The phylum Mollusca contains over 50,000 species including snails, slugs, clams, oysters, octopuses and squid. They have bilateral symmetry, a mantle that secretes a calcium-based shell, and a muscular foot. Habitats include marine, freshwater and land. Reproduction is usually sexual with external or internal fertilization. The three main classes are Gastropoda (snails and slugs), Bivalvia (clams and oysters), and Cephalopoda (octopuses and squid). Gastropods have a coiled shell and radula, Bivalves have two shells, and Cephalopods are highly intelligent

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.