2. Description
▪ The molluscs (Americans spell it as
‘mollusks’) constitute one of the largest phyla
of animals, with close to 112,000 species to
be found worldwide. The scientific study of
mollusks is known as malacology . The
mollusks first appeared during the Cambrian
period of the evolutionary calendar. The
world mollusca has its root in the Latin
word molluscus meaning ‘thin shelled’ which
in turn is derived from the root Latin
3. ▪ This is a description of their main
characteristic -all mollusks have a soft
body, which ( with the exceptions of
from certain families) is generally
protected by an external, calcium-
shell. In the octopuses and sea-slugs
completely absent where as in squids
it is located internally.
4. Main characteristics of
Mollusca
▪ All modern mollusks have the same
fundamental body plan. There are
three distinct body zones: a head-
foot, a visceral mass and a mantle.
Some common features are :
5. 1)Bilaterally symmetrical.
2)Body has more than two cell layers, tissues
and organs.
3)Body without cavity.
4)Body possesses a through gut with mouth
and anus.
5)Body monomeric and highly variable in form,
may possess a dorsal or lateral shells of
protein and calcareous spicules.
6)Has a nervous system with a circum-
oesophagal ring, ganglia and paired nerve
chords.
7)Has an open circulatory system with a heart
and an aorta.
8)Has gaseous exchange organs called ctenidial
6.
7. The Class Bivalvia
• The bivalves (meaning "two-
shells") are perhaps the
most well known mollusks
simply because of their
history as a source of food.
Clams, mussels, oysters and
scallops are all bivalves.
• The bivalve may either
burrow through the bottom,
or attach itself to the
substrate with glue-like
strings called "byssal
threads.“
• Most bivalves feed by
filtering organic particles from
water, and therefore do not
have a radula.
• The shell is generated by
the mantle from the inside.
Pearls are made by clams,
oysters and mussels
8.
9. The Class Cephalopoda
• Although we usually think of Mollusks as
benthic (bottom dwelling), the
cephalopods have taken to a nektonic
(swimming), rather than benthic,
existence in the ocean.
• The cephalopods may not seem very
closely related to the other mollusks, but
physiologically, they are similar in internal
construction.
• Perhaps the most obvious difference
between most cephalopods and other
mollusks is the apparent lack of a shell.
• Cephalopods have the most well
developed nervous systems of all
mollusks, as well as the most well
developed eye.
• The cephalopod eye is one of the most
notable examples of convergent
evolution in all of the animal world
10. The class of Gastropoda
million years ago. No
other class has as many
species (more than 50,000)
or are as widely collected.
• The Class Gastropoda
includes the snails and
slugs. Most gastropods
have a single, usually
spirally coiled shell into
which the body can be
withdrawn, but the shell is
lost or reduced some
important groups.
• Gastropods are
characterized by "torsion,"
a process that results in
the rotation of the visceral
mass and mantle on the
foot.
• Gastropods are dioecious,
and some forms are
hermaphroditic.
Hermaphroditic forms
exchange bundles of sperm
to avoid self-fertilization
11. The class of
Monoplacophora
• Monoplacophorans are small
and limpet-like, having a single,
cap-like shell. Some organs
(kidneys, heart, gills) are
repeated serially,.
• The word Monoplacophora is
derived from three Greek
words monos=one, plax=plate
and pherein=bearing. So
it includes animals having only
one plate. It does not have any
order.The characteristics of it
are listed as below.
Examples :- Neopilina galatheae
12. ▪ They have bilaterally symmetrical and segmented body.
They have shell made up of single piece or valve.They have
head without eyes and tentacles.They have ventral and flat
foot.They have mantle which encircle the body as
a circular fold of the body wall.They have external and
serially arranged gills.They have five pairs of serially
arranged nephridia.They have sex organs in two separate
bodies.
13. The Scaphopoda
distinctive group
of mollusks commonly
known as the "tusk
shells" because their
shells are conical and
slightly curved to the
dorsal side, making
the shells look like
tiny tusks
• Scaphopods live their
adult lives buried in
sand or mud, with
their head end pointed
downwards. Only the
narrow posterior end
of the shell sticks up
into the seawater for
water exchange and
waste expulsion.
14. ▪ The scientific name Scaphopoda means "shovel
foot," a term that refers to the "head" of the animal,
which lacks eyes and is used for burrowing in
marine mud and sediments.The most distinctive
feature of scaphopods is that the tubular shell is
open at both ends, not just one end as in most
mollusks.
15. Significance
▪ Mollusks, which include land snails, freshwater
snails, mussels, clams, and slugs, are vital to
healthy ecosystems. Land snails, for example,
not only provide food for a host of small
mammals and birds, but they play vital roles
in recycling forest nutrients. Freshwater snails
also provide food for fish, including native
trout and salmon, and are also important
recyclers of plant and animal waste,
essentially keeping water clean and healthy.
16. ▪ Mollusks are of general importance within food chains and as
members of ecosystems. Certain species are of direct or indirect
commercial and even medical importance to humans. Many
gastropod species, for example, are necessary intermediate
hosts for parasitic flatworms (classTrematoda, phylum
Platyhelminthes), such as the species that cause
schistosomiasis in humans. Most bivalves contribute to the
organic turnover in the intertidal (littoral) zones of marine and
fresh water because, as filter feeders, they filter up to 40 liters
(10 gallons) of water per hour.
