employ
external fertilization (fertilization by chance!)
Tentacled molluscs and some snails have
internal fertilization
Some are hermaphrodites
Molluscs - reproduction
Oysters –
hermaphrodites that
change sexes each
season – sometimes
they are the males,
sometimes the
females.
Defence Mechanisms Gastropods
Slugs hide under rocks
or only come out at
night
Some gastropods are
poisonous – brightly
colored to warn off
predators
Some nudibranchs save
the nematocysts from
the cnidarians they ate,
and sting their predators
with them
Poisonous !
Cephalopods – Defence
Mechanisms
Octopi move
rapidly
backwards with
their jet
propulsion of
water
Some release
foul-tasting
black ink
Some can
change colours
Squid – 8 arms + 2 long
tentacles
Annelids – Characteristics that
unify
Segmented worms
Coelomates
Trochophore larvae
protostome
Segmentation
Annelids – Feeding
Filter feeders – use
gills to capture food
particles from water
Others use pharynx
to either suck in food
or to extend out and
get it.
Some annelids have
jaws on their
pharnyx
Christmas tree worm

Nereis – (clam worm)
A Polychaete – has jaws for eating
and fighting
Respiration
Earthworm cuticle 
Aquatic annelids use
gills
Terrestrial annelids
exchange gases
through their skin –
their skin must
remain moist for this
purpose
Cuticle on the
earthworm helps
prevent desiccation
Feather-duster worm
– gills exposed (red)
- tentacles for gathering food
Internal
Transport
Closed circulatory system
Dorsal surface has blood
moving toward the head,
ventral surface moves
blood away from the head
Each segment has a ring
vessel connecting the
ventral with the dorsal
Large ring vessels at
anterior end act as a
“heart” to pump the blood
If no heart is present, the
bodies muscular
contractions move the
blood around
Excretion
Solid wastes out the
anus
Metabolic (liquid)
wastes are
eliminated by
nephridia (like the
nephrons in our
kidneys)
2 nephridia/segment
Response
Brain on dorsal
surface
Nerves travel from
the brain, around
the gut and a main
nerve cord travels
down the ventral
surface
Brain = #11
Ganglia = #12
Ventral Nerve cord = #13
Sense organs
Spaghetti worm underground
With tentacles exposed
Best developed in free
living annelids
Most of these have
sensory tentacles,
statocysts,
chemoreceptors, and
“eyes”
Some form images,
some are light-detectors
Some have sensory
organs that detect
vibrations (like our
tympanic membranes)
Self Defence
Tube worms

Burrow into
ground or
swim away
Setae (oneway bristles)
anchor
worms in
the soil
Marine fan
worms
make tubes
of CaCO3 to
hide in
Self-Defence
Marine
fireworms
have
poisonous
bristles
(burns)
Carnivorous
annelids
have jaws to
bite their
predators
Movement
longitudinal muscles
shorten the body
Circular muscles
contract to decrease the
diameter of the worm
Coordination of these
sets of muscles allow
the worms to wiggle,
burrow and swim
Reproduction
Some reproduce by
budding
Some have separate
sexes and have external
fertilization
Palolo worms
congregate at the
surface and release
sperm or eggs at the
same time
Feast for the predators
(humans included!)
Earthworms are
hermaphrodites and
exchange sperm
Sperm is stored in
sacs until eggs are
ready
Mucus ring is secreted
by the clitellum –
sperm and eggs are
deposited there and
the ring slips off the
worm, forming a
cocoon for the
fertilized eggs
Common Characteristics in all
Annelids
Unidirectional Digestive System
Closed Circulatory System

First sign of a ‘heart-like’ structure
Nephridia used for excretion
Primitive Brains present
Class Polychaeta –
bristleworms
Mainly marine worm
Have padddle-like
appendages with
bristles
Show cephalization
More Polychaetes
Polychaetes
Class Oligochaeta Earthworms and Relatives
Earthworms +
Freshwater worms
Aquatic – eg tubifex
(red, thread-like worms
sold for fish food)
Oligo – means few (as
in few bristles)
Castings are excellent
fertilizer
Class Hirudinea
Commonly known as
the LEECHES
Fresh and saltwater
species (~300)
Few land species
Muscular proboscis
is forced into skin
Others slice skin
with their jaws
Leeches
Hirudin – anticoagulating chemical
is used to keep
host’s blood flowing
Some also use an
aesthetic so the host
does not know they
are there
Used for medicinal
purposes
Annelids aerate the
soil by burrowing
Return nutrients to
the soil completing
decomposition of
material
Many annelids are
the basis of food
chains
Internal Anatomy
Identifying practice: