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

Trochus Murex

• 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 dorso-

ventrally 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) Order(ii)

Intergripalliata Sinupalliata

Eg: Lamellidens ,Unio 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