Phylum:
Mollusks
Three Classes of Mollusks
1. Class Gastropoda – snails, slugs
2. Class Bivalvia – clams, oysters, mussels,
scallops
3. Class Cephalopoda – octopi, squids and
cuttlefishes
Basic information
1. Over 100,000 living species
2. Marine, fresh water, and land
3. Most Bilateral symmetry
Body Plan
• Divided into two regions:
1. head/foot
2. visceral mass
Head/foot
• Head (contains mouth and variety of sensory
structures)
• Foot (muscular organ used for locomotion)
Visceral Mass
1.
2.
3.
4.
heart
digestion
excretion
reproduction
Mantle
1. Covers and protects the visceral mass
2. Secretes shell
mantle cavity
• Location of gills
• Space between the mantle and the visceral
mass
nervous system
• Ganglia: paired cluster of nerve cells
1. locomotion
2. feeding
3. process sensory information (light, touch)
Feeding
• Radula: flexible, tongue like strip of tissue
covered with tough abrasive teeth that point
backward
Feature
Gastropoda
Bivalvia
Cephalopoda
External Shell
0-1
Two
None (except
nautilus)
Head
Yes
No
Yes
Radula
Yes
No
Yes
Locomotion
Crawl
sessile
Rapid swimming
Class Gastropoda
Basic Information
•
•
•
•
Most diverse class of mollusks
90,000 species
Snails, abalones, conches= single shell
Slugs and nudibranchs= no shell
Torsion
• Visceral mass twists around 180° in relation to
the head
• Twisting results in mantle cavity, gills, and
anus to the front of the animal
• Gastropod is now able to with drawl its head
into mantle cavity when threatened
Movement
• foot secrets a substance allowing animal to
glide over surfaces (Slime Trail)
Open circulatory system
• HEMOLYMPH (blood in an organism with open
circulation) does not remain in vessels
1. Collected from gills or lungs
2. Pumped through heart
3. Released directly into spaces in the tissues
a) Fluid filled spaces or blood cavity
Bivalvia: Clams
Other Bivalvia: scallops, mussels,
oysters
Characteristics
1.
2.
3.
4.
5.
Sessile
Filter feeders
No radula
No cephalization
Aquatic
Anatomy of Valves
• Shell is divided into two
halves (valves)
• Connected by a hinge
• Adductor muscles
– Contract: close valves
– Relax: open vlaves
Clam Sensory information
• 3 pairs of ganglia
– mouth (cerebral)
– digestive system (visceral)
– Foot (pedal)
Foot
• Foot helps
burrow in the
sand or mud
Water flow
• water enters through
incurrent siphon
• Water exits through
excurrent siphon
Steps in digestion: Filter Feeders
1. Cilia in gills set up water current
2. Gills filter water for small organisms
3. Palps: flaplike structures that surround &
guide food into the clam's mouth
4. Food then enters the mouth
Steps in Digestion: Filter Feeders
4. Stomach: Digestion begins
5. Digestive glands: digested particles are
absorbed
6. Intestines: collects and removes digestive
wastes
7. Waste are passed through the rectum and
excreted through anus
Function of Gills
1. Trap food particles
2. Exchange oxygen and carbon dioxide
Growth rate
• Umbo oldest part
of the clam
• Growth rings
CLASS
CEPHALOPODA
Most advanced class
Cephalopod
“Head-foot”
• foot is concentrated in the head region
• foot is modified into arms and tentacles
equipped with suckers
• Foot also forms funnel (siphon) for expelling
water, allowing movement by "jet
propulsion"
Squid
Cuttlefish
The major distinction between the squid and octopus
Squid suction cups are armed with hooks or sucker rings (or a combination of
the two). Octopus have simple suction cups without secondary armature.
Squid
Suction
Cups
Tentacle club of
Architeuthis,
showing circularsaw-like sucker
rings.
tentacle club of
Mesonychoteuthis,
with swiveling hooks.
Profile of Mesonychoteuthis tentacle club, showing
hooks.
Suction cups of Haliphron atlanticus, the giant
gelatinous octopus (the world's largest species of
octopus), lacking secondary armature.
Giant Squid
Can reach
length of up to
60 feet and
weight of
more than 3.5
tons.
Architeuthis dux
Nautilus
Octopus
Locomotion
• Jet propulsion by using
siphon to force water out
• Crawling – octopus
*Also uses siphon
External Features
• Octopus: 8 arms with either one or two rows of
suction cups (but never hooks or sucker rings),
• Squid/cuttlefish: 10 appendages containing suction
cups (contain either hooks or sucker rings)
• 8 arms
• 2 tentacles
Ink
–All except nautilus have ink sack
which empties into rectum; ink
contains the pigment melanin
(same pigment as human skin)
–Released when the animal is
alarmed. The animal quickly
departs from the scene leaving the
ink as a decoy to the predator.
Color Changes
• Chromatophores – pigment cells that expand and
contract to produce color change.
– Used as danger signals, protective coloring, and for courtship.
The Blue Ringed Octopus (found in shallow coral and rock pools of Australia): It’s
poisonous saliva is 10,000 more potent than cyanide. This octopus is only the size of
a golf ball but carries enough poison to kill 26 humans in minutes.
Feeding
1. Fish, other
mollusks,
crustaceans, worms
2. Beak like jaws and
radula tear prey into
pieces
3. Octopus and
Cuttlefish have
poison in saliva
Circulatory
• Closed circulatory system
• Blood delivers oxygen and
nutrients directly to organs
through veins and arteries
Nervous System
• Well-developed brain; eyes which are
similar in construction to vertebrate eyes
• FYI: The giant squid has the largest eye of any animal, either living or
extinct. In a 55-foot specimen the diameter was 15.74 inches. In
comparison, a blue whale's eye has the diameter of 4.70 inches, and
humans have an eye diameter of .94 inches.
Reproduction
1. Male or Female
2. The male uses arm to take sperm from own
mantle cavity and insert into females mantle
cavity
3. The female lays ~100 eggs and guards them until
they hatch (approx 50 days)
4. Frequently the mother dies soon after the eggs
hatch because during the guarding of the eggs she
is unable to eat. 
Squid Vs Octopus