•Molluscs live in a wide variety of habitats from
tropical regions, to polar regions, at altitudes of
7000 meters.
•Basically every water habitat from the high
mountain lakes to the bottom of the ocean.
•Its size ranges from minute planktonic and
interstitial
•species to 20 m long giant squid, and includes
both
•animals without eyes or brain and those with
highly developed eyes and brain (among
invertebrates).
•Features of Phylum Mollusca:
•Bilaterally symmetrical.
•Majority are aquatic animals with a few being
terrestrial.
•Body soft, unsegmented, without a body ‘
cavity other than that provided by blood
sinuses.
•Body monomeric and highly variable,
essentially with an anterior head bearing eyes
and sensory tentacles, a large flat ventral foot
and a posterior mantle cavity.
•Presence of a protective external dorsal
shell. In some cases, it may be internal and
covered by tissue or secondarily reduced or
lost.
•The visceral mass remains enclosed by a
thick muscular fold of the body wall called
mantle.
•A toothed, chitinous, tongue-like ribbon, the
radula is present which assists in feeding.
•The respiratory organs are in the form of one or more
ctenidia gills housed in the mantle cavity and pulmonary
sacs.
•An open blood system with a heart enclosed by the
pericardium is present, through which the intestine also
passes.
•Excretory organs include a pair of saclike ‘kidneys’,
opening proximally into the pericardium and discharging
into the mantle cavity
•Nervous system comprises of a circumoesophageal ring
and two pairs of ganglionated longitudinal cords with
various modifications
•Sexes are usually separate, a few forms
are hermaphrodite.
•Typically with a single pair of gonads,
discharging the gametes into the mantle
cavity.
•Cleavage is spiral.
•Development indirect via trochophore
and veliger larval stages or secondarily
direct.
Classification of Phylum Mollusca
•There are six classes of which three are more
prominent
•Class 1 Pelecypoda or Bivalvia
• Habitat: mostly marine
•Laterally compressed body enclosed within a pair
of shell valves
•Symmetry: bilateral and the body is laterally
compressed.
• No distinct head
•Examples: Mussels, Unio, Mytilus,
Class 2 Gastropoda
•Habitat: These are either aquatic or terrestrial
•They possess a spiral shell.
•The foot is large and flat.
•Head is well developed with tentacles and
eyes.
Examples: Hilex (Snail), Limax (Slug), Pila
(Apple snail)
Class 3 Cephalopoda
•Habitat: mostly marine.
•They are adapted for swimming.
•The foot is modified into eight to ten long
tentacles in the head region.
•The shell is either external, internal or
absent.
•Examples: Octopus, Loligo, Sepia, spirula,
Nautilus
Class 4 Monoplacophora
•Habitat: exclusively marine
•Head is present without eyes and
tentacles
•Gills are external
•Excretion: by serially arranged
nephridia
Class 5 Amphineura
•Habitat: mostly marine
•Body is elongated
•Eye and tentacles are absent
•Radula is present
• Examples; Chiton, Neomenia
Class 6 Scaphopoda
•Habitat: marine
•Body is bilateral symmetry elongated and
enclosed in tusk shell
•Foot is reduced
•Excretion: a pair of Kidney
Examples: Cadulus, Dentalium
Respiratory Structures in Pila:
•Pila commonly known as pond snail or apple snail
is a gastropod mollusc. They inhabit fresh water
pond and lake.
•They live most of the time in water but can also
thrive well on land. So they are really amphibious
in nature and exhibit double mode of respiration
i.e., absorb oxygen from water during aquatic
respiration and utilize atmospheric air while living
on land.
Aquatic Respiration:
•Pila performs aquatic respiration by gill.
•Gill:
•In Pila a single ctenidium is situated
on the dorsolateral wall of the
branchial or right chamber of the
mantle cavity.
•The gill consists of
numerous triangular
lamellae or leaflets,
arranged in a single
row running parallel to
one another along the
central axis of the gill.
• This type of gill is
called monopectinate
type
•The basal end of each lamella is
attached to the pallial epithelium
of the mantle wall and the other
end hangs freely in the branchial
chamber.
• In the middle of the gill, the
lamellae are large in size, while it
decreases towards the two ends
•Histologically each branchial
lamella is composed of two layers
of epithelia supported by muscle
fibres and connective tissues.
•Each epithelial layer consists
of three types of cells:
• (i) Ciliated columnar cells,
• (ii) Non- ciliated columnar
cells and
•(iii) Glandular cells .
•Both afferent and efferent
blood vessels lying along the
axis of the gill supply blood to
the lamellae through fine
vessels and capillaries.
•The efferent vessel carries
oxygenated blood to the heart
Mechanism of Aquatic Respiration in Pila:
•In aquatic respiration, a current of water
containing oxygen is drawn in by the left
siphon into the mantle cavity.
•The water then flows over the gill by the
help of ciliated epithelia.
• Exchange of gases take place between
the water and the blood vessels of gill.
•The water is expelled from the mantle cavity
through the right siphon. Two fleshy projections
over the foot, called nuchal lobes form the
respiratory siphon during aquatic respiration in
Pila
Aerial Respiration in Pila:
•If the oxygen concentration of water gets
reduced, Pila comes out of water and
respire by taking in air with the help of
pulmonary sac.
• A comb-like sense organ called
osphradium, close to the left nuchal lobe
helps the animal to estimate the oxygen
concentration in water.
Pulmonary Sac:
•The pulmonary sac is a closed cavity hanging from the
dorsal wall of the mantle in the pulmonary chamber or left
chamber of mantle cavity .
•The sac has an opening called the pneumostome in the
pulmonary chamber — which is guarded by two valves.
•The dorsal wall of the pulmonary sac is highly vascular
and helps directly in gaseous exchange.
•Histologically, the sac is composed of outer epithelial
layer, middle muscular and vascular connective tissue
layers and inner endothelial layer.
Mechanism of Aerial Respiration:
•It takes place in two ways:
•(i) In Water: Pila can respire through the
pulmonary sac while it remains in water.
• To inhale the atmospheric air, it comes to
the surface of the water.
• It then expands the size of the left nuchal
lobe, both in length and breadth and rolls up
to form an elongated respiratory tube.
•The outer end of the tube extends
beyond the level of water and sucks in
air from the atmosphere.
•The inner end of the tube comes in
contact with the opening of the
pulmonary sac.
• The alternate contraction and dilation of
the mantle wall and the pulmonary sac
help in the process of respiration.
•Following the exchange of oxygen and
carbon dioxide, from air to the blood vessels
of pulmonary sac and from blood vessels to
air, the expelled air goes out of pulmonary
chamber through the respiratory tube.
• During this exchange, the branchial chamber
remains completely separated from the
pulmonary chamber by the epitaenia which
comes in contact with the roof of the mantle
On Land:
•During dry season, when the water level gets reduced, or
while living on land, Pila does not use any respiratory
tubes or siphons.
•The pulmonary sac directly becomes filled up with
atmospheric air to facilitate aerial respiration.
•The shell remains closed completely with the help of
operculum during the time of aestivation, when Pila
respires with the little amount of stored air kept in the
pulmonary sac.
•Hemocyanin, dissolved in plasma acts as the respiratory
pigment in Pila.
