For post lab discussion

1. Phylum Mollusca

2. Phylum Mollusca
• Mollusk means soft-bodied.
• A mollusk has no internal skeleton but many have shells
that act as an external skeleton, providing protection.
• Most mollusks usually have some version of a head, soft
body, and foot.
• One important part of a mollusk's body is the mantle,
which is the fleshy tissue that lines the inside of the shell.
– This part is responsible for shell growth and color, and it assists
in other functions such as respiration.
– Growth of the shell occurs by the mantle absorbing calcium
carbonate from the sea water.
– The color of the shell is due to pigment cells in the mantle.
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3. • (Latin.mollis = soft)
• over 80 000 species.
• soft, unsegmented body, consisting of an anterior head, a
dorsal visceral mass and a ventral foot.
• The body is more or less surrounded by a fleshy mantle (an
outgrowth of the body wall) and nearly all species in the
group secrete a lime shell that covers and protects the
body.
• All, except the class Bivalvia, have a ribbon-like rasping
tongue (radula - unique to this phylum) with small chitinous
teeth that processes the food.
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4. • Most mollusks are free living, but slow moving
creatures, showing a close association with
the substrate.
• Some attach to rocks or shells, others burrow,
others float, octopuses and squids swim
freely.
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5. Classes of Mollusca
• Bivalvia
• Gastropoda
• Cephalopoda
• Aplacophora
• Monoplacophora
• Polyplacophora
• Scaphophoda
(http://coa.acnatsci.org/conchnet/c-101a2.html)
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6. Characteristics of Mollusc
• Body usually short and partially or wholy
enclosed by a fleshy outgrowth of the body
wall called the mantle, which may be variously
modified. Between the mantle and the
visceral mass is a mantle cavity containing
components of several systems (secondarily
lost in a few groups).
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7. • A shell (if present) is secreted by the mantle
and consists of one, two or eight parts.
• The head and the ventral muscular foot are
closely allied (the foot being variously
modified for burrowing, crawling, swimming,
or food capture).
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8. • The digestive canals are complete and
intricate with ciliary canals for the sorting of
particles.
• The mouth with a radula bearing traverse
rows of minute chitinous teeth to rasp food ,
except in Bivalvia.
• The anus opening in the mantle cavity. A large
digestive gland and often salivary glands are
present.
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9. • The circulatory system is open, except in
Cephalopoda and usually includes a dorsal
heart with one or two atrias and one ventricle.
This is situated in a pericardial cavity. An
anterior aorta and other vessels and many
blood spaces (hemocoels) exist in the tissues.
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10. • Respiration occurs via one to many uniquely
structured ctenidia (gills) in the mantle cavity
(secondarily lost in some), by the mantle
cavity, or by the mantle.
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11. • Excretion by kidneys (nephridia), one or two
or six pairs, or only a single one. They usually
connect to the pericardial cavity and they exit
in the mantle cavity.
• The coelom is reduced to the cavities of the
nephridia, gonads and pericardium.
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12. • The nervous system is typically a
circumesophageal nerve ring with:
– multiple pairs of ganglia and
– two pairs of nerve cords (one pair innervating the foot
and another the visceral mass).
• Many poses organs for smell, or touch, or taste.
• Eyespots or complex eyes present.
• A statocyst for equilibration present.
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13. • The sexes are usually separate(some are monoecious, a
few are protandric).
• Gonads add up to four, two or one, all with ducts.
• Fertilization occurs externally or internally.
• Most species are oviparous.
• Egg cleavage determinate, spiral, unequal and total
(meroblastic in Cephalopoda).
• Trochophores and veliger larvae form, or a parasitic
stage occurs(Unionidae), or the development is direct
(Plumonata, Cephalopoda).
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14. • Unsegmented (except Monoplasophora).
• Symmetry bilateral or asymmetrical.
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15. Class POLYPLACOPHORA
• Polyplacophora ("many plate
bearers")
• contains the Chitons
– easily recognizable because of
their shells that are split into
eight dorsal plates that cover
the centre of their bodies.
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16. Class GASTROPODA
• Gastropods, which include univalves, are
mollusks usually covered by a single shell,
• Some gastropods, such as slugs, have no shell
at all. Many of the subclass Opisthobranchia
(sea slugs, sea hares, nudibranches etc.) have
forsaken their gills and shells.
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17. Class GASTROPODA
• The class Gastropoda is the largest in the phylum and
includes the snails, whelks, winkles and sea slugs.
Subclass
PROSOBRANCHIA
= Snails, limpets
Subclass
OPISTHOBRANCHIA
= Sea slugs
Gastropod anatomy
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18. Class GASTROPODA
• Most of these species belong to the subclass,
Prosobranchia, and have a spiral shaped shell, a well
developed head that poses tentacles and a radula,
and a large flat foot for motion.
• The primitive members are herbivores that rasp
seaweeds and micro algae.
• Around 40 000 known species.
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19. Class GASTROPODA
• The shelled gastropod body
emerges from an opening,
called an aperture, to eat
and move. The outer edge
of the aperture is called a
lip.
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20. Class GASTROPODA
• Each coil of a
gastropods shell is
called a whorl, with
the last and usually
the largest whorl
containing the body,
thus called the body
whorl. All whorls
above the body whorl
make up the spire.
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21. The calcareous layers of the shell are covered
by a thin, glossy, proteinaceous periostracum.
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22. Gastropod means stomach-foot
because the foot is large and
is the most prominent feature
in most cases. The rest of the
body basically contains the
stomach, which rests on the
foot.
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23. • As with bivalves, most gastropods breathe by
taking in oxygen from the water through
siphons.
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Univalves, unlike bivalves, have a head with
tentacles, which have sense organs that can
detect shadows and movement.
Anterior pair of tentacles
shorter
Posterior pair tentacles
longer
eyes at distal tip

24. • Many gastropods have
a trap door, or operculum,
attached to their foot,
which is actually a thin
piece of shell.
– This operculum seals the
aperture closed when the
animal retreats into its shell,
providing protection from
predators and from drying
out.
operculum is a thin teardrop-
shaped disk of flexible protein
(conchiolin)
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25. 25

26. 26

27. Class GASTROPODA
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28. 28

29. Class BIVALVIA (Pelecypoda)
• Includes:
– Mussels
– clams
– oysters
• Has a shell that consists of two
parts, but unlike the Brachiopoda
(Lamp shells) the two parts are
hinged together dorsally and then
extends downwards, laterally on
either side of the body, thereby
encasing it.
Interior view of right valve showing
the muscle scars.
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30. Class BIVALVIA
• Bivalve means two shells or valves.
– These two shells are attached at a hinge where
one or more strong muscles inside the shell keep
it tightly closed, and a rubbery ligament (hinge
ligament) near the hinge holds the halves
together and open.
– Scars can sometimes be seen on the inside of
some shells where the muscles were once
attached.
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31. Concentric lines
Recede from an elevated
point near the hinged margin
Umbo
(UMBO)
Approximating
points of umbos
of opposite valves
Beak
Ridges/folds or
spines
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32. Interior view of right valve showing
the muscle scars.
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33. Interior view of right valve showing
the muscle scars.
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34. Pallial sinus
Pallial line
Point of attachment of
pallial retractor muscle
to the shell.
A P
V
Serrations Insertion area of mantle
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35. Cardinal teeth
Lateral teeth
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36. Class BIVALVIA
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37. Class BIVALVIA
• Most of the bivalves poses large gills for the purposes
of respiration and filtering out of small food particles.
Bivalve with left valve
and mantle removed.
(Heavy arrows - path of
water current,
Dashed arrow - path of
filtered particles).
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38. Class BIVALVIA
• Most are sedentary suspension feeders
– Depend on ciliary currents produced by the gills to
bring food materials.
• Some are deposit feeders
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39. Class BIVALVIA
• A bivalve breathes by circulating water within
its shell, which brings in oxygen. As the water
leaves the shell, it carries with it carbon
dioxide and other wastes.
• Some bivalves have siphons, where water
enters and leaves through.
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40. Class BIVALVIA
• Bivalves lack a radula.
• one or two pairs of
gills(ctenidia) or
branchia
for respiration.
• Many species burrow in the
sand or mud and poses
large, wedge-shaped feet
for this.
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41. Class BIVALVIA
• Mussels attach to rocks by means of a beard-
like byssus, while oysters and their allies
cement one of their valves to the rock face.
• Around 20 000
known species.
Section through the visceral mass
showing the internal organs.
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42. Class BIVALVIA
Clam anatomy
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43. Inside of the right shell showing scars where muscles are attached
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44. Class BIVALVIA
A
P
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45. mantle
Gills
(Ctenidia)
Byssal threads
Foot
Anterior
adductor
muscle
Posterior
adductor
muscle
Anterior retractor muscle
Posterior retractor
muscle
Visceral
mass
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46. Class CEPHALOPODA
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47. Class CEPHALOPODA
• Octopus, cuttlefish and squid
Dorsal view of a squid (loligo)
in swimming position.
The tentacles and arms are
held together and functions
as a rudder.
Cuttlefish (sepia) seizing a shrimp
with the use of its tentacles.
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48. Class CEPHALOPODA
• Instead of a foot
they poses eight or
ten long tentacles
armed with suckers.
Anatomy of a squid - loligo
(ventral view with the mantle cut open).
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49. 49

50. • Cephalopods are divided even further into the
eight-armed octopods (octopuses) and the 10-
armed decapods (cuttlefish and squid).
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51. Class CEPHALOPODA
• Most of the cephalopods are active and predatory swimmers
posesing jaws and radula.
• This class includes the deep-sea giant squids (20m long)
which were the inspiration for many a mythical story about
sea monsters.
• The Cephalopods poses eyes as complex as those of humans,
and a greater capacity for learning than any other
invertabrates.
• Around 650 known species.
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52. Sagittal section of nautilus.
Nautilus : the only
shelled cephalopod.
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53. 53

54. Reference:
• http://library.thinkquest.org/26153/marine/mollus
ca.htm
• http://oceanica.cofc.edu/shellguide/molluskframe
set.htm
• http://www.applesnail.net/content/anatomy/respi
ration.php
• http://www.applesnail.net/content/anatomy/shell.
php
• Hickman Jr. C.P. and et al., 2007. Animal Diversity
4th edition. Boston: McGrawHill
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