Phylum Mollusca

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Phylum Mollusca

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Introduction

• Meaning soft body
• Examples clams, oysters, mussels, scallops,
  snails, slugs, squid, octopus, limpets
• Habitat fresh and marine water / slugs and
  snails can be terrestrial
• One of the largest of all animal phyla 50 000
  110 000 living species (depending on who is
  counting)

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Evolution

• It is believed that molluscs share an ancestor
  with annelid worms.
• Both most likely evolved from a primitive
  flatworm.
• The molluscan evolutionary pathway and how all of
  the organisms are related is still relatively
  unknown.
• DNA sequencing will give further detail

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Unique Characteristics

• Body that is divided into 3 regions
• Head absent in some
• - have a mouth, appendages, and sensory organs
• Foot muscular in squid and octopus it is
  modified into tentacles
• Mantle tissue that functions to surround and
  protect internal organs and secretes the shell

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Unique Characteristics

• Other unique parts
• Mantle Cavity a space that houses the molluscan
  gills, and is closed off in terrestrial slugs and
  snails and made into lungs.
• Radula teeth like structures in the esophagus
  that are used for feeding. (not present in
  bivalves)

Abalone Radula
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The Radula
Radulas from different mollusks are adapted for
different functions. Some radulas are adapted
for scraping algae others are adapted for
scraping flesh. These are electron micrographs
from different mollusk species.
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Gumboot chiton showing the foot of the chiton
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Common Characteristics

• Bilateral symmetry
• 3 cell layers
• Coelom
• Complete digestive system with a digestive gland
  surrounding the stomach

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Classification

• Are several classes of mollusks
• Class Polyplacophora
• Class Bivalvia
• Class Gastropoda
• Class Cephalopoda

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Class Polyplacophora Chitons

• Chitons can be identified by having a shell that
  is formed from a series of 7 to 8 separate
  plates. Because the shell is multisectioned, the
  body can bend to conform to a wide variety of
  underlying substrate shapes.

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Class Bivalvia

• Includes clams, oysters, mussels, scallops
• Have 2 valves

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Class Gastropoda

• Means belly foot or stomach foot
• Eg marine snails, nudibranchs, slugs, and
  terrestrial snails, limpets

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Olive Snail

Purple-ring Top Snail
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Class Cephalopoda

• Meaning - head foot
• Eg squid and octopus
• Modified foot into tentacles and/or arms

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Giant Squid Beak
Giant Squid radula, found inside the beak
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The suckers of a giant Pacific Octopus. The
largest octopus on record was over 23 feet long!
Octopus Beak
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You will learn that molluscs have open
circulatory systems. This is NOT true for
cephalopods. Cephalopods have closed circulatory
systems. This allows them to be larger and much
faster than all others.
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LIFE ACTIVITIES OF CLASS BIVALVIA A CLAM
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The Anatomy of A Clam
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Ingestion, Digestion, and Elimination 

• Incurrent siphon contracts bringing in water and
  plankton ? through mantle ? gills
• Mucus in gills traps food, cilia move the food
  (gills have cilia) ? mouth
• Palps (fleshy-lips) around mouth sort the food
  and pass it onto the mouth
• Food then moves down the esophagus into the
  stomach where digestion occurs

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Ingestion, Digestion, and Elimination

• Digestive glands secrete enzymes for digestion
• Nutrients enter intestine and are absorbed
• Wastes pass into the rectum ? through the anus ?
  excurrent siphon (solid wastes exit)

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Excretion

• 2 kidneys filter N-wastes from the body and the
  fluid surrounding the heart ? outside mantle
  cavity ? excurrent siphon (liquid wastes exit)

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Respiratory System Gills are used for

• For gas exchange
• To filter and trap food in aquatic molluscs
• Snails and slugs have a highly vascularized
  mantle cavity that is otherwise modified to form
  a lung

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The Valve - AKA Shell

• Made up of CaCO3, produced by the mantle for
  protection and camouflage
• The bivalve shell is made up of 2 pieces

Note Not all mollusks have shells, many
mollusks, such as octopus, slugs and nudibranchs
have lost their shells
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The Circulatory System

• Open system in all molluscs except for
  Cephalopods which have a closed system
• Consists of blood vessels, heart and sinuses
  (open spaces), the blood is NOT confined to
  vessels
• Nutrients and O2 in blood are pumped by heart
  into sinuses where body tissues / organs are
  bathed

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Reproduction

• Most mollusks are dioecious (separate sexes),
  with external fertilization
• Most mollusks have a larval stage
• Depending on species, fertilization may internal
• Slow-moving species like snails may be
  hermaphrodites
• Some slugs may exhibit interesting but strange
  reproductive behaviour eg. apophallation