Questions and Answers
What is the characteristic feature of the body of a mollusk?
It has a bilateral symmetry
What is the function of the mantle in mollusks?
It is responsible for shell formation and gas exchange
What is the name of the tongue-like structure bearing transverse rows of minute chitinous teeth in mollusks?
Radula
What is the outer layer of the shell of a mollusk composed of?
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What is the characteristic feature of the radula in mollusks?
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How many species of mollusks are known?
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What is the name of the phylum that includes mollusks?
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What is the function of the ctenidia in mollusks?
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What is the characteristic feature of the inner layer of the shell of a mollusk?
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What is the term for the space between the mantle epithelium and the visceral mass body wall?
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Study Notes
Phylum Mollusca
- Highly diverse group of animals, with some members having no sense organs or brains, while others have advanced features like the octopus
- Approximately 100,000 known species
- Characteristics: bilateral symmetry, cephalization, triploblastic, coelomate, cells organized into tissues, tissues into organs, and organs into organ systems
Body Structure
- Body consists of an anterior head, ventral foot, and a dorsal visceral mass
- Covered by a fleshy outgrowth of the body wall called a mantle
- Mantle is responsible for shell formation and gas exchange
- Mantle cavity is the space between the mantle epithelium and the visceral mass body wall
Shell
- Highly variable, secreted, and lined by the mantle
- Composed of three layers: outer periostracum (protein), middle prismatic layer (ostracum), and inner nacreous layer (hypostracum)
Radula
- A tongue-like structure bearing transverse rows of minute chitinous teeth
- Used for feeding, with the chitinous "teeth" moved forwards and backwards to break off small pieces of food
Classes of Mollusca
Polyplacophora (Chitons)
- Marine
- Shells have 8 plates
- Foot is broad and ventral
- Radula present
- Head is poorly developed
- Mantle is thick and fleshy, extending beneath the plates
- Many bipectinate gills (6-80 pairs) and one pair of nephridia
Bivalvia (Pelecypoda)
- Body enclosed in a two-lobed large mantle
- Shell consists of two lateral valves of variable size and form, with a dorsal hinge
- Head is rudimentary
- Mouth has labial palp, but no radula or cephalic eyes
- Gills are plate-like
- Posterior mantle margin forms siphons
- All are suspension filter-feeders
Gastropoda (Slugs and Snails)
- Largest and most varied class of mollusks, with over 75,000 extant species and 15,000 fossil forms
- Move using a muscular foot
- Can be terrestrial (snails and slugs), freshwater, or marine
- Shell varies greatly, from spired helical coiled to planispirals or cap-like
- Head is well-developed with a mouth, tentacles, eyes, and other sense organs
- All young gastropods undergo torsion during embryonic development
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