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					The beginning of a
    Introduction to Molluscs
• The molluscs (British spelling) or mollusks
  (American spelling) are members of the very
  large and diverse phylum of invertebrate
  animals known as Mollusca.
• Phylum Mollusca: Mollis-soft
• The scientific study of molluscs is known as
• Class Polyplacophora (chiton), gastropoda
  (snail, abalone), bivalvia (scallop, mussel, clam,
  oyster), cephalopoda (nautilus, squid, octopus)
   Three distinct body zones
• All modern mollusks have the same fundamental
  body plan. There are : a head-foot, which contains
  both the sensory and motor organs; a visceral mass,
  which contains the well-developed organs of
  digestion, excretion, and reproduction; and a mantle,
  a specialized tissue formed from folds of the dorsal
  body wall, that hangs over and enfolds the visceral
  mass and that secretes the shell.

• Mollusks are also characterized
  by a toothed tongue, the
  radula, composed primarily of
  chitin. The radula serves both
  to scrape off algae and other
  food materials and also to
  convey them backward to the
  digestive tract. In some
  species, it is also used in

• Nervous System and Sensory Capability -
  Octopuses are Smart Suckers!?
• Mollusks have a relatively complex nervous system, which
  varies from species to species reaching the height of complexity
  at the octopus.
• The octopus is thought to be among the most intelligent of all
  invertebrates, with a mental capacity likened to that of a
  domestic cat.
• Sensory ability in some mollusks (notably the cephalopods) is
  considerable, with a variety of organ systems, as well as large,
  complex eyes-The eyes of the giant squid are the largest in the
  animal kingdom, approaching the size of dinner plates.
• It has recently been demonstrated that squid can successfully
  locate and capture transparent prey in the water by means of a
  specialized polarization vision.

 Digestion and Excretion
• The digestive tract is complete and ciliated, with a
  mouth, anus and complex stomach.
• The pattern of the stomach varies according to the
  mollusks diet.
  – Food is taken up by cells lining the digestive glands
    arising from the stomach, and then is passed into the
  – Undigested materials are compressed and packaged,
    then discharged through the anus into the mantle cavity
    and are carried away from the animals in the water
  – This packaging of wastes in solid form prevents fouling of
    the water passing over the gills.
                         Respiration and Circulation
•   Excluding cephalopods, mollusks have an open circulatory system, meaning
  the blood does not circulate entirely within vessels but is collected from
  the gills, pumped through the heart, and released directly into spaces in
  the tissues from which it returns to the gills and then to the heart.
• closed circulatory system (meaning that the blood never leaves the network of
  arteries, veins and capillaries)
• Such a blood-filled space is known as a hemocoel ("blood cavity").
   – In the mollusks, the hemocoel has largely replaced the coelom, which is
       reduced to a small area around the heart and to the cavities of the
       organs of reproduction and excretion.
• Cephalopods, whose vigorous activities require that the cells be supplied with
  large quantities of oxygen and food molecules, have a closed circulatory
  system of continuous vessels and accessory hearts that propel blood into the
• The presence of discrete respiratory and circulatory systems has led to an
  improved capacity for oxygen uptake and distribution, and hence an increase
  in body mass.
• Molluscans include the largest of living invertebrates with the cephalopods,
  including the monstrous Giant Squid Architeuthis, which can reach a total
  length, including tentacles, of 18 meters.

• CLASS: Gastropoda
• Largest class of molluscs
• Most gastropod shells are built as
  a series of spirals called whorls
• They occupied a number of life
  modes, most lived in water:
  typically shallow marine areas
  but they can also live in fresh
  water and some forms survive
  on dry land.
• The most striking feature is the
  coiled shell, forms because the
  internal organs are twisted.
• The soft body parts are lined
  throughout the shell and
  sections could be extended at
  will: Head, foot and siphons.
• The HEAD extended out of the
  extended out of the majority of
  the shell APERTURE.
• The head consists of
  TENTACLES for sensing and
  below this is the MOUTH.
• It has one valve = UNIVALVE
  and is coiled vertically and
  usually spirals to the right
• The tip, or apex of the shell is
  the smallest whorl laid down
  by the snail in the early part
  of its life
• The shell is secreted by the
  MANTLE and grows along the
• Each completed coil is called
  a WHORL.
• The line along which the
  whorls meet is called the
• The LAST WHORL is called
• The spire may be high,
  pointed with many whorls or
  short with a few whorls.
• Sometimes the shell can be
  flattened giving a planispiral
  form (similar to ammonites).
• As the snail grows, it lays down the
  intermediate whorls which form the
  spire of the shell
• The final large spiral is the body
  whorl, terminating at the
• The aperture is elongated into an
  anterior notch or siphonal canal,
  which harbors the incurrent
  respiratory siphon in the living snail
  (takes in water).
• Some snails posses a touch proteinaceaous oval-
  shaped structure called the operculum, which is
  carried on its broad foot and is used to shut the snail
  snugly into its shell.
• When disturbed, the snail retracts into its shell by first
  pulling in its head, then its foot, with the operculum
  brought in last to seal off the shell

• Projecting anteriorly is the elongated siphon used to
  carry water to the gills for respiration.
• Special chemosensory organs are located near the gills
• The tentacles are chemosensory and touch sensitive;
  the eyes are light-sensitive and can detect movement

   Shell-less gastropods
• Sea slugs
• Chemical and biological defenses instead of shell
   – Poison glands
   – Prickly/sharp structures
   – Noxious mucous secretions
   – Distasteful milky substance
   – Vivid dye
• “butterflies of the sea” b/c of bright colors to warn predators
• Rhinophores-chemosensory tentacles
• Cerata -inside each contains specialized cnidosacs in which
  are located undischarged nematocysts taken from their prey

• PHYLUM: Mollusca

• CLASS: Bivalvia
  Water pollution
• Because bivalves are filter feeders, they tend to
  accumulate pollutants and in many places they are
  collected and analyzed as a means for monitoring
  water pollution.
• Pollution effects on bivalve mollusks are
  particularly apparent in coastal waters, though
  ocean dumping can contaminate offshore stalks
  as well.
• Coastal pollution has in recent decades become a
  significant and growing problem, calling for
  greater understanding of the effects of man’s
  activities on habitats and increased awareness of
  this aspect of environmental degradation.         20
• The best known
  examples of
  bivalves will be
  clams, mussels,
  scallops and

Interesting Facts About Bivalves
 • Some oysters may shed over one million eggs in
   a season. Only about one of these lives to
 • Some oysters alternate their gender. Male one
   year, female the next!
 • Some scallops (Pectens) have dozens of eyes.
   They help the scallop to see predators, so it will
   know whether to swim away or clamup!
 • The ocean quahog can live to be 220 years old.
 • Boring clams (Family Xylophagidae "wood eater"
   in Latin) can sink a ship! They are often called
   "ship worms", even though they aren't worms at
Interesting Facts About Bivalves 2
• Most molluscs are capable of making pearls when
  foreign substances enter their shell. They coat the
  foreign substance with shelly material. Some clams
  can grow pearls as big as golf balls. Freshwater
  clams are often used as nuclei, around which pearls
  can grow small, round pieces of the clam's shell are
  placed inside a Pearl Oyster and they are coated with
  iridescent nacre and form a pearl.
• The largest known bivalve harvested was a Tridacna
  gigas which weighed 330kg and was 1.4m (nearly four
  feet) in length!
• Some molluscs, such as the oysters, change sex. They
  start off life as a male and they usually end up life as
  How real pearls are made?
• Natural pearl is formed when an accidental intruder
  enters a mollusk`s shell.
• To defend itself, layer after layer of a substance
  called nacre grow and form like onion skins around
  the particle. Those are semi-opaque layers that
  consist of the calcium carbonate that makes up the
  pearl and becomes a coat for the particle.
• Inevitably, natural pearls vary in shape because the
  process depends on the shape of the piece being
  coated by nature.


• Anyone that has ever slurped down an oyster, worn a
  pearl around their neck, or enjoyed the half-shells
  found on the beach has come into contact with
  bivalves. But bivalves are much more than that. The
  second most diverse group of molluscs behind
  gastropods, bivalves are one of the most important
  members of most marine and freshwater ecosystems.
  In fact, there are well over 10,000 described species
  of bivalve, found from the deepest depths of the
  oceans, to the streams in your backyard.
• Throughout history, bivalves have been one of the
  most important marine animals to humans. They have
  been used for food, jewelry, decoration, even money.

                  Locomotion in bivalves
• Most bivalves are sedentary and many are adapted for
  burrowing into soft sediments using the laterally
  compressed foot. A few genera, e.g. Teredo, are
  capable of boring into wood, and some are even able to
  bore into rock, e.g. Zirphaea, Lithophaga.
• Although most bivalves are sedentary, some, especially
  members of the scallop family, are able to swim
  actively over short distances, usually to escape
  potential predators.
• The scallops swim by rapidly ejecting water from the
  mantle cavity by rapid clapping together (adduction) of
  the shell valves, which forces water out on either side
  of the hinge. Scallops usually swim in short bursts, but
  may cover several metres at a time.                     28
External Anatomy

           Bivalve Morphology
• The soft body is held between two
  shells (valves)
• The valves act as protection from
  carnivores and wave energy.
• Bivalves are bilaterally symmetrical-
  more or less mirror images.
             Bivalve Shell
• Umbo is the raised, oldest part of the
  shell and is used to find surfaces



           Bivalve Shell
• Shell show regular spacing of external
  growth rings on shells and their progressive
  crowding as the animals grow older
• These are added as the bivalve grows much in
  the same way as the growth rings on a tree.
• If the growth season is good then the rings
  will be more widely spaced.


            Bivalve Shell
• Bivalves have an incurrent and excurrent
  siphon to bring in water containing food
  & O2 and for excess water & wastes to

•The hinge
ligament holds       Bivalve Shell
the two halves of
the shell
•Acts as a spring
forcing the valves
apart when the
muscles relax.

Internal Anatomy

Scissor tips or a screwdriver & can be used
     to open the shell

              Adductor Muscles

Strong adductor muscles help open &
close the valves and must be cut to
examine the interior of the clam

      Inside Shell Lining
• The inside of the
  shell is pearly and
  smooth to protect
  the body
• Oysters coat
  irritants with a
  substance called
  “mother of pearl”

Teeth located on the dorsal edge of
  the shell lock to keep shells from

               Mantle cavity


Gills hang outside visceral mass (body) in the
mantle cavity & remove O2 as H2O flows over

           Bivalve morphology 5
• This is the fleshy part that
  extended out towards the edge of
  the valves.
• The mantle makes a CaCO3
  (limestone) shell
• This is a muscular organ, which
  can extend out of the shell and is
  used by the bivalve to move
  through the sediment.
              Water Flow
• Incurrent siphons aid in bringing in water
  with food and oxygen
• Cilia direct water into the gills, while labial
  palps direct water into the mouth

               Cilia on the gills pulls in water
                VISCERAL MASS -
contains respiratory, digestive, & reproductive systems


         Nervous System

• Consists of 3
  pairs of ganglia
• Has 2 pairs of
  nerve cords
  extending from

Released into Mantle Cavity to be removed
           by excurrent Siphon:

•Nitrogen waste
from kidney
• Digestive waste
from anus
• Sperm or egg (if
• Larva (if internal