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Sunday, 5 November 2017

General Characteristics of Phylum Mollusca


           General Characteristics of
                   Phylum Mollusca

Mollusca (Molluscus- soft bodies)
Study of Mollusca is called Malacology.
The term Mollusca was first applied by
Aristotle to cuttle fish.
Lamarck coined the term Molluscus.
Study of molluscan shell is called
The animals of this phylum are soft-bodied,
non-metameric, triploblastic coelomates and
fundamentally bilaterally symmetrical
invertebrates with a thin fleshy envelope the
mantle, around the visceral (internal organs
of the body) which may secrete calcareous
They are sluggish animals.
General characters
1. Widely distributed over the world.
Mostly aquatic (marine and few are fresh-
water) and some are terrestrial living in
damp soil.
2. Body is soft and unsegmented (except
3. Bilaterally symmetrical except
gastropodas (asymmetrical) because the
shell is coiled.
4. Schizocoel or Haemocoel cavity found
as coelom.
5. Triploblastic
6. Organ system level of body
7. Body is divisible into head (absence in
pelecypoda and scaphopoda), dorsal
visceral mass, ventral muscular foot and
Mantle- is a dorsal glandular fold of the
body wall. It is thick and muscular and
encloses a mantle cavity.
Head- anterior part of the body, which
contains mouth, eyes and tentacles and
other sense organs.
Foot-is ventral in position usually thick and
muscular and generally forms the main
locomotory organs.
Visceral mass- on dorsal side contains
digestive, circulatory and reproductive
organ of the body and it forms a hump or
8. Body is covered by a hard calcareous
structure, made up of calcium carbonate,
called shell. In some molluscus, it is
internal or reduced or even absent
9. Locomotion takes place by ventral
muscular foot.
10. Digestive system is well developed
with a hard chitinous structure, called
11. Respiration takes place through one
or more gills or ctenidia, lungs (pulmonary
sac) or general body surface in the
terrestrial forms.
12. Circulatory system is closed or open
13. Head consists tentacles and
compound eyes.
Presence of one pair tentacles except
octopus where tentacles are modified into
14. Excretion takes place by paired
metanephridia (kidney).
15. Nervous system consists of many
paired ganglia, connectives and nerves.
16. Sense organs are eyes, tentacles,
osphradium and statocyst.
17. Sexes are usually separate but some
are monoecious.
Pila- sexual dimorphism.
Limax- hermaphrodite
Helix- hermaphrodite
18. Fertilization is external (in sessile
group such as in oyster) or internal.
19. Development may be direct or
indirect. Larva is trochopore or veliger or
parasitic stage called glochidium larva.
Things to remember
1. Neopilina is considered as connecting
link between Annelida and Mollusca. It is
primitive with segmented body.
2. 2 largest animal phylum after
3. Unio contains a digestive gland called
Green gland , which is comparable to liver
of vertebrates.
4. Statocysts- for maintaining equilibrium
of the body.
5. Osphradia- chemoreceptor to test
chemical nature of the water.
6. Excretion by a pair of kidneys or
organs of Bojanus and pericardial gland
or Keber’s organ.
7. Octopus is known as devil fish. It is a
cephalopod having 8 arms and without a
8. Heart is myogenic.
9. Ink glands are present in cephalopoda
that are concerned with protection.
10. Blood consists of copper containing
pigment called haemocyanin. So blood
color is blue or green.


-Triploblastic/three body layers
-Bilaterally symmetrical/non-metamerically
segmented coelomates
-The moluscan body consists of three parts;
the head, foot and visceral mass
-The head is well defined and bears, two
tentacles and two eyes
-The foot is ventral, muscular and flat
-The visceral mass is covered by the mantle,
a fold of skin
-The mantle cavity contains: two glands, the
hypobranchial gland, the anus, the kidney
openings and the osphadium-a chemoreceptor
-The coelom is reduced
-The perivisceral cavity contains blood and is
therefore not a coelom but a haemocoel
-The blood system is open
The heart consists of two auricles and one
-Excretion is by two-tubular nephridia
-Nervous system is circum-pharyngestring
frequently concentrated into the cerebral and
pleural /ganglia
-Occur in the sea, freshwater and land
-E.g snails, clams, squids and octopuses