CLASSIFICATION OF MOLLUSCA
• The word Mollusca is derived from the latin
word mollis which means soft bodied.
• GENERAL CHARACTERISTICS
• It is the second largest phylum of invertebrates
consisting of more than 80,000 living species
and about 35,000 fossil species.
• The adults are triploblastic, bilaterally
symmetrical animals with a soft unsegmented
body. However, the bilateral symmetry may be
lost in some adult mollusc.
• Majority of them are enclosed in a calcareous
shell. The shell may be external or in a few
molluscs it may be internal, reduced or
absent.
• They have a well marked cephalisation.
• The body is divisible into head, mantle,
foot and visceral mass.
• The visceral mass is enclosed in a thick
muscular fold of the body wall called
mantle which secretes the shell.
• Body cavity is a haemocoel.
• The buccal cavity contains a rasping
organ called radula. It helps the animal
in feeding. In some of the molluscs,
however, it is secondarily lost.
• A ventral muscular foot is an adaptation
for locomotion, primarily for crawling.
In some, it is secondarily modified for
swimming or burrowing.
• Paired gills or ctenidia are the primary
aquatic respiratory organs. Some of the
molluscs may possess pulmonary sac
for breathing atmospheric oxygen.
• Blood vascular system is of the open type
with dorsal heart and a few blood
vessels. However cephalopods show a
tendency towards a closed system.
• Respiratory pigment is haemocyanin.
• Excretion is by a pair of kidneys or
nephridia or organ of Bojanus.
• An olfactory organ, the osphradia is
situated at the base of the gills. The
osphradia tests the purity of water that
enters the mantle cavity.
• They are hermaphrodite.
• Development is generally indirect having
larval stages but direct development is
also found in some molluscs. Trocophore
larva, veliger larva and glochidium
larva are the characteristic larvae of
mollusc that have an indirect
development.
Eg: Mussel, Oyster,Snail,Cuttlefish,Chiton
• Molluscs are classified into seven classes
according to their symmetry, shell, mantle,
gills, nervous system and radula.
1.Gastropoda
2.Amphineura
3. Solenogastres 4. Pelecypoda
5. Scaphopoda
6. Cephalopoda
7. Monoplacophora
Class 1: Gastropoda
Characters
1. Molluscs with distinct head bearing
tentacles and eyes.
2. A ventral flat muscular foot .
3. Univalve shell which is spirally coiled.
4. The mantle which secretes the shell is a continuous fold
enclosing the visceral mass,which is also spirally coiled.
5.The disposition of the internal organs shows asymmetry which
is well marked in the nervous system
Classified in to 2 subclasses
• Sub class I:Streptoneura
(Prosobranchiata)
1.Presence of shell & operculum
2.The head bears a single pair of
tentacles.
3.The mantle cavity opens anteriorly.
4.The ctenidium lies infront of the heart.
5.Sexes mostly separate & the males have
usually a copulatory organ.
• Order:1 Aspidobranchiata (Diotocardia)
Example:Patella(Limpet)
Patella
• Order 1: Diotocardia
1.Bilateral symmetry are seen in the presence of two
ctenidia,two kidneys & two auricles.
2. Ctenidia are bipectinate & free anteriorly.
Example:Patella(Limpet)
• Order 2: Pectinibranchiata (Monotocardia)
Examples: Pila globosa( Apple snail)
Turritella
Xancus
Conus
Trochus
Murex
Turritella
Xancus
Conus
Pila globosa
Murex
Trochus
• Characters of Monotocardia
1.Bilateral symmetry is completely lost.
2.There are presence of only a
single auricle ,ctenidium & a single
kidney.
3.Ctenidium is monopectinate and
attached to the mantle throughout its
length.
4. Sexes are separate.
• Sub Class II: Euthyneura
1.Visceral loop is untwisted.
2. Mantle cavity is displaced far
backwards.
3. Head bears two pairs of tentacles.
4.No operculum.
5.Sexes are united.
Order 1: Opisthobranchiata
Example :Aplysia(Sea hare)
• Characters:
1. All marine with aquatic respiration
2. Mantle cavity is idely open & occupies
a posterior position.
3. Shell shows a tendency to reduction.
4.Ctenidium also disappear.
• Order 2: Pulmonata
Example : Limnaea (Pond snail),Achatina
1. Mantle cavity is reduced.
2. The shell is often reduced.
3. Operculum is never present.
4. They are terrestrial but some live in fresh
water.
• Class 2: Amphineura
It includes the chitons which live in
the sea between tide-marks, generally attached
to rocks. They are sluggish animals feeding on
sea weeds. Their body is elongate and dorsoventrally compressed. A flat sole like foot
extending along the whole length of the body
serves for creeping & adhering to the
substratum.
CHITON
• A calcareous shell is present on the dorsal side
& it is composed of eight overlapping plates.
There is no distinct head. Body is bilaterally
symmetrical & covered by a mantle. The
mouth & anus are at the opposite ends of the
body.
• There are numerous pairs of bipectinate
ctenidia lying on either side of the body in the
mantle groove between the mantle & foot .
Eyes and special sense organs are absent.
Sexes are separate.
• Class 3 : Solenogastres eg. Neomenia
Worm like degenerate marine, without
mantle, foot or shell. Body is covered with a
skin containing calcareous spicules.Ctenidia
are absent. Alimentary canal is straight.
Hermaphrodite animals.
Neomenia
Dorsal view
Ventral View
• Class 4: Pelecypoda
Bilaterally symmetrical with a
shell of 2 valves , wedge shaped foot.
Distinct head is absent, no tentacles, body
is laterally compressed and covered with
mantle. The gills are lamellate.Coelom is
reduced. The alimentary canal is coiled.
Horny jaws are absent. Eyes are absent.
Sexes are separate.
• Sub Class 1:Protobranchiata
The gills or ctenidia are single pair in
the form of a plume with a central axis and a
row of flattened filaments on either side.The
foot has a flat sole adapted for creeping.
Eg: Solen
SOLEN
Sub Class 2: Filibranchiata
A pair of lamellate gills,in which the
filaments are reflected .Twoadductor muscles
are present.The foot has a byssus glandwhich
secrets byssus threads for atttachment.
Eg: Mytilus(sea mussel),
Lithodomus
MYTILUS
LITHODOMUS
• Sub Class 3: Pseudolamellibranchiata
GillsLamellate.Only one adductor is
present.foot is feebly developed.shell are
unequal.
Eg:Ostrea(edible oyster)
OSTREA
(EDIBLE OYSTER)
Sub Class 4: Eulamellibranchiata
GillsLamellate.shell equivalved.Both
adductors are well developed.
order (i)
Intergripalliata
Eg: Lamellidens ,Unio
Order(ii)
Sinupalliata
Eg: Cardium,Pholas
LAMELLIDENS
UNIO
CARDIUM
PHOLAS
• Sub Class 5: Septibranchiata
Gills in the form of a perforated
muscular partition extening between the
visceral mass and the mantle.
Eg:Poromya.
Class 5: Scaphopoda(Tusk shells)
It forms Small group of
Mollusca,consisting of 3 genera. All
marine,living in the sand at great
depths.Mantle folds are fused ventrally, so
as to form a tube,shell secreted by the
mantle.No distinct head.Sexes are
separate.Eyes are absent.
Eg:Dentalium
DENTALIUM
Class 6:Cephalopoda
It is an active swimmer .It feeds on animals like
crabs,prawns,small molluscs and marine worms.It has
no external shell,But internal shell is present.
• Sub Class I:Dibranchiata
It has the mouth is surrounded by eight or ten
arms bearing suckers on their oral faces.The funnel is
complete tube .An ink sac is present.
The Dibranchiata are divided into two orders,
Decapoda and Octopoda
• Order 1:Decapoda
There are ten arms ,two of which
(tentacular arms) are longer than the rest
and retractile into special pouches.The
arms bear pedunculated suckers.
Eg:Sepia ,Loligo
SEPIA
• Order 2: Octopoda
They have only eight arms.The body is short
and rounded aborally.Suckers have no horny
rings.The shell is absent.
Eg:Octopus
OCTOPUS
• Sub Class:Tetrabranchiata
Body is enclosed in a spirally
coiled many chambered shell,lying all in one
plane.
2 orders:
1. Nautiloidea – Eg: Nautilus
It is nocturnal and lives at moderate
depths swimming or crawling with its arms.
2.Ammonitoidea – Extinct
NAUTILUS
• CLASS MONOPLACOPHORA:
• Gr. Monos means one; plax means plate;
pherein means bearing.
• 1. The class consists of a small group of living
species which are marine.
• 2. They possess a single, bilaterally
symmetrical shell which varies in shape from a
flattened, shield-like plate to a short cone.
• 3. The repetition of gills, retractor muscles,
auricles and nephridia is a characteristic
feature of living species.
4. They possess a broad, flat foot with pedal
retractor muscles.
5. The fossil species show three to eight pairs of
muscle scar.
Example: Neopilina
FOSSIL SHELL OF MONOPLACOPHORA
1.Match the animal with its class and choose the correct answer
1.Chiton
a) Solenogastres
2.Dentalium
b) Amphineura
3.Pila
c) Scaphopoda
4.Neomenia
d) Gastropoda
A)1d, 2c, 3b, 4a B) 1b, 2c, 3d, 4a C) 1c,2d,3a,4b
D) 1d,2b,3c,4a
2.The organ in molluscs which helps in testing the purity of
water is
a.Ctenidium b. Kidney c. Osphradium d.Ommatophore
3.Worm-like degenerate marine molluscs belong to the
class
a.Amphineura b. Solenogaster c. Gastropoda d.
Cephalopoda
4.Neopilina belongs to the class
a.Amphineura b. Monoplacophora c. Gastropoda d.
Scaphopoda
5.Give an example for Scaphopoda .
6.Mention any two characteristic features of
Solenogasters
7.What is osphradium?
8.Mention any two features of the order Octopoda
9.Mention the functions of byssus thread
10.Match the order with example and choose the correct
answer
1. Aspidobranchiata
a.Achatina
2. Pectininibranchiata
b. Patella
3. Ophisthobranchiata
c. Pila
4. Pulmonata
d. Aplysia
A)1a, 2b, 3c, 4d B) 1d,2b,3c,4a
C) 1c,2d,3a,4b D) 1b, 2c, 3d,
11.Chitons are able to roll their body, because of
a) long, soft body b) adhesive foot c) overlapping shell
plates
d) dorso-ventrally flattened body
12. Give an example for Monoplacophora.
13. Give an example for Gastropoda
14. An economically important pelecypod group is
a) Snail b) Mussels c) Oysters
d) Chitons
15. A mollusc that has unique defense mechanism by ink gland
is
a) Pila b) Sepia c) Oyster d) Murex