EXPLANATION OF PHYLUM MOLLUSCA....PPT BEST OF LUCK

1. Phylum Mollusca
Lecture-7

2. General Characteristics
• 50,000 or more species of molluscs.
• Bilateral symmetry.
• Body enclosed by a blanket-like mantle that secretes a shell made of calcium or some other
stiff structure.
• A mantle cavity between the mantle and the internal organs; the anus, reproductive, and
excretory ducts open into the mantle cavity.
• A ventral, muscular foot that is highly modified among the various groups of molluscs.
• Head withy sensory organs and a mouth.
• Aquatic mollusks have gills. Terrestrial mollusks have lungs.
• Herbivorous or carnivorous. All of them with a complex digestive system.
• Most of them are hermaphrodites.
• In separate sexes individuals, there is no sexual dimorphism.
• Most of them lay eggs from which a larvae emerges. It will go under metamorphosis.
• Have a true heart
• Simple to complex nervous system

3. Body Plan

4. Body Plan

5. Habitat and niche
Some molluscs habitats include:
• Marine (mostly)
• Freshwater (a few)
• Land (a few)
Molluscs niches (life-style):
• Most live on the ocean bottom and are mostly sedentary
• Some are free-swimming (the cephalopods)

6. Reproduction
Sexual
• Most are dioecious,
• some hermaphrodites (gastropods)
• Internal or external fertilization
• Fertilized egg develops into trochophore larva:
• Free swimming, planktonic larva that feeds on tiny plants.
Asexual
• Some regeneration

7. Ecological Roles
Molluscs fill a wide variety of ecological niches
• Predators,
• Filter feeders,
• Food source
• Break down decaying plant matter
• Filter feeding bivalves can be an indicator of water quality
• Shipworms (not worms) burrow through wood boats and docks
• Terrestrial molluscs are agricultural pests

8. Classes of Molluscs
Class Gastropoda
• Most popular class of molluscs.
• Consists of snails and snail-like creatures.
Specific characteristics:
• have a dorsally located shell (often coiled).
• They have a well-developed radula.
• 4 tentacles (2 optic and 2 sensory)
• Visceral mass (organs) almost always located inside the shell. Their visceral mass is rotated
180 degrees during development.
• They are herbivores or predatory (carnivorous).
• Habitats include: ocean, lake, river bottoms, coastal shores, and land. Terrestrial have lungs
and aquatic have gills.

9. Classes Gastropoda
Limpet Snail
Slug

10. Class Bivalvia
• They have no head.
• They have two shells held together by powerful muscles.
• They have a ventrally located foot that sticks out between the two valves.
• They tend to burrow into soft mud or sand or attach to rocks or other shells.
• Habitats include marine and freshwater.
• Large gills are used for respiration and filter feeding.
• Water enters and exits through siphons.
Classes of Molluscs

11. Classes of Molluscs
Class Cephalopoda
• Contains the largest molluscs.
• Specific Characteristics:
• May lack a shell (like an octopus)
• Shell may be reduced to a stiffening rod (like the squid or
nautilus)
• The foot is highly modified to form a group of tentacles around
the mouth.
• They are found in deep and shallow waters along many coasts.
• Squids & Nautilus are free-swimming and move very quickly.
• Octopuses are found among rocks or crawling on the bottom of
the ocean.
• Complex brain, two lateral eyes, excellent eyesight.