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Phylum Mollusca. Coelom. Acoelom. Tissues. Who are the Members of the Phylum Mollusca ?. Mollusca can be broken down into seven classes: Gastropoda Pelecypoda Cephalopoda Aplacophora Monoplacophora Polyplacophora Scaphopoda. Mollusca Classes cont.

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who are the members of the phylum mollusca
Who are the Members of the Phylum Mollusca?
  • Mollusca can be broken down into seven classes:
    • Gastropoda
    • Pelecypoda
    • Cephalopoda
    • Aplacophora
    • Monoplacophora
    • Polyplacophora
    • Scaphopoda
mollusca classes cont
Mollusca Classes cont.
  • Although there are seven classes, three classes dominate the phylum
  • These classes are:
    • Gastropoda
      • Snails and sea slugs
    • Pelecypoda (Bivalvia)
      • Clams, mussels, scallops, and giant clams
    • Cephalopoda
      • Octopi, squid, cuttlefish
identifying characteristics
Identifying Characteristics
  • Bilateral Symmetry
  • Coelom
  • Cephalization with sensory organs
  • 3 layers of tissue in embryo
      • Protosome development
        • Mouth develops before the anus
  • Complex organ systems
    • Digestive system
      • Many have a radula which is a chitinous tongue used for feeding
    • Circulatory system (closed in Cephalopods)
    • Respiratory system includes the use of gills to extract O2 and get rid of waste
identifying characteristics continued
Identifying Characteristics continued
  • Non-segmented
  • Soft body and Mantle
  • Head, muscular foot and viseral mass
    • Head
      • Contains sensory organs, mouth, and cerebral ganglia(extremely primitive brain)
    • Muscular Foot
      • Used for locomotion
      • Can be anything from long tongue like structures for crawling, to tentacles for capturing
    • Visceral Mass
      • Contains organs and systems
characteristics continued
Characteristics continued
  • second largest animal phylum next to arthropods
  • soft bodies with either an internal or external shell ( but some lack shells altogether ( octopi))
  • found in salt/ fresh water and on land
  • large size differences
  • Sexual Reproduction (have separate sexes)
all forms of mollusks have a body composed of at least three distinct parts
All forms of Mollusks have a body composed of at least three distinct parts:
  • Visceral Mass

- The soft bodied portion that contains the internal organs

  • Foot

- A strong, muscular portion used for locomotion which is adapted as tentacles in cephalopods

  • Mantle
    • A membranous, sometimes muscular covering that envelops but does not completely enclose the visceral mass
    • Can secrete a shell

Illustration of the general features of all mollusks

  • bilaterally symmetrical
  • all have soft bodies that contain the organs
  • 3 basic parts: Foot, mantle, visceral mass (and possible shell)
  • Foot: used for movement and for feeding (how it moves or feed differs for different animals)

example: snails used their foot to creep over rocks and plants, octopuses use their tentacles to capture prey

  • Mantle: fold of skin covering the body organs that helps to develop the shell of the animal by secreting calcium carbonate
  • Shell: hard layer of the animal composed mainly of calcium carbonate
  • Visceral mass: Lies under the mantle and contains the internal organs
  • range from herbivores, carnivores to filter feeders, with fewer being detritus feeders ( decayed matter ) and others are parasites ( live off host )
  • mode of feeding is different depending on the animal
  • use the radula to feed, tongue like organ with many rows of teeth

For herbivores the radula is used to scrap food off rocks ect

  • For carnivores the radula is used to drill holes in the shells of other mollusks and suck out the soft bodies ( snails )
  • For filter feeders use gills to capture food found in water ex: scallops, no radula usage
respiration and internal transport
Respiration and Internal Transport
  • Aquatic mollusks use gills located inside their mantle cavity
  • Land mollusks use adapted mantle cavity lined with blood vessels, "skin breathing“
  • Open circulatory system in sessile and slow moving mollusks. A system made up of heart and vessels where blood flows to and from the vessels and into internal body cavities called sinuses that bath the body tissue
  • Closed circulatory system used by fast moving members (octopi and squids). Blood is ALWAYS in vessels and never leave them ( humans have a closed system )
excretion and response
Excretion and response
  • Undigested food ----> anus
  • Nephridia - special organs that remove cellular metabolic waste and ammonia ( poisonous) from body fluids.
  • Sessile and burrowing members have simple nervous systems with
    • nerve ganglions – a group of nerve cells forming a nerve center
    • a few nerve cords
    • simple sense organs
  • Cephalopods ( octopuses ) have a well developed brain. Can be trained.
  • Most have separate sexes and external fertilization
  • Most have trochophore larva ( swim in open water and feed on tiny floating plants )
  • In tentacled mollusks and some snails fertilization is internal
  • Some snails and oysters are hermaphroditic and engage in cross fertilization
  • -The process of Torsion is unique to Gastropods that leads their body to coil to one side during embryonic development
  • What happens:
  • -Torsion describes the twisting of the visceral mass
  • -This leads the digestive tract to be coiled and the anus is near the mouth at the head end of the animal
the gastropods belly foot
The Gastropods : “belly”“foot”
  • Usually are univalves(one shell)
  • Large, muscular foot
  • Well defined head
  • Muscles move in a rippling motion for movement
  • Most possess gills but some have lungs
  • Have a radula to help obtain food
  • Most snails have a lid-like part called an operculum on the back of the foot so they can draw their bodies into their shell and close off the opening

Underside of a Limpet

gastropod characteristics
Gastropod Characteristics
  • Gastropoda (stomach foot)
  • Examples: Snails
  • Are univalves (one shell).
  • Respiration in mantle cavity with gills
  • Respiration in land snails use blood
  • Response to touch and light. Eyes on stalks extending from foot.
  • Radula - rasp-like tongue, used to scrape algae or holes if a carnivore.
  • Move by means of a large muscular "foot" on their ventral (stomach) side
  • Sea hare can excrete "purple ink" when threatened by a predator.
  • Sea butterflies can swim rapidly.
  • Bright colored slugs and nudibranchs are protected because they are poisonous.
  • Only mollusks to invade the land.
  • Exhibit torsion - twisting "U-shape" of internal organs.
gastropods in love
Gastropods in Love….

.. and a lonely garden slug….

class polyplacophora
Class Polyplacophora

The Chitons

  • Exclusively marine
  • Have 8 dorsal plates
  • Primitive with segmentation have radula scrapers to feed on algae
the bivalves two shell
The Bivalves : “two”“shell”
  • Possess 2 shells that can be tightly closed with strong muscles
  • Strong, muscular foot and use it for locomotion
  • No head or teeth
  • Obtain food and oxygen by using a muscular siphon that brings water to the gills which filter out O2 and plant cell material
  • Often attach themselves to surfaces
  • Possess an open circulatory system


class bivalvia
Class Bivalvia


  • Contain 2 hinged shells (valves)
  • Filter feeders, sessile
  • Use gills for respiration &filter feeding
  • Includes: clams, oysters, scallops, mussels



Above left: Elephant trunk clams, named for their large foot

Above right: an opened oyster commonly enjoyed raw or cooked as a delicacy

Lower right: Scallop showing off its many primitive eyes

class cephalopoda


Class Cephalopoda
  • These are the most highly evolved invertebrates
  • Have vertebrate eye Capable of learning
  • Very motile predators - largest invertebrate
  • Shell greatly reduced


the cephalopods head foot
The Cephalopods: “head”“foot”
  • Bilaterally symmetrical
  • Prominent head, arms and tentacles
  • Have developed brain “intelligent “ creatures
  • Arms that take in food surround a hard, strong, beak-like jaws that tear the prey and secrete a poison to paralyze it
  • Closed circulatory system with hemocyanin (like hemoglobin)
  • Move by jet propulsion
  • Can change color with the use of chromatophores
  • Possess advanced vision
  • Secrete an ink cloud when in danger

Caribbean Reef Squid

Tentacles of a giant squid

Calamari: a deep fried squid dish

Squid eye and gills


Octopus on the beach

Octopus beak

Swimming Octopus

Octopus arm in detail

Octopi are extremely intelligent creatures and have been known to learn to escape from their aquariums when in captivity through very small cracks at the top.
ecological roles of mollusks
Ecological Roles of Mollusks
  • Mollusks are part of the aquatic food chain (see picture to left)
  • Many mollusks are keystone species, if their numbers decrease the decline in other species of marine life will follow
  • Filter feeders help to purify the water
  • Many bivalves are reef forming and provide an ecosystem for other aquatic species
  • Reef builders stabilize banks and bottom sediments

Starfish eating (digesting) a mussel

squid fact
Squid Fact

The largest squid ever caught was a 10 m long ColossalSquid off the coast of New Zealand and weighed in at 495 kg. Colossal Squid are larger than the giant squid found off the Mexican coast and are thought to grow in size to a whopping 13 meters!