· Johston coined the term Mollusca (Mollusks = soft) meaning soft-bodied animals.
· It is the second largest phylum after Arthropoda.
· Study of Mollusca = Malacology
· Study of the shell of mollusca = Conchology

VVI Examples of Phylum Mollusca for Entrance Exams are:
  • Neopilina (Connecting link between Annelida and Mollusca)
  • Pila (Land snail)
  • Aplysia (Sea horse)
  • Patella (Limpet)
  • Cyprea (Cowrie)
  • Helix (Sand snail)
  • Unio (Freshwater mussel)
  • Mytilus (Marine or sea mussel)
  • Pecten (Scallop)
  • Ostrea (Oyster)
  • Spondylus (Edible oyster)
  • Pinctada (Pearl oyster)
  • Teredo (Shipworm) destruct the wood in seawater.
  • Sea hare
  • Octopus (Devilfish)
  • Sea mice or Sea mica (Chiton)
  • Isochiton
  • Chaetopleura
  • Dentalium
  • Cadulus
  • Sepia (Cuttlefish)
  • Sea lemon
  • Unio (Freshwater mussel)
  • Squids (Loligo)

Characteristics of Phylum Mollusca

· Triploblastic, soft-bodied, coelomate animals with organ system level of body organization and tube within a tube body plan.
· Habitat – Mostly marine and freshwater animals but some species are terrestrial living in damp places.

A. External features
· Body is unsegmented except Neopilina (The connecting link between Annelida and Mollusca).
· Bilaterally symmetrical except Gastropods which are asymmetrical due to coiling/torsion.

B. Body divisions
The body is divisible into 4 parts.
a) Head – Bears mouth, tentacles, and other sense organs.
b) Ventral muscular foot – For locomotion
c) Visceral mass/hump – Digestive and circulatory system.
d) Mantle/pallium
· Thick, muscular, glandular fold of body wall.
· Mantle cavity is present between the body and mantle.
· A hard calcareous shell is secreted by the mantle.

Shell may be External (eg. Pila), OR Internal (Example: Sepia, Slug, Sea hare), or absent (Octopus, Sea lemon).

· Body wall consists of a single layer of ciliated epidermis followed by unstriped muscles occurring in a bundle.
· They are haemocoelomate. The true coelom is represented by pericardial, nephridial, and gonadial cavities.

C. Digestive system
· Alimentary canal is complete; mostly straight.
· Mouth guarded by an operculum.
· The buccal cavity is provided with a rasping organ called Radula having transverse rows of 7 teeth (odontopore).
· Digestion is completed in hepatopancreas.

D. Respiratory system
· In aquatic forms by comb-like gills called ctenidia.
· Terrestrial form by pulmonary sac (lung-like).
· Also takes place by Mantle or body surface.

E. Circulatory system
· Open type (exception: closed type in Sepia) with many large blood spaces in the tissues.
· Blood is blue in colour due to the presence of a copper-containing respiratory pigment called Haemocyanin.
· Dorsally situated heart and few blood vessels are also present.
· Heart is Myogenic; 2 chambered in Pila, 3 chambered in Unio.

F. Excretion
· A pair of metanephridia/kidney.
· In Unio, the excretory organ is called as Organ of Bojanus and Keber's organ.
· Ammonotelic.

G. Nervous system
· Consists of the brain, paired cerebral, pleural, pedal, stellate, and visceral ganglia, joined by nerve connectives and commissures.

H. Sense organs
· Eyes – Photoreceptors, located on the tip of ommatophores.
· Statocysts – Organ of balancing (equilibrium).
· Tentacles – Tactile in function.
· Osphradium – Test the chemical nature of ingoing water current. 
Associated with visceral ganglion

I. Reproductive system
· Sexes are usually separated but some are hermaphrodites.
· Development is indirect with a larval stage – Trochopore, Veliger, or glochidium larva.

Classification of Phylum Mollusca

Based on foot, shell and mantle, classified into six classes:

a) Monoplacophora
i) Marine, bilaterally symmetrical, almost extinct.
ii) Univalved shell (i.e. shell composed of a single piece)
iii) Head without eyes and tentacles
iv) Annelidian characters: Internal segmentation, presence of nephridia (6 pairs) for excretion.
v) Molluscan characters: Presence of foot, mantle, shell, Respiration via gills (5 – 6 pairs)

Example: Neopilina (The connecting link between Annelida and Mollusca), also called a Living fossil.

b) Amphineura (Amphi = both; neuron = nerve)/Polyplacophora
i) Bilaterally symmetrical and dorsoventrally flattened body.
ii) Exclusively marine in shallow water.
iii) Their body is covered by the shell (8 plates).
iv) Their mouth and anus are terminal.
v) Foot is ventral and flat.
vi) There is a non-ganglianated nerve ring present around the mouth and two pairs of interconnected nerve cords.
vii) Indistinct head without eyes and sensory tentacles.
viii) Separate sexes, Trochophore larva.

Examples: Chiton (Sea mice), Isochiton, Chaetopleura etc.
· Chiton has 2 nerves in the head.

c) Scaphopoda
(Scapha = boat; podos = foot)
i) Scaphods are commonly called Tusk shell/Tooth shells.
ii) Marine, bilaterally symmetrical.
iii) Foot is a conical used for burrowing (Burrowing Mollusca).
iv) Body is enclosed in a tubular shell that opens at both ends.
v) Eyes, true tentacles, and gills are absent and a distinct head is lacking.
vi) Paired kidneys but single gonad; dioecious.
vii) Trochophore larva is formed during their development.
viii) Mostly buried in the substrate to feed on foraminiferans.

Examples: Dentalium, Cadulus

d) Gastropoda (The largest class of Mollusca)
(gaster = belly; podos = foot)
i) Marine, freshwater, terrestrial.
ii) Shell if present is univalve and spirally coiled.
iii) Asymmetrical (visceral mass is coiled), larvae are bilaterally symmetrical.
iv) Well-developed head with eyes, tentacles.
v) Foot is large and flat present at belly; used for creeping.
vi) Buccal mass contains Radula.
vii) Development is indirect; Trochophore and Veliger stages are formed during the development.
viii) Torsion occurs in development.

Examples: Pila (Land snail)
Aplysia (Sea horse)
Patella (Limpet)
Cyprea (Cowrie)
Helix (Sand snail)

e) Pelecypoda or Bivalvia or Lamellibranchiata
(Pelekus = hatchet; podos = foot)
i) Foot is laterally compressed and hatchet-shaped (tongue-shaped).
ii) Aquatic and mostly marine.
iii) Bilaterally symmetrical with two mantle lobes.
iv) Shell is composed of two valves or shells (Bivalvia).
v) No distinct head; eyes, tentacles, radula absent.
vi) These respire by mantle and gills that are made of lamella (Lamellibranchiata).
vii) Dioecious
viii) Development takes place through Glochidium or Veliger larva.

· They are commonly called Pearl forming groups, so considered as a most economic class.
· When the foreign particles get lodged in the mantle; mantle cells secrete a layer of Nacre which encircles the foreign particles forming pearls over a number of years.
· Pearl is said to be an inside–out shell.

Examples: Unio (Freshwater mussel)
Mytilus (Marine or sea mussel)
Pecten (Scallop)
Ostrea (Oyster)
Spondylus (Edible oyster)
Pinctada (Pearl oyster)
Teredo (Shipworm) destruct the wood in seawater.

f) Cephalopoda or siphonopoda
(kephale = head; podos = foot)
i) Exclusively marine; bilaterally symmetrical and carnivores.
ii) Head and foot regions are combined and modified into a structure with eyes, tentacles (arms) radula, and siphon.
iii) Shell may be present or absent; if present it may be external or internal.
iv) Dioecious and development are direct.
v) Have ink-gland that shoots a jet of ink cloud to escape from prey.

Examples: Octopus (Devilfish)
· Doesn't have any shell.
· Presence of powerful suckers; 8 arms/tentacles.
 Sepia (Cuttlefish) – Internal shell
Loligo (Squid or sea arrow) – 10 arms
· Giant Atlantic squid (Architeuthis) is the largest of all invertebrates.
· Cephalopods have a modified mantle cavity where water is expelled with a force and hence the animal moves by Jet Propulsive Force.

Previous Post Next Post

Main Tags