LESSON 10: THE MOLLUSKS
BODY PLAN
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Mollusks mainly belong to eucoelomates animals.
Meaning they are:
o Animals having a true coelom.
o Animals have a body cavity that is completely
lined with mesoderm.
o The mesoderm can develop into muscle
layers and internal skeletal elements.
Aside from that, mesodermal layers can also lead to the
development of arteries and veins, such as their
circulatory system, and it can also be developed to line
the digestive tract, which allows them to eat and digest
the food they have eaten.
Well-developed bilateral symmetry.
So, the majority of your mollusks follow a bilateral
symmetry. Meaning if they are cut into half, the other half
is the same as the other, or it mirrors the other side.
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Most are unsegmented – a few primitive forms seem
to show some segmentation.
Most mollusks (except bivalves) have well-developed
head bears, mouths, and various sense organs.
In spite of the wide variety diversity of group most
share some basic features in their body plan:
Mantle – secretes shell or becomes outer
body covering itself.
Visceral mass – most internal organs are
embedded in tissue rather than being
surrounded by the body cavity.
Shell – (internal or external).
Radula – specialized feeding organ in the
mouth.
Foot – usually used for locomotion.
Most of mollusks contain as basic as this body plan
(mantle, visceral mass, shell, radula, and foot).
ANIMALS – MOLLUSCS
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Mollusks are sometimes referred to as molluscs.
They are the second largest phylum of animals in
terms of number of known species.
The most versatile body plan of all animals ranges
from fairly simple organisms to some of the most
complex and specialized of invertebrates.
Includes snails, limpets, clams, mussels, chitons,
octopus, squid, oysters, slugs, nautilus, and tooth
shells.
The phylum is divided into 8 different classes, but
90% of species are in only two: bivalves and snails.
FEATURES AND CHARACTERISTICS
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Feeding and Digestion
o Detritus feeders, burrowers, borers, grazers,
carnivores, filter feeders, etc.
Detritus feeders feed on detritus or decaying matter,
which contains small microorganisms that allow them to
have the protein and other nutrients they need for survival.
Burrowers live mostly in water or most of them in the soil
or sand.
o
In the mouth is a tongue-like rasping organ called
a radula.
Radula helps them with the food that they have captured.
o
Most internal organs, including the digestive
system, are embedded in solid tissue called the
visceral mass instead of being free inside a body
cavity (coelom).
Since they have a coelom, the food that they eat goes
there.
o
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Complete digestive tract, which is divided into
discrete, functionally specialized regions:
Foregut (start) – includes mouth and radula
up into the esophagus. Basically, they receive
the food that they eat.
Midgut (middle) – which is the stomach and
associated with digestive glands.
Hindgut (bottom) – usually for intestines,
which is usually long, and this happens in the
formation of feces or those things that are not
able to digest and are ready to be excreted
from the body.
Respiration
o Most mollusks
(ctenidia).
have
folded,
ciliated
gills
They are thin, feathery sheets of tissue covered with cilia
also used for feeding in bivalves.
o
Circulation
o They have an open circulatory system in most.
o Blood is not entirely contained within vessels,
unlike humans, which have blood vessels and
work good for slow-moving animals.
o They have a simple heart with a few vessels with
2 auricles, and 1 ventricle extending from heart
that is connected to aorta.
o Reduced coelom becomes a haemocoel
surrounding heart.
The haemocoel actually contains haemocyanin, which
contains copper, which is why some of the animals or
mollusks have blue blood. Haemocyanin, which contains
copper with iron, makes animals red blood, but the
majority of the animals have blue blood.
Blood contains several kinds of cells.
However, this is different mostly. Most mollusk follow this
structure, however for
o
Cephalopods have a closed circulatory.
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Endocrine System
o Well-developed endocrine system which controls
egg laying and growth.
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Nervous System
o CNS is a ring of ganglia in head area with paired
nerves and ganglia extending to other parts of the
body.
o Although the mollusks have a relatively simple
nervous system; cephalopods are considered the
most “intelligent” of all invertebrates;
Of course, this phylum actually contains or belongs to
octopuses or octopi.
o
Rival some mammals in ability to reason and
learn unlike most mollusks, cephalopods are
active predators.
They are active predators because they can recognize
their prey with their sharp eyes, and they can actually
learn by just watching.
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Some mollusks breathe through their skin many
terrestrial snails lack gills (pulmonates).
>> mantle is modified into a saclike “lung” for
breathing air.
Examples: through earthworms. Since their skin is
permeable, you can absorb oxygen from their skin and
breathe through it. Sometimes they are called pulmonate.
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Reproduction and Development
o Most mollusks are dioecious; some especially.
o Gastropods are monoecious (hermaphrodites) or
can reproduce independently.
o Many marine forms produce characteristics larva,
which is called trochophore.
o In some gastropods and bivalves, a second larval
form develops called veliger.
A veliger is a free-swimming organism that has a foot,
shell, and mantle usually.
o
Some freshwater bivalves produce a parasitic
larva = glochidium.
This glochidium is actually attached to the gills of host
fishes, and they feed on the blood of fishes in order to
survive.
o
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In cephalopods, many freshwater snails and
some bivalves development is direct, ie. No
larval stage.
Evolution of Molluscs
o New molecular evidence indicates an
evolutionary relationship between molluscs,
rotifers, and annelids have occurred that is why
they are kind of similar to each other.
o
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Molluscs branded off the main animal line about
545 millions years ago.
Plant and animal life on land would not arise for
another 100 millions year ago.
CLASSIFICATION OF PHYLUM MOLLUSCA
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Class: Aplacophora;
Solanogastres
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Class: Monoplacophora
now
Caudofeveata
and
Class: Polyplacophora (chitins)
Class: Scaphopoda (tusk shells, tooth shells)
Class: Bivalvia (mussels and clams)
Class: Gastropoda (snails and slugs)
Class: Cephalopoda (octopus and squid)
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Class Monoplacophora
o Means “one plate” that is why the name is
MONOplacophora.
o There are 25 known species.
o Small; 3 mm to 3 cm.
o Fossils known from Cambrian and was believed
to be an extinct group until discovered near Costa
Rica in 1952.
o Small, single, low rounded shell with ventral
creeping foot which superficially resemble limpets
(gastropods).
o Unlike other molluscs; show some evidence of
segmentation. Some organs are serially repeated
in body:
3-6 pairs of gills
8 pairs of muscles
3-7 pairs of kidneys (nephridia)
o
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Class Aplacophora
o Has around 370 species that are known to
mankind.
o Features may be closer to ancestral mollusks
than any other modern group.
o All marine; most are small, 2-140 mm long,
meaning they can only be seen at the microscope
and cannot be seen by the naked eye.
o Soft-bodied, worm-like, no shell, no distinct head.
o Body covered with calcareous scales or spicules
known as small spines.
o Some are burrowers in muddy sediments.
>> feed on protozoa and microorganisms and
detritus.
o Others don’t burrow and live on the sediment and
feed on larger organisms, including cnidaria.
o Radula is present in some, absent in others.
o Some have gills for respiration.
o No nephridia.
o Monoecious or dioecious.
Have radula for scraping food.
Ladle-like nervous system similar to flatworms.
Eg. Neopilina species.
This was once believed that the mollusks ancestor was
segmented wormlike animal, but current research
discredit this claim, thus remaining it as a separate
phylum.
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Class Polyplacophora (Chitons)
Differences
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Have a unique shell composed of eight
overlapping plates (valves) that provide
protection.
Their body is flattened and elongated, and they
have a foot for clinging to rocky surfaces.
Similarities
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Marine animals that mainly graze on algae and
other organisms.
They have a radula for feeding and a simple
nervous system.
They have a eight segmentation which is their most
common trait. Quite similar to monoplacophora, however
it has an 8 segmentation.
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Class: Schaphopoda (Tusk Shells, Tooth Shells)
Differences
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Feeding
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Most feed on algae using radula to scrape algae
from surface.
One predatory species (Placiphorella Velata)
captures small invertebrates by “jumping” on
them.
Since they are called tusk shells or tooth shells,
they are called with their name based on how they
look like. Have a tubular shell that is open at both
ends.
One end is buried in the sediment, while the other
extends into the water column. They have a
tentacle-like structure called a captacula used to
capture food particles.
Similarities
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Scaphopods are marine organisms that feed on
detritus and small organisms by extending their
captacula into the water to catch particles.
Feeding and Digestion
o
Feeds mainly on detritus and protozoa caught by
cilia on foot or using tentacle-like captacula.
Something unique with them is their captacula which is
used to eat.
o
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Respiration
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Gills suspended in mantle cavity along sides of
thick flat muscular foot water is pumped across
gills within the “tube.”
At low tide when exposed, can breathe air to
some degree.
Nervous System
o
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2 long nerve cords extend along body.
Osphradia in mantle cavity near anus samples
chemical in water.
Reproduction
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Sexes separate (dioecious) in most.
Often with complex courting behaviors.
Producing trochophore larva as their early stage
for their offspring.
Mucous covered tentacles extending from head
captacula may also serve a sensory function.
Radula carries food to gizzard for crushing.
Respiration
o
o
o
o
No gills.
Mantle wraps around viscera and fuses to form
tube.
Movement of foot and cilia pump water through
tubular mantle cavity.
Gas exchange occurs across surface of mantle.
Nervous System
o
o
No eyes.
Captacula may serve some sensory function.
Reproduction
o
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Dioecious.
Produces trochophore larva at the early stage of
development.
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Class: Bivalvia
They are bivalves, which means that they 2 valves that is
bilaterally similar to each other.
Differences
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Have a two-part hinged shell that covers their
body.
They lack a distinct head and radula.
Use their muscular foot to burrow in sediment or
attach to surfaces. Their gills are modified for both
respiration and filter-feeding.
Similarities
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o
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Typically, sedentary filter feeders obtaining food
by pumping water through their gills and trapping
food particles.
They are mainly marine, but some species inhabit
freshwater environments.
The general term for those seashells that are living in
water are gastropods or gastropoda.
They are different from hermit crabs since hermit crabs
are just actually using the shell or the empty shells in order
to in order to house their body, but they are not
necessarily gastropods.
Class: Gastropoda
Differences
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Are characterized by their single, coiled shell (in
most species) and a distinct head with eyes and
tentacles.
They have a muscular foot used for crawling.
Exhibit torsion, a process during development
that results in the twisting of the body, causing the
anus and mantle cavity to be located above the
head.
Similarities
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They are the largest and most diverse class of
mollusks.
They are found in marine, freshwater, and
terrestrial habitats.
They have a radula, a specialized feeding organ
with rows of teeth used to scrape food from
surfaces.
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Class: Cephalopoda
Also known as cephalopods. From the name cephalo,
meaning they have a head.
Differences
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The most advanced and intelligent class of
mollusks because in here you can see your
nautilus, squids, and octopus or octopi.
They have a well-developed head with complex
eyes and a ring of tentacles around their mouth.
o
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They lack an external shell, which is very different
from the other classes of mollusks (except for
some squids with internal shells).
Use a specialized siphon to propel themselves
through water.
Similarities
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Highly active predators, preying on various
marine organisms.
Have a closed circulatory system and a welldeveloped nervous system, making them capable
of complex behaviors and learning.