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Questions and Answers
Which layer of the molluscan shell is composed of an organic substance called conchiolin?
Which layer of the molluscan shell is composed of an organic substance called conchiolin?
- Nacreous layer
- Prismatic layer
- Periostracum (correct)
- Extrapallial layer
What is the primary purpose of nacre production in molluscs?
What is the primary purpose of nacre production in molluscs?
- To facilitate locomotion
- To enhance buoyancy
- To attract mates
- To produce pearls as a by-product of defense (correct)
Which of the following best describes the function of chromatophore organs in cephalopods?
Which of the following best describes the function of chromatophore organs in cephalopods?
- To aid in jet propulsion
- To capture prey
- To facilitate respiration
- To change color for camouflage or communication (correct)
How are the majority of molluscs classified in terms of reproductive strategies?
How are the majority of molluscs classified in terms of reproductive strategies?
What is the composition of the prismatic layer in a molluscan shell?
What is the composition of the prismatic layer in a molluscan shell?
Which structure in cephalopods is primarily modified for jet propulsion?
Which structure in cephalopods is primarily modified for jet propulsion?
The 'pen' in cephalopods serves what function?
The 'pen' in cephalopods serves what function?
What is a significant function of the radula in molluscs?
What is a significant function of the radula in molluscs?
Which of the following accurately describes the shell structure of Bivalvia?
Which of the following accurately describes the shell structure of Bivalvia?
Which adaptation is specific to Cephalopoda that enhances their predatory capabilities?
Which adaptation is specific to Cephalopoda that enhances their predatory capabilities?
What reproductive strategy is observed in many molluscs?
What reproductive strategy is observed in many molluscs?
In terms of shell layers, which layer is typically the outermost in molluscan shells?
In terms of shell layers, which layer is typically the outermost in molluscan shells?
Which anatomical feature is characteristic of Gastropoda?
Which anatomical feature is characteristic of Gastropoda?
What is the primary function of the siphons in Bivalvia?
What is the primary function of the siphons in Bivalvia?
Which characteristic distinguishes Cephalopoda from other molluscan classes?
Which characteristic distinguishes Cephalopoda from other molluscan classes?
How do molluscs vary in their diets?
How do molluscs vary in their diets?
Which structure in Bivalvia is responsible for moving water into and out of the mantle cavity?
Which structure in Bivalvia is responsible for moving water into and out of the mantle cavity?
What is a notable adaptation of Cephalopoda that differentiates them from other mollusks?
What is a notable adaptation of Cephalopoda that differentiates them from other mollusks?
Which reproductive strategy is commonly found in Mollusca?
Which reproductive strategy is commonly found in Mollusca?
Molluscan shells are usually composed of multiple layers. Which of the following correctly describes one of these layers?
Molluscan shells are usually composed of multiple layers. Which of the following correctly describes one of these layers?
Which statement accurately describes the anatomy of Gastropoda?
Which statement accurately describes the anatomy of Gastropoda?
In which group of mollusks is the foot significantly modified for diverse functionality?
In which group of mollusks is the foot significantly modified for diverse functionality?
What is the primary function of ciliated molluscan gills (ctendia) housed in the mantle cavity?
What is the primary function of ciliated molluscan gills (ctendia) housed in the mantle cavity?
How do Bivalvia prevent fouling of their gills?
How do Bivalvia prevent fouling of their gills?
The mantle cavity in molluscs houses respiratory organs called ctenidia.
The mantle cavity in molluscs houses respiratory organs called ctenidia.
Gastropoda are characterized by their coiled shells and an extensive siphon structure.
Gastropoda are characterized by their coiled shells and an extensive siphon structure.
In Cephalopoda, the foot is extensively modified into arms and tentacles for locomotion and feeding.
In Cephalopoda, the foot is extensively modified into arms and tentacles for locomotion and feeding.
The visceral mass is a distinct structure that contains the shell in molluscs.
The visceral mass is a distinct structure that contains the shell in molluscs.
Ciliary function in molluscan gills includes waste elimination and providing a water current.
Ciliary function in molluscan gills includes waste elimination and providing a water current.
The pallial groove is a feature found in the class Bivalvia.
The pallial groove is a feature found in the class Bivalvia.
Bivalvia possess a significant shell structure known as the valve to aid in water movement.
Bivalvia possess a significant shell structure known as the valve to aid in water movement.
The prismatic layer of a molluscan shell is primarily composed of organic substances called conchiolin.
The prismatic layer of a molluscan shell is primarily composed of organic substances called conchiolin.
In molluscs, the mantle is only present in those with shells.
In molluscs, the mantle is only present in those with shells.
Pearl production in molluscs occurs when foreign objects are enclosed in layers of nacre as a defensive measure.
Pearl production in molluscs occurs when foreign objects are enclosed in layers of nacre as a defensive measure.
Chromatophore organs in cephalopods are responsible for producing sound.
Chromatophore organs in cephalopods are responsible for producing sound.
The mantle in molluscs is associated with secreting materials for both shell formation and pearl production.
The mantle in molluscs is associated with secreting materials for both shell formation and pearl production.
The outermost layer of a molluscan shell is known as the nacreous layer.
The outermost layer of a molluscan shell is known as the nacreous layer.
Most molluscs are monoecious, meaning they have both male and female reproductive organs.
Most molluscs are monoecious, meaning they have both male and female reproductive organs.
The retractor muscles in cephalopods play a critical role in the movement of the arms and tentacles.
The retractor muscles in cephalopods play a critical role in the movement of the arms and tentacles.
The extrapallial space in molluscs is involved in nutrient absorption.
The extrapallial space in molluscs is involved in nutrient absorption.
Mollusca can only be found in aquatic habitats.
Mollusca can only be found in aquatic habitats.
The radula is present in all classes of Mollusca.
The radula is present in all classes of Mollusca.
Mollusca exhibit a size range from 1 mm to 18 m.
Mollusca exhibit a size range from 1 mm to 18 m.
Bivalvia typically have a single dorsal shell plate.
Bivalvia typically have a single dorsal shell plate.
Cephalopoda are primarily characterized by their coiled shells.
Cephalopoda are primarily characterized by their coiled shells.
The mantle in molluscs is responsible for secreting the shells.
The mantle in molluscs is responsible for secreting the shells.
The foot in molluscs is always developed for swimming.
The foot in molluscs is always developed for swimming.
Detritovores are a type of herbivore found within the molluscs.
Detritovores are a type of herbivore found within the molluscs.
Most Molluscs are dioecious, meaning they have separate sexes.
Most Molluscs are dioecious, meaning they have separate sexes.
Simultaneous hermaphrodites in Molluscs produce eggs and sperm by separate individuals.
Simultaneous hermaphrodites in Molluscs produce eggs and sperm by separate individuals.
Crassostrea virginica is an example of a protandrous hermaphrodite that can change from female to male.
Crassostrea virginica is an example of a protandrous hermaphrodite that can change from female to male.
Trochophore larvae may have both feeding and non-feeding stages.
Trochophore larvae may have both feeding and non-feeding stages.
Free-swimming Veliger larvae develop directly from fishes.
Free-swimming Veliger larvae develop directly from fishes.
Reciprocal copulation is a common mating strategy among some simultaneous hermaphrodites in Molluscs.
Reciprocal copulation is a common mating strategy among some simultaneous hermaphrodites in Molluscs.
Bivalve larvae predominantly exhibit free-swimming stages and never directly metamorphose.
Bivalve larvae predominantly exhibit free-swimming stages and never directly metamorphose.
The gonad in simultaneous hermaphrodites is called the ovotestis.
The gonad in simultaneous hermaphrodites is called the ovotestis.
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Study Notes
Introduction
- Mollusca are a diverse phylum with approximately 93,000 species.
- Molluscs are found in various habitats, from high altitudes to abyssal depths.
- They are characterized by a range of sizes, from 1 mm to 18 m long.
- Locomotion varies from sessile to swift, and their diets include herbivores, detritovores, filter feeders, and predatory carnivores.
Defining Molluscs
- A “typical” mollusc is a hypothetical representation, as each class has unique characteristics.
- Defining characteristics include:
- Mantle: Specialized dorsal tissue that secretes shells.
- Radula: Tongue-like feeding organ with teeth (except Bivalvia).
- Foot: Muscular ventral body wall for locomotion or clinging.
Molluscan Classes
- Polyplacophora: Found in aquatic/semi-terrestrial habitats. They have 7-8 dorsal plates as shells and a single large foot.
- Gastropoda: Found in aquatic/terrestrial/semi-terrestrial habitats. They have a coiled shell and a foot that extends from the shell.
- Bivalvia: Found in aquatic environments. They have two lateral plates (valves) as their shells. Their foot is used for burrowing.
- Cephalopoda: Found only in aquatic environments. Their shells are reduced or absent, and they have modified feet into arms and tentacles.
Body Form & Function
- Molluscs share a basic body plan with a foot or head-foot section, a visceral mass, and a shell.
- Respiratory organs: Ctenidia (molluscan gills) are located in the mantle cavity and function in respiration, preventing fouling, waste elimination, gamete dispersal, and sensory detection.
Polyplacophora
- Possess a ventral pallial groove where ctenidia are located.
- Mantle is located along the outside of the shell plates, and the foot extends from the ventral side.
Gastropoda
- Shell is coiled and covered by the mantle.
- Mantle cavity contains the ctenidium and is located above the foot.
Bivalvia
- Shell is composed of two valves.
- Foot is used for burrowing.
- Mantle cavity contains ctenidia and is connected to the outside by siphons (for water intake and expulsion).
Cephalopoda
- Foot is modified into arms and tentacles for locomotion and feeding.
- Mantle is muscular and lacks a true shell.
- Mantle is used for jet propulsion, and they possess chromatophores for color change.
Radula
- Radula is a toothed, tongue-like organ used for feeding in most molluscs.
- Composed of an odontophore (a cartilaginous structure) and retractor and protractor muscles.
- Radula teeth are arranged in rows on the ribbon-like radular membrane.
Molluscan Shells
- Mollusc shells have three distinct layers:
- Periostracum: Outermost layer made of conchiolin, secreted by the mantle edge.
- Prismatic Layer: Middle layer composed of calcium carbonate and a protein matrix, secreted by the mantle edge.
- Nacreous Layer: Innermost layer composed of nacre (CaCO3 in a protein matrix), secreted by the mantle surface.
Pearl Formation
- Pearl formation is a defensive mechanism.
- When a foreign object enters the extrapallial space between the mantle and shell, the mantle secretes nacre around it.
- Concentric layers of nacre form a pearl if the object is fully enclosed by the mantle.
- In most cases, the object remains attached to the shell, forming a nacreous deposit.
Reproduction & Development
- The majority of mollusc species are dioecious (separate sexes).
- Fertilization occurs externally for most, while some gastropods and cephalopods have internal fertilization.
- Development includes a larval stage, with variation in larval types depending on the species.
Mollusca Overview
- Phylum Mollusca represented by approximately 93,000 species.
- Diverse in size, locomotion and diet.
- Molluscs can be found in a wide range of habitats, from 7,000 meters above sea level to abyssal depths.
Defining Characteristics
- Mantle: specialized tissue that secretes the shell.
- Radula: tongue-like organ with teeth, used for feeding (absent in Bivalvia).
- Foot: ventral body wall muscles used for locomotion or clinging
Major Classes
- Polyplacophora (chitons) : 7-8 dorsal plates for a shell.
- Gastropoda (snails, slugs) : coiled shell.
- Bivalvia (clams, mussels): two lateral plates for a shell.
- Cephalopoda (squid, octopus, nautilus) : reduced or absent shell, modified into arms and tentacles.
Body Form and Function
- General body plan:
- Foot or Head-Foot portion
- Visceral mass portion
- Shell
- Respiratory organs: ciliated molluscan gills (ctendia), housed in the mantle cavity.
- Shell structure:
- Periostracum (outer, conchiolin layer)
- Prismatic layer (middle layer of calcium carbonate and protein matrix)
- Nacreous layer (inner calcareous layer of nacre)
- Pearl production: foreign objects in extrapallial space are enclosed in nacre.
Reproduction and Development
- Majority of molluscs are dioecious (separate sexes).
- Exceptions:
- Simultaneous hermaphrodites.
- Protandrous hermaphrodites (e.g. Crassostrea virginica) change sex from male to female.
- Gamete exchange:
- Internal fertilization (copulation).
- External fertilization (gametes released into water).
- Trochophore larvae: free-swimming larvae with a prototroch (cilia) for feeding.
- Veliger larvae: free-swimming larvae with two shell valves.
- Direct metamorphosis: some species develop directly into a juvenile (eg. polyplacophorans).
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