Download Phylum Mollusca

Chapter 12
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Characteristics of Mollusca

Protostomes - Embryonic blastopore
becomes mouth, schizocoelous coelom
formation and spiral embryonic cleavage.

Have a coelom – fluid filled body cavity lined
with mesoderm

Head-foot – body part that contains head
and is responsible for locomotion
Land Slug
cuttlefish
More Characteristics of Mollusca

Viseral mass – contains the organs of digestion,
circulation, reproduction, and excretion
 Mantle – attaches to the viseral mass, may
secrete a shell
 Mantle cavity – is between the mantle and the
foot, functions in gas exchange, excretion,
elimination, and release of reproductive products



Most have a radula – contains rows of
posteriorly curved teeth used to scrap and
gather food.
Open Circulatory System (except – Cephalopoda)
Bilateral Symmetry
Three Major Classes
 Class Gastropoda
 (snails, limpets, slugs)
Austrorhytida capillacea
Aequipenctin irradians
 Class
Bivalvia
 (clams, oysters, mussels, scallops)
 Class
Cephalopoda
 (octopuses, squid, cuttlefish, nautiluses)
Hapalochlaena nierstraszi
Other Classes of the Phylum
Mollusca

Class Polyplacophora

Class Scaphopoda

Class Monoplacophora
Chiton
Tusk Shell
Neopilina

Class Aplacophora
Solenogaster
Number of Species
Relationships to Other Animals

Protostomes
 Similarities in embryological development in
mollusca and annelids
○ The Trochophore larvae stage is very similar
and hard to distinguish
Origin of the Coelom

Schizocoelous coelom formation
 The splitting of mesoderm to form the
coelom
 The Coelom is found in molluscan
development as small cavities around the
heart, pericardial cavity, the nephridia, and
the gonads.
Anatomical Features of Molluscs

Three major Body Regions
 Head-foot
 Visceral mass
 mantle
Anatomical Features of Molluscs

Head-Foot
 The elongated portion with an anterior head,
containing a mouth and certain nervous and
sensory structures, and an elongated foot
used for attachment and locomotion
Anatomical Features of Molluscs

Visceral Mass
 Contains the organs of digestion, circulation,
reproduction, and excretion is found dorsal
to the head-foot.
Anatomical Features of Molluscs

Mantle
 Usually attaches to the visceral mass and
may secrete a shell
Anatomical Features of Molluscs

Mantle Cavity
 Opens to the outside of the body and
functions in gas exchange, excretion,
elimination of digestive waste and releases
reproductive products.
Anatomical Features of Molluscs

Radula
 Consists of chitinous belt and rows of
posteriorly curved teeth used to scrape food.
Class Gastropoda

The Snails, Limpets, and Slugs
Limpet – Lottia insessa
Snail - Bradybaena
similaris
Slug – Ariolimax californicas
Class Gastropoda
Largest class of molluscs
 Found in marine, freshwater, and
terrestrial habitats
 Also found on plates in France

Helix pomatia – French Delicacy
Features of the Class Gastropoda

Torsion – is the 180̊, counterclockwise twisting of
the visceral mass, mantle and mantle cavity. This
position twists the gills, anus, and openings from
the excretory and reproductive systems just
behind the head and nerve cord, and twists the
digestive tract into a U shape.
More on Torsion in Gastropods
Torsion in gastropods
Takes place in all gastropods usually during late veliger stage.
Rotation of visceral mass and overlying mantle and shell 180o with respect
to foot and head.
Gut ends up U-shaped and incipient organs are switched left to right.
Many gastropods remain torted. Opisthobranchs become detorted (untwist).
3 Advantages of Torsion
1. Allows the head to retreat into the
shell first. Allowing the gastropod to
protect its head from predators first.
 2. Allows clean water to enter the
anterior of the mantle cavity.
 3. Allows the snail’s sensory organs to
be orientated into the direction the snail
is moving.

Disadvantage of Torsion

The anus and nephridia empty dorsal to
the head
 The solution – some snails have notches or
openings in the mantle that allow waste to
exit posterior to the head
 The solution – some snails undergo
detorsion by untwisting 90̊ allowing the
mantle cavity to open to the right side of the
body.
Class Gastropoda – Shell Coiling
Early fossils of gastropoda have a shell
coiled in one plane creating a
cumbersome shell.
 Modern snails are asymmetrically coiled
into a more compact form.

Achatina fulica – Giant
African Snails
Class Gastropoda - Locomotion

Ciliated Flattened Foot, covered with
Gland Cells.
 Small gastropods – use cilia to propel over
mucus
 Larger gastropods – use waves of muscular
contractions
 Aquatic gastropods modified foot for
clinging.
Limpets move through muscular undulations of the foot
Class Gastropoda and Locomotion
Gastropods and
polyplacophorans.
Crawling
Direct and indirect waves.
Mucus alternately acts as a glue and allows sliding.
Some gastropods use cilia (moon snails).
Class Gastropoda – Feeding and
Digestion
Most feed by scraping algae and small
organisms from substrate using their
radula
 Some are herbivores that feed on
plants, scavengers, parasites, or
predators

Radula – Class Gastropoda
(Left) Radula of the deep sea limpet, Neomphalus fretterae, Family Lepetidae
(Right) Radula of the slit shell mollusc, Scissurella crispata, Family
Scissurellidae
Scanning electron micrographs; reprinted courtesy of Dr. Carole S. Hickman,
Museum of Paleontology, University of California, Berkeley CA.
Snail Radula in Action

http://www.youtube.com/watch?v=mLVD
wlrSq5U
Class Gastropoda - Digestion
The digestive tract is ciliated
 Food is suspended in a mucus mass called
protostyle which extends into the stomach
and is rotated by the cilia
 Enzymes are released from the digestive
gland to process food.
 The intestines holds waste in the form of
fecal pellets.

Class Gastropoda – Other
Maintenance Functions

Gas Exchange
 Occurs in the mantle cavity
○ Modern gastropods have one gill – due to
coiling
○ Siphon – is a rolled extension of the mantle
 Extends to the surface of the substrate to bring in
water
 Land Snails – gills lost or reduced
○ Have richly vascular mantle for gas exchange
with air
Class Gastropoda – Other
Maintenance Functions

Open Circulatory System –
 Blood leaves vessels and directly washes
over cells in the sinus spaces
 Heart consists of a single ventricle and two
auricles – unless there is a shell (coiling)
then one auricle
Class Gastropoda – Other
Maintenance Functions

Hydraulic skeleton
 Gastropods contract muscles to push blood
into structures that help push the snail
forward
Class Gastropoda –
Other Maintenance
Functions

Sensory Structures
 Nervous system has 6
ganglia in the head and foot
 Osphradia – chemoreceptors
anterior wall of mantle
 Eyes – may be at base or
ends of tentacles
○ Simple photoreceptors
○ May consist of lens and
cornea
Class Gastropoda – Other
Maintenance Functions

Kidney Functions
 Modern Gastropoda have one nephridia that
consists of a sac of highly folded walls
where waste is modified and certain ions
and organic molecules are reabsorbed.
○ Aquatic species excrete ammonia
○ Terrestrial snails convert ammonia to uric acid,
less toxic and can be excreted in semisolid
form to conserve water
Class Gastropoda – Reproduction
and Development
Marine Snails – dioecious (separate
males and female species) – discharge
gametes into the sea for external
fertilization.
 Here is a video of a Marine Snail
Spawning:

 http://www.youtube.com/watch?v=KQdYDJU
3JHM
Class Gastropoda – Reproduction
and Development

Terrestrial Snails – monoecious
(hermaphroditic – having both male and female
sex organs)
 Various forms of mating depends on species
○ Some internal some external
○ Some Develop as male first then later in life
as female
○ Eggs need a moist place to mature
 Here is a Video of Terrestrial Snails Mating:
 http://www.youtube.com/watch?v=8ynjLPnF
_2c&p=688A33FFB1266BA4&playnext=1&i
ndex=6
Class Gastropoda - Development

Marine Gastropods –
 Free Swimming trochophore larva
 Develop into free swimming veliger larva
Veliger Larva –
Have foot, eyes,
tentacles and shell
Torsion occurs during
this stage
Gastropod Diversity

Subclass – Prosobranchia – 20,000
species
 Mostly Marine Species
Gastropod
Diversity
Video: Nudibranchs
http://www.youtube.com/watc
h?v=nHVoV0MVwSc

Subclass Opisthobranchia – less than
2,000 species
 Marine Slugs – foot modified as lobes for
swimming
 Interesting Fact – Due to loss of shell some
develop the defense system of acquiring
undischarged nematocytsts from their cnidarian
prey which they use to ward off predators
Gastropod Diversity

Subclass Pulmonata – 17,000 species
 Terrestrial and Marine snails and terrestial
slugs
Class Bivalvia

30,000 species

Loss of radula and head

Includes:
 Clams
 Oysters
Bay Scallop
 Mussels
 Scallops


Shell Consists of Two Valves
Many are edible, some form pearls, valuable in
removing bacteria from polluted water because
they are filter feeders
Class Bivalvia – Shell and Associated
Structures






Valves – Two Convex halves of the shell
Hinge – on dorsal side
Teeth – keep shell from twisting found on anterior
end of shell
Umbo – swollen area near shell’s anterior
Embiologically shell forms as a single structure
Near Hinge there is more protein than calcium
carbonate, allows for muscular opening and
closing of shell
Class Bivalvia – Shell and
Associated Structures
Class Bivalvia – Shell and Associated
Structures
Adductor muscles at either end of the
shell close the shell. This is important to
defend against predators.
 Here are some clams escaping a
predator:
 http://www.youtube.com/watch?v=ALaM
oS_vvNE

Class Bivalvia Pearl formation
Shell
Developing pearl
Epithelium
Irritant lodged between shell and mantle
Layers of nacre secreted around foreign material
43
Class Bivalvia – Buried in sand
Bivalves
Burrowing
Byssal threads
Swimming
Class Bivalvia – Gas Exchange


Gills form folded sheets
Cilia move water into the mantle cavity through an
incurrent opening of the mantle. A siphon is an
extension of the mantle and is a mechanism for
buried bivalves to breath and filter feed.
Class Bivalvia – Gas
Exchange
Siphons sticking above sand
Class Bivalvia – Gas Exchange

After entering the gills water moves into the
vertical water channels in the gills called water
tubes. Once in the water tubes the water with
dissolved oxygen is in close proximity with the
blood and diffusion takes place for the exchange
of gases. Water exits through the excurrent
opening in the mantle.
Class Bivalvia - Feeding





Suspended organic matter enters incurrent siphon.
Gland cells on gills and labial palps secrete mucus to entangle
particles.
Food in mucous masses slides to food grooves at lower edge of
gills.
Cilia and grooves on the labial palps direct the mucous mass into
mouth.
Some bivalves feed on deposits in sand.
Class Bivalvia – Nervous System




Nervous system has three
pairs of widely separated
ganglia connected together.
Sense organs are poorly
developed.
 Statocysts in the foot.
 Osphradia in the mantle
cavity (chemoreceptive).
 Pigment cells on the mantle.
Some mantle eyes have a
cornea, lens, retina and
pigmented layer.
Tentacles may have tactile and
chemoreceptor cells.
Class Bivalvia – Nervous System

Scallops have a row of small blue eyes along
the mantle edge. Each eye has a cornea,
lens, retina, and pigmented layer.
Class Bivalvia - Reproduction
Bivalves usually
have separate
sexes.
 Zygotes develop
into
trochophore,
veliger, and
spat (tiny
bivalve) stages.

Class Bivalvia - Reproduction

In freshwater clams,
fertilized eggs develop
into glochidium larvae
which is a specialized
veliger.
 Glochidia live as
parasites on fish and
then drop off to
complete their
development.
 Here is a video of a host
being captured by a lure. The
glochidia are injected into the
host.
 http://www.youtube.com/watc
h?v=BNKv07KjrFY&NR=1
Bivalve Diversity
Live in nearly all aquatic habitats
 Can attach themselves using:

 Byssal threads
 Cementation
Can burry into sand
 Some are boring and can secrete acid to
create a home in limestone

Ecological Impact of Non-native
Species - Zebra Mussel
Environmental Pest
 Ballast water of ships
from Europe in 1986
 Attack be secreting
adhesive byssal
threads

 Each other
 Other mussels
 Man made objects
○ Pipes, plumbing
54
Zebra Mussel
Live in high densities
 Feed on
phytoplankton
 Reproduce rapidly

55
Zebra Mussel
Attach to native
mussels
 Killed all native
mussels in Lake Erie

56
Distribution of Zebra Mussel
57
Class Cephalopoda

Includes
 Octopuses
Cuttlefish
 Squid
 Cuttlefish
 Nautiluses
Nautilus
Squid – Humboldt
octopus
Class Cephalopoda
The most complex molluscs
 Anterior portion of their foot modified
into a circle of arms used for

 Prey capture
 Attachment
 Locomotion
 Copulation

Foot is also incorporated into a foot
used for jet like locomotion
Class Cephalopoda
Head is in line with the visceral mass
 Mantle is muscular and encloses all of
the body except the head and tentacles.

 Acts as a pump to bring large quantities of
water into the mantle cavity
Class Cephalopoda




Cephalopod fossils go back to Cambrian (570 mya)
times.
The earliest had straight cone-shaped shells.
Later examples had coiled shells similar to Nautilus.
Ammonoids were a very successful group, some had
quite elaborate shells.
Class Cephalopoda - Shells

Shells of
Nautilus and
early nautiloid
and ammonoid
cephalopods
were made
buoyant by a
series of gas
chambers.
Class Cephalopoda - Shells

Nautilus shells differ
from those of a
gastropod because
they are divided into
chambers. The
animal lives in the
last chamber. A cord
of living tissue
extends through
each chamber.
Class Cephalopoda - Shells

Cuttlefishes have a small curved shell,
completely enclosed by the mantle.
Cuttlebone from Cuttlefish
shell

Used to supplement birds tortoise diet
with calcium
Class Cephalopoda - Shells

In squid, the shell has been reduced to a
small strip called the pen, which is enclosed
in the mantle.
Class Cephalopoda

Mantle Shell Absent in Octopus
Class Cephalopoda - Locomotion

Cephalopods
swim by expelling
water from the
mantle cavity
through a ventral
funnel.
 They can aim the
funnel to control
the direction they
are swimming.

Normally they crawl
over substrate
Class Cephalopoda – Feeding
and Digestion
Locate their prey by sight and capture their
prey with tentacles that have adhesive
cups
 In squid the margins are reinforced with
protein and sometimes possess small
hooks

Colossal Squid Tentacles
Class Cephalopoda – Feeding
and Digestion
All have jaws (beak like structure for
tearing food) and radula (rasps food)
 Colossal Squid Beak:

Class Cephalopoda –
Octopus
Most Octopuses feed on snails, fish and
crustasions
 They have salivary glands that inject
venom into prey
 Check out the abilities of this octopus
 http://www.youtube.com/watch?v=Q36_
8s5z6S8&feature=channel

Class Cephalopoda

Cephalopods have a closed circulatory
system.

Cephalopods exhibit greater excretory efficiency
because of the closed circulatory system. A close
association of blood vessels with nephridia allows
waste to filter and secrete directly from the blood
into the excretory system.
Class Cephalopoda

Nervous and sensory
systems are more
elaborate in
cephalopods than in
other molluscs.
 The brain is the largest of
any invertebrate.

Most cephalopods have
complex eyes with
cornea, lens, chambers,
and retina.
Class Cephalopoda – Communication

Visual signals allow cephalopods
to communicate.
 Movement of body and arms
 Color changes effected by
chromatophores (cells in the
skin containing pigment
granules).
○ Chromatophores can change
shape alternately dispersing
and concentrating pigment.
Cuttlefish and Camouflage

http://www.youtube.com/watch?v=2x8v1mxpR0
Class Cephalopoda

Most cephalopods have an ink sac that
secretes sepia, a dark fluid containing the
pigment melanin.
 When a predator tries to attack, the animal ejects
the ink into the water where it hangs between the
animal and the predator screening a quick escape.
Octopus with secretes sepia
Class Cephalopoda - Reproduction



Sexes are separate in
cephalopods.
Juveniles hatch directly
from eggs – no freeswimming larvae.
One arm of male is
modified as an
intromittent organ, the
hectocotylus.
 Removes a
spermatophore from
mantle cavity and
inserts it into female.
Cuttlefish and mating

http://www.youtube.com/watch?v=fR7Dq
f0vzzQ
Squid in San Diago

http://www.youtube.com/watch?v=sn7yx
3DFthI
Interesting video about an
Octopus and its features
http://www.youtube.com/watch?v=4Tcnq
2iYJJo&feature=related
 Video from Discovery

Mollusca Taxonomy

Major classes
 Polyplacophora
○ Chitons
Chiton tuberculatus lives in
the rocky intertidal
zones eat algae with
its radula
8 shell plates
Polyplacophoran Anatomy
POLYPLACOPHORA
- 800 species
- primarily intertidal
- restricted to rocky environment
POLYPLACOPHORANS - EXTERNAL FEATURES
- 8 plates embedded in a tough mantle
- Mantle edge stiffened - called the girdle
- The mantle may have spicules embedded in it.
POLYPLACOPHORANS - EXTERNAL FEATURES
girdle
- Broad muscular foot
- girdle and foot can act as
suction cup
- suction good to keep chiton
attached to substrate (rocky shore)
POLYPLACOPHORANS - EXTERNAL FEATURES
girdle
WATER FLOW:
gills
foot
inhalent chamber: region
between girdle and ctenidia.
exhalent chamber: region of
mantle cavity between ctenidia and
foot, mantle cavity.
HOW DO GILLS WORK?
exhalent flow
inhalent flow
POLYPLACOPHORAN RADULA
Iron oxides on teeth
- Radular teeth have iron oxides on teeth
- Iron oxides help teeth not wear down
- Important on the rocky shore intertidal
Chiton Spawning in Alaska
http://www.youtube.com/watch?v=SvQcMbgUWU&feature=related
 Dioecious (have separate male and
female sexes)

Chiton Filtering Water
 http://www.youtube.com/watch?v=v3fljev
ww9Y&feature=related

Class Scaphopoda
Called Tooth Shells or Tusk Shells
All burrowing marine animals
Conical shell that is open at both ends
Head and foot can project from shell
Incurrent and Excurrent allows water to enter and exit
through apex of shell
Have Radula and Tentacles
Dioecious have separate sexes
Class Monoplacophora
Neopilina
This class was only known from fossils until 1952 when
Neopilina was dredged up from a depth of 3,520m off the
Pacific Coast of Costa Rica.
Have an Undivided Arch Shell
Broad Flat foot
Foot-retractor Muscle
Serially repeated pairs of gills
Dioecious
Class Aplacophora
Solengaster
Divided into two subclasses
Subclass Neomeniomorpha the solengasters lack a shell and crawl on their ventral
surfaces
Have a ladder like nervous system that suggests relations to the flatworms
Subclass Chaetodermomorpha
Wormlike molluscs live in ventrical burrows on the deep sea floor
They have spicules on the body wall
They lack the following molluscan traits: shell, crystalline style, statocysts, foot and
nephridia
Mollusc
Phylogeny
Mantle
secreting shell
 Muscular foot
 Chambered
heart

Phylogeny

The first molluscs probably arose during
Precambrian times.
 Diverse molluscs found in the early
Cambrian.

It is likely that molluscs split off from the
line that led to annelids after coelom
formation, but before segmentation
appeared.
Phylogeny

“Hypothetical Ancestral
Mollusc”
 Probably lacked a shell
or crawling foot.
 Probably small (about 1
mm).
 Likely was a worm-like
organism with a ventral
gliding surface.
 Probably possessed a
dorsal mantle, a
chitinous cuticle and
calcareous scales.
Reference

Zoology by Miller and Harvey
 6th Edition
References – Pictures and
diagrams





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


cas.bellarmine.edu
eol.org
biology.ualberta.ca
www.southtexascollege.edu
science.kennesaw.edu
elearning.najah.edu
library.thinkquest.org
Howstuffworks.com
biology.fullerton.edu
www.bumblebee.org
More References
depts.washington.edu
 reefkeeping.com
 snailstales.blogspot.com
 http://people.bu.edu/veliger/
 www.geochembio.com
 http://cas.bellarmine.edu/tietjen/images/
phylum_mollusca.htm
 http://ofseaandshore.com/news/northse
abeach/northseabeach.php
