Molluscs

Kingdom: Animalia

Phylum: Mollusca

Class: CephalopodaClass: Gastropoda

Class: Bivalvia

Class: Polyplacophora

Characteristics

• Diverse species

• Coelomates- have a

true coelom

– Cavity completely surrounded by mesoderm

Reproduction

• Trocophore Larvae

– Shared with annelids

• Shows evolutionary link

• Trocophore hatches

from egg case

– Cilia – used for swimming and for eating

– Movement allows for distribution of species

Body Plan• Divided into two main

regions

– Head-foot

• Head - mouth and sensory structures

• Foot – large muscular organ used for locomotion

– Visceral mass

• Above head-foot

• Contains heart and organs for digestion, excretion and reproduction

• Covering this is a layer of epidermis called mantle

Respiration and Gas Exchange• Mantle

– Secretes one or more shells

containing CaCO3

– Protects organism but also

reduces surface area for gas exchange

– Overcome by adaptation called gills

• Provide large surface area with rich blood supply

• Gills located in Mantle Cavity

– Hollow space under shell

Nervous system

• Bilateral symmetry

– Head region is

reduced in several classes

• Ganglia

– Clusters of nerve cells

located in head-foot and connected by two

long nerve cords to visceral mass

Circulatory system• Open

• Hemolymph –

circulatory fluid

– Does not remain within blood vessels

– Once filled with O2, it travels to heart and then

is pumped throughout spaces in tissue

• Hemocoel – blood

cavity, these fluid-filled

spaces

Digestive System

• Main Feeding Adaptation = Radula

– Flexible, tongue-like strip of tissue covered with tough, abrasive teeth (chitin) that point backward

• Modified for different

functions

– Cutting, scraping, drilling, harpooning

• One-way, complete gut

Class Cephalopoda

• Name means “head-

foot” – foot near head

• Adaptation:

Specialized for free-

swimming, predatory

existence

Main Characteristics• Often have jaw-like beaks

– Sometimes with poison glands

• Radula present – behind beak– used to move food into gut

• Locomotion - Foot modified into 8-10 arms or tentacles and a siphon– Siphon is part of mantle used

for propulsion

• Skeletal: Shell absent – peninside squid is remnant

• Fins on end of body for steering

• Very complex sensory, nervous, locomotion and behavior systems

Sketch

Types of Cephalopods• Squid

• Octopus

• Nautilus

• Cuttlefish

Class Gastropoda• “Belly foot”

• Include snails, abalone, conchs, slugs, nudibranchs

• Most with single shell or no shell

• Torsion - larvae have visceral mass twist around 1800

– This brings the mantle cavity, gills and anus to the front above the head

• Allows for head to be withdrawn into cavity

Unique Reproduction characteristic

• The Gastropoda and Bivalvia trocophore larva transforms into a second larval stage– Veliger larva

• Has a foot, shell

• Velum – unique feeding organ made of two large ciliated lobes

• Eventually eyes and tentacles appear to aid in settling on substrate

Pictures of Gastropods

More picts

http://www.life.uiuc.edu/slugcity/movie/navanax_eat_he

m.mpg

http://www1.chapman.edu/~wwright/Navanax4.mov

More PicturesChestnut Cowrie

Common Limpet

Class Bivalvia• Clams, oysters, and scallops

• Bivalves - shell is divided

into two halves or valves connected by hinge

• Adductor muscle - closes

shells by contracting (what we

eat)

• Shell made of three layers

– One - protect from acidic conditions

– One - CaCO3 to strengthen

– One - smooth, iridescent to protect soft body

• If sand gets in, mantle covers it

in smooth iridescent material –

a pearl

Feeding and Respiration• Most bivalves are sessile

• Some use muscular foot to

pull down into sand

• Adaptation: Filter feeders

– no radula

• Ctenidia = Gill

– functions as trap for food particles and CO2 and O2

transporter

Nervous system

• Lack distinct head

• Nervous system – (small change)

3 pairs of ganglia

– Near mouth, digestive system and foot

• Nerve cells receive info

from sensory cells near

mantle edge

– Can include eye spots

Clams• Bivalves that live in

mud or sand

• Mantle cavity is sealed except for pair of hollow, fleshy tubes called siphons

• Incurrent siphon –water enters

• Excurrent siphon –water leaves

• Clams have separate sexes– Eggs fertilized internally

Other bivalves• Mussels - special

way to attach to

substrate

– using byssal threads

• Scallops - can move

through water by

rapidly opening and

closing valves

Class Polyplacophora• Called Chitons

• Herbivores - radulascrapes algae off rocks

• **Shell is modified into 8 plates

• The mantle usually covers the edges of the plates.

• Plates allow to bend around crevices and still stay attached

• Live in intertidal zones