Phylum: Mollusks

Three Classes of Mollusks

1. Class Gastropoda – snails, slugs2. Class Bivalvia – clams, oysters, mussels,

scallops3. Class Cephalopoda – octopi, squids and

cuttlefishes

Basic information

1. Over 100,000 living species 2. Marine, fresh water, and land3. 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 Mass1. heart2. digestion3. excretion4. reproduction

Mantle

1. Covers and protects the visceral mass2. Secretes shell

mantle cavity

• Location of gills• Space between the mantle and the visceral

mass

nervous system

• Ganglia: paired cluster of nerve cells1. 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 heart3.Released directly into spaces in the tissues

a) Fluid filled spaces or blood cavity

Bivalvia: Clams

Other Bivalvia: scallops, mussels, oysters

Characteristics

1. Sessile2. Filter feeders3. No radula4. No cephalization5. 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 current2. Gills filter water for small organisms3. Palps: flaplike structures that surround &

guide food into the clam's mouth4. Food then enters the mouth

Steps in Digestion: Filter Feeders

4. Stomach: Digestion begins5. Digestive glands: digested particles are

absorbed6. Intestines: collects and removes digestive

wastes7. Waste are passed through the rectum and

excreted through anus

Function of Gills

1. Trap food particles2. Exchange oxygen and carbon dioxide

Growth rate

• Umbo oldest part of the clam

• Growth rings

CLASS CEPHALOPODAMost 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 ofthe two). Octopus have simple suction cups without secondary armature.

Tentacle club of Architeuthis, showing circular-saw-like sucker rings.

tentacle club of Mesonychoteuthis, with swiveling hooks.

Squid Suction Cups

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 SquidCan 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 Female2. 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