WESTERN INSTITUTE OF TECHNOLOGY (BS BIOLOGY-1ST YEAR)
GENERAL ZOOLOGY (NS4)
1st Semester
By: Maria Yzabelle Cabalum
PHYLUM MOLLUSCA
Mollusca, meaning “soft-bodied,” is one of the largest phylums in the animal kingdom. The word mollusc
(or mollusk) derives from the Latin word “mollis,” which means “soft.”
There are about 200,000 species of mollusk worldwide accounting for nearly a quarter of all marine life.
Familiar phylum species include snails, squid, octopus, clams and oysters.
Characteristics
Theyareinvertebrates
Coveredwithprotectivemantlethatmayormaynotformahard,calciumcarbonateshell
Amantlethatsecretsanexoskeletonorshellfurtherdistinguishesmollusksfromotheranimals.
HaveamuscularfootformovementwhichismodifiedintotentaclesforCephalopods.
Symmetry-bilaterallysymmetrical
Theadulttissuesarederivedfromthreelayersofembryonicgermtissuewhicharemesoderm
and ectoderm. That is why animals in this phylum are triploblastic
BodyCavity-trueandfullylinedcoelom(truecoelom)
Cephalization-haveadistinctheadwithsenseorgans&brain
Thebodyconsistsofvarioustissuesandorgan-systems,whichperformtheactivityofthebody
that is why they have organ grade of organization.
Mostmollusksaredioecious(separatesexes)
Somearehermaphroditic
Thelifecycleofmanymollusksincludesafreeswimming,ciliatedlarvalstagecalleda
TROCHOPHORE
Mollusk Body Plan
All mollusks have a similar body plan with three main parts:
1. Muscular foot - used in locomotion
2. Visceral mass – covered with a soft epithelium that contains digestive, circulatory,
respiratory and reproductive organs.
3. Mantle – outer layer that houses the gills and in some secretes a protective shell over the
visceral mass.
Most mollusks have separate sexes with gonads located in the visceral mass.
Mantle Cavity
The space between the mantle and the visceral mass (body organs) is called the mantle cavity.
The respiratory organs (gills or lungs) are generally housed here.
Head-Foot Region
Snails and slug feed using a flexible tongue-shaped structure known as a radula.
Hundreds of tiny teeth are attached to the radula.
Radula is used to scrape algae off rocks or to eat the soft tissues of plants.
Most mollusks have well developed head ends with sensory structures that may be simple light
detectors or complex eyes (cephalopods).
,Phylum Mollusca
o Mostmollusksaremarine
o Somegastropodsandbivalvesinhabitfreshwater
o Afewgastropods(slugs&snails)areterrestrial
Shells
Foundinsnails,bivalvemollusks,chitons,andnautilus
Madeofcalciumcarbonate(limestone)
Secretedbythemantle.
Oneofthefunctionsofthemantleistosecretetheshell.
Somebivalveslikeoystersandmusselspossesstheuniqueabilitytosecreteanddeposita
calcareous nacre or “mother of pearl” around foreign particles that may enter the mantle cavity
Internal Structures and Function
Nervous system
o ismadeupofpairedganglia,connectives,andnerves.
o Theventralmuscularfoothelpsinlocomotion.
Excretory System
o theyhaveapairofkidneys(metanephridia-atypeofexcretoryglandswithaciliatedfunnel
opening into the body cavity.
Reproduction
o Sexesareseparateandreproductionisthroughsexualreproduction.
Digestive System
o well-developedandcomplex.Ithasaradula,whichisaraspingstructurethathaschitinousteeth.
It is used in feeding
Circulatory system
o Manymolluskshaveanopencirculatorysystemwithapumpingheart,bloodvesselsandblood
sinuses except cephalopods (squid & octopus), they have a closed circulatory system with a
heart, blood vessels and capillaries.
Respiration
o Mostmolluskshaverespiratorystructurescalledgills.Gillsarespecializedpartsofthemantlethat
consists of a system of filamentous projections that contain a rich supply of blood for the
transport of gases.
o Inland,snailsandslugs,themantlecavityappearstohaveevolvedintoaprimitivelung.
Major Mollusk Classes
Four major classes of mollusks:
ClassPolyplacophora–thechitons
ClassGastropoda–snails&slugs
ClassBivalvia–clams,mussels,oysters
ClassCephalopoda–octopus&squid
, Class Polyplacaphora
Animals in the class Polyplacophora (“bearing many plates”) are commonly known as “chitons”
and bear an armor-like eight-plated shell.
Calcareous spines may be present on the girdle to offer protection from predators.
These
animals possess a radula that is modified for scraping.
Chi to n
Class Gastropoda
Gastropodaisthelargestofthemolluskclasses."stomachfoot"70,000namedspecies.
Includesnails,slugs,seahares,seaslugs,seabutterflies.
Marine,freshwater,terrestrial.
Slugslackashell!
Gastropoda includes shell-bearing species as well as species with a reduced shell.
These animals are asymmetrical and usually present a coiled shell.
Shells may be planospiral (like a garden hose wound up), commonly seen in garden
snails, or conispiral, (like a spiral staircase), commonly seen in marine conches
Class Bivalvia
Bivalvemolluskshavetwoshells(valves).
Mussels,clams,oysters,scallops,shipworms.
Mostlysessilefilterfeeders.
The overall morphology is laterally flattened, and the head region is poorly developed.
Bivalves has siphon- a tube-like structure through which food and water enters and leaves the body.
ClassBivalvia–Locomotion
o Bivalvesmovearoundbyextendingthemuscularfootbetweentheshellswhich
extends out between the shells and is used for locomotion.
o Theyswimbyclappingtheirshellstogethertocreatejetpropulsion.
GENERAL ZOOLOGY (NS4)
1st Semester
By: Maria Yzabelle Cabalum
PHYLUM MOLLUSCA
Mollusca, meaning “soft-bodied,” is one of the largest phylums in the animal kingdom. The word mollusc
(or mollusk) derives from the Latin word “mollis,” which means “soft.”
There are about 200,000 species of mollusk worldwide accounting for nearly a quarter of all marine life.
Familiar phylum species include snails, squid, octopus, clams and oysters.
Characteristics
Theyareinvertebrates
Coveredwithprotectivemantlethatmayormaynotformahard,calciumcarbonateshell
Amantlethatsecretsanexoskeletonorshellfurtherdistinguishesmollusksfromotheranimals.
HaveamuscularfootformovementwhichismodifiedintotentaclesforCephalopods.
Symmetry-bilaterallysymmetrical
Theadulttissuesarederivedfromthreelayersofembryonicgermtissuewhicharemesoderm
and ectoderm. That is why animals in this phylum are triploblastic
BodyCavity-trueandfullylinedcoelom(truecoelom)
Cephalization-haveadistinctheadwithsenseorgans&brain
Thebodyconsistsofvarioustissuesandorgan-systems,whichperformtheactivityofthebody
that is why they have organ grade of organization.
Mostmollusksaredioecious(separatesexes)
Somearehermaphroditic
Thelifecycleofmanymollusksincludesafreeswimming,ciliatedlarvalstagecalleda
TROCHOPHORE
Mollusk Body Plan
All mollusks have a similar body plan with three main parts:
1. Muscular foot - used in locomotion
2. Visceral mass – covered with a soft epithelium that contains digestive, circulatory,
respiratory and reproductive organs.
3. Mantle – outer layer that houses the gills and in some secretes a protective shell over the
visceral mass.
Most mollusks have separate sexes with gonads located in the visceral mass.
Mantle Cavity
The space between the mantle and the visceral mass (body organs) is called the mantle cavity.
The respiratory organs (gills or lungs) are generally housed here.
Head-Foot Region
Snails and slug feed using a flexible tongue-shaped structure known as a radula.
Hundreds of tiny teeth are attached to the radula.
Radula is used to scrape algae off rocks or to eat the soft tissues of plants.
Most mollusks have well developed head ends with sensory structures that may be simple light
detectors or complex eyes (cephalopods).
,Phylum Mollusca
o Mostmollusksaremarine
o Somegastropodsandbivalvesinhabitfreshwater
o Afewgastropods(slugs&snails)areterrestrial
Shells
Foundinsnails,bivalvemollusks,chitons,andnautilus
Madeofcalciumcarbonate(limestone)
Secretedbythemantle.
Oneofthefunctionsofthemantleistosecretetheshell.
Somebivalveslikeoystersandmusselspossesstheuniqueabilitytosecreteanddeposita
calcareous nacre or “mother of pearl” around foreign particles that may enter the mantle cavity
Internal Structures and Function
Nervous system
o ismadeupofpairedganglia,connectives,andnerves.
o Theventralmuscularfoothelpsinlocomotion.
Excretory System
o theyhaveapairofkidneys(metanephridia-atypeofexcretoryglandswithaciliatedfunnel
opening into the body cavity.
Reproduction
o Sexesareseparateandreproductionisthroughsexualreproduction.
Digestive System
o well-developedandcomplex.Ithasaradula,whichisaraspingstructurethathaschitinousteeth.
It is used in feeding
Circulatory system
o Manymolluskshaveanopencirculatorysystemwithapumpingheart,bloodvesselsandblood
sinuses except cephalopods (squid & octopus), they have a closed circulatory system with a
heart, blood vessels and capillaries.
Respiration
o Mostmolluskshaverespiratorystructurescalledgills.Gillsarespecializedpartsofthemantlethat
consists of a system of filamentous projections that contain a rich supply of blood for the
transport of gases.
o Inland,snailsandslugs,themantlecavityappearstohaveevolvedintoaprimitivelung.
Major Mollusk Classes
Four major classes of mollusks:
ClassPolyplacophora–thechitons
ClassGastropoda–snails&slugs
ClassBivalvia–clams,mussels,oysters
ClassCephalopoda–octopus&squid
, Class Polyplacaphora
Animals in the class Polyplacophora (“bearing many plates”) are commonly known as “chitons”
and bear an armor-like eight-plated shell.
Calcareous spines may be present on the girdle to offer protection from predators.
These
animals possess a radula that is modified for scraping.
Chi to n
Class Gastropoda
Gastropodaisthelargestofthemolluskclasses."stomachfoot"70,000namedspecies.
Includesnails,slugs,seahares,seaslugs,seabutterflies.
Marine,freshwater,terrestrial.
Slugslackashell!
Gastropoda includes shell-bearing species as well as species with a reduced shell.
These animals are asymmetrical and usually present a coiled shell.
Shells may be planospiral (like a garden hose wound up), commonly seen in garden
snails, or conispiral, (like a spiral staircase), commonly seen in marine conches
Class Bivalvia
Bivalvemolluskshavetwoshells(valves).
Mussels,clams,oysters,scallops,shipworms.
Mostlysessilefilterfeeders.
The overall morphology is laterally flattened, and the head region is poorly developed.
Bivalves has siphon- a tube-like structure through which food and water enters and leaves the body.
ClassBivalvia–Locomotion
o Bivalvesmovearoundbyextendingthemuscularfootbetweentheshellswhich
extends out between the shells and is used for locomotion.
o Theyswimbyclappingtheirshellstogethertocreatejetpropulsion.
