K.2 Phylum Mollusca
General Characteristics of Molluscs
Q: What are the three main characteristics of molluscs?
A:
Mantle (secretes shell)
Muscular foot (movement)
Soft body (visceral mass)
Q: What is the mantle?
A: A specialized tissue that secretes calcium carbonate to form the shell.
Q: What is the visceral mass?
A: The soft internal organs of the mollusc.
Q: What is the function of the foot in molluscs?
A: Locomotion.
Q: What type of skeleton do molluscs have?
A: Exoskeleton (external shell), not an endoskeleton.
Q: What is the main trade-off of having an exoskeleton?
A: Protection vs mobility.
🐚 Bivalves (Clams, Mussels, Oysters)
Q: What is a bivalve?
A: A mollusc with two shells hinged together.
Q: What is the lifestyle of most bivalves?
A: Sessile or slow-moving filter feeders.
Q: How do bivalves feed?
A: By filtering water for food particles.
Q: What are siphons used for?
A: Bringing water in (incurrent) and pushing water out (excurrent).
Q: What happens in the bivalve siphon system?
A: Water brings in oxygen and food, and removes waste.
Q: What are labial palps?
A: Structures that help direct food into the digestive tract.
Q: Why can bivalves survive in intertidal zones?
A: They can close their shells to retain water.
Q: What is nacre?
A: A smooth shell lining that can form pearls.
🫀 Circulation in Bivalves
Q: Do bivalves have a closed or open circulatory system?
A: A partially open circulatory system.
Q: What is the function of the two-chambered heart in bivalves?
A: Pumps blood into tissues via sinuses.
Q: What are sinuses in bivalves?
A: Open spaces where blood bathes tissues.
Q: How do bivalves move using their foot?
A: By extending it into sand, swelling it with blood, and pulling themselves forward.
🐚 Reproduction in Bivalves
Q: How do bivalves reproduce?
A: By broadcasting gametes into water.
Q: Are bivalves male, female, or both?
A: Separate sexes (dioecious).
Q: What larval stage do bivalves produce?
A: Trochophore larva.
Q: Why is the trochophore stage important?
A: It suggests evolutionary links with annelids.
🐌 Class Gastropoda (Snails & Slugs)
Q: What animals are gastropods?
A: Snails and slugs.
Q: What does “gastropod” mean?
A: “Stomach-foot.”
Q: What is the main difference between snails and slugs?
A: Snails have shells; slugs do not.
Q: What is the function of the snail mantle?
A: Produces shell material as the snail grows.
Q: How do slugs protect themselves?
A: Camouflage and foul-tasting secretions (“ink”).
Q: What is the gastropod foot used for?
A: Crawling using muscular movement and mucus.
Q: What do gastropods eat?
A: Mostly plants (herbivores).
Q: What is a radula?
A: A tongue-like structure with tiny teeth used for scraping food.
🧠 Gastropod Nervous System
Q: Do gastropods show cephalization?
A: Yes, they have a distinct head region.
Q: What sensory structures do gastropods have?
A: Light-sensitive eyes on retractable antennae.
Q: Is the gastropod circulatory system open or closed?
A: Open circulatory system.
🐙 Class Cephalopoda (Squid, Octopus)
Q: What does “cephalopod” mean?
A: “Head-foot.”
Q: What is the main feature of cephalopods?
A: Highly developed head and tentacles with suckers.
Q: What type of circulatory system do cephalopods have?
A: Closed circulatory system.
Q: Why do cephalopods need a closed circulatory system?
A: To support fast movement and high energy demand.
Q: How do squids and octopuses differ in movement?
A: Squids swim rapidly; octopuses crawl more and swim occasionally.
Q: What defense mechanism do cephalopods use?
A: Ink release.
👁 Cephalopod Eyes
Q: Why are cephalopod eyes advanced?
A: They form clear images using a lens and retina.
Q: What is the structure of a cephalopod eye?
A: Fluid-filled chamber with a lens focusing light on a retina.
Q: Do cephalopods have eyelids?
A: No, but they have iris-like structures controlling light entry.