Phylum Mollusca
By: Christa Jewett, Instructor
“For since the creation of the world God’s invisible qualities – His eternal
power and divine nature – have been clearly seen, being understood from
what has been made, so that people are without excuse.” Romans 1:20
Taxonomy
• Kingdom Animalia: Animals
• Phylum Mollusca: Mollusks
– Class Gastropoda: snails, slugs, limpets, sea hares
– Class Bivalvia: scallops, clams, mussels and oysters
• Note: There are a few other classes but we
will not be focusing on them during our
discussion today.
General Information
• There are about 85,000 different species of mollusks.
• Mollusks are invertebrates, most of which have an external
skeleton called an exoskeleton.
• They are highly diverse and inhabit numerous
environments worldwide including terrestrial, freshwater
and marine biomes.
• The three main defining features of mollusks are:
– A mantle with a significant cavity used for breathing and
excretion
– The presence of a radula
– A nervous system that includes two or three nerve chords, a
brain and various sensory systems which may include eyes and
mechanisms to detect chemicals, vibrations and touch.
General Information
• Mollusks are also known for being brilliant
multi-taskers with many internal organs
having dual functions in multiple systems. For
example, in bivalves, the gills are responsible
for respiration but also for creating a water
current in the mantle cavity, important for
excretion and reproduction.
• Mollusks represent a commercially important
food source for humans.
Class Gastropoda
• This group of animals includes snails, slugs, limpets and sea
hares.
• The scientific name for this class is derived from two words:
Gastro meaning stomach and Pod meaning foot. This name
was given to this class of animals because it appears that
they crawl on their bellies.
• There are over 40,000 different species of Gastropods so
there are exceptions to the general characteristics of this
class. For example, some may not have shells or a
developed foot.
• There are also a wide variety of species. Some are
voracious predators and some are benign grazers which
feed on algae.
Class Gastropoda
• The late embryonic stage undergoes a peculiar twisting
of 180 degrees called torsion so that many of the
posterior internal organs face forward into the mantle
cavity.
• They all have only one shell
• Animals have a distinct head and wear their shells on
their backs.
• Most have tentacles and eyes.
• Most have a radula.
• Most have a well-developed foot used for mobility.
Class Gastropoda: External Features
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Whorl
Body Whorl
Inner Lip
Outer Lip
Aperture
Operculum
Anal Canal
Siphonal Canal
Geography Cone Snail
Photo Credit: Kerry Matz National Institute of General Medical Services
(http://en.wikipedia.org/wiki/File:Conus-geographicus.jpg)
Cone Snail
Photo Credit: David Burdick (http://en.wikipedia.org/wiki/File:Conus_eating_a_fish.jpg)
Atlantic Moon Snail
Photo Credit: Ryan Somma (http://en.wikipedia.org/wiki/File:Atlantic_Moon_Snail_001.jpg)
Purple Sea Snail
Nudibranch
Photo Credit: © 2010 Dan Norton
Sea Hares
Photo Credit: Genny Anderson (http://en.wikipedia.org/wiki/File:Aplysia_californica.jpg)
Spanish Dancer
Head
Photo Credit: Dr. James P. McVey, NOAA Sea Grant Program
Class Bivalvia
• There are over 10,000 different species of Bivalves so
there are exceptions to the general characteristics of
this class.
• They are the second most diverse class of mollusks,
other than Gastropods.
• The scientific name for this class is derived from two
words: bi meaning two and valve meaning half of the
shell. All of these animals have two shells joined by a
hinge.
• These animals have two shelly halves with a hinge at
the top.
• They do not have a head.
Class Bivalvia
• Most have a hatchet-shaped foot which they use to bury
themselves in the sediment. They then extend a long siphon up to
the surface of the sea floor which is used for feeding and
respiration.
• Most bivalves are filter feeders, using large gills to draw water into
their bodies and capture small plankton with their gills. Very few
species are predatory.
• They widely vary in size with the “monster” Giant Clam that can
weigh more than 500 lbs.
• Some are sedentary and live attached to a substrate, some burrow
into the sand and some live on the benthos and swim.
• The age of bivalves can be determined by counting annual growth
rings on the shell.
Class Bivalvia: External Features
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Beak or umbo
Pallilal line
Lunule
Adductor muscle scars
Taxodont teeth
Cardinal Tooth & Anterior Lateral Tooth
Hinge
Ocean Quahog
Photo Credit: NOAA (http://www.nmfs.noaa.gov/fishwatch/species/ocean_quahog.htm)
Giant Clam
Photo Credit: Drow_male (http://en.wikipedia.org/wiki/File:Tridacna_gigas.001_-_Aquarium_Finisterrae.JPG)
Giant Clam
Photo Credit: Nick Hobgood (http://en.wikipedia.org/wiki/File:Tridacna_crocea.jpg)
Giant Clam
Photo Credit: Nick Hobgood (http://en.wikipedia.org/wiki/File:Tridacna_gigas.jpg)
Life Cycle
• Mollusks reproduce sexually. Some species
may be hermaphroditic but they do not selffertilize.
• Fertilized eggs develop into a larval stage and
then settle out onto the sea floor before
maturing into adults.
Character of God
How can we see the
character of God
in these animals?
You tell me! 