Phylum Mollusca

Question 1

What are the key features of Molluscs?

Answer 1

• 4-part body plan: head, foot, mantle, & visceral mass
• Cephalised, with bilateral symmetry
• Reduced coelom, unsegmented
• Open circulatory system
• Metanephridia (“kidneys”)

Question 2

What are the features (layers) of the mollusc shell, and what advantages/disadvantages does the shell provide?

Answer 2

• Secreted by the mantle
• 3 layers: Periostracum, prismatic layer, nacreous layer
• Provides protection but restricts growth

Question 3

What is the mantle cavity and what does it contain?

Answer 3

A space formed by a folded dorsal body wall. Often has flaps for many purposes.
- Usually covered by shell
- Contains the ctenidia (gills) and openings to the gut, gonads, and kidneys.

Question 4

What are ctenidia?

Answer 4

A particular type of gill. Functions in gas exchange. Comprised of a central shaft with ciliated feathery elements.

Question 5

What kind of circulatory system do (most) molluscs have?

Question 6

What is the radula?

Question 7

What are the key characteristics of the class Polyplacophora (chitons)?

Answer 7

• All marine
• Shell made of 8 overlapping plates
• Girdle covers plate edges
• Separate sexes
• Simple nervous system (“shell eyes”)
• Herbivorous
• Mantle cavity forms pallial groove
• Unidirectional waterflow: from head to anus

Question 8

What are the key characteristics of the class Gastropoda?

Answer 8

• Extremely diverse: marine, freshwater and terrestrial
• Characterised by torsion: twisting of the animal’s body relative to the foot, so the mantle cavity lies over the head.
• Asymmetrical
• Diecious or hermaphroditic
• Sensory structures on tentacles and foot

Question 9

What is torsion, why does it occur and what issues does it pose to gastropoda?

Answer 9

Visceral mass twists counter-clockwise, to where the mantle cavity, ctenidia and anus are moved behind the head. The gut is twisted into a U-shape and nerve chords are crossed in a figure eight.
This creates a sanitation issue, and basal gastropods, have found a solution through holes in the shell, where water exits. Intermediate gastropods have moved their anus and nephridiopores to the right and their ctenidia to the left. Water flows in through the right side.
This occurs due to their shell, if it was to be symmetrical it would be bulky and they would be uneven. An asymmetric shell is compact and stable, although it reduces the amount of room.

Question 11

In sea slugs (derived gastropods) what are rhinophores and what does their body plan/life history look like?

Answer 11

Mantle cavity lost/reduced; “de-torted”
Rhinophores = chemosensory tentacles
Secondary gills/cerata
Hermaphroditic, with internal fertilisation
Mostly carnivorous, with highly specialised diet
Have radula + jaw

Question 12

In land snails/slugs (derived gastropods) what is their body plan/life history?

Answer 12

No ctenidia
Slime used for moving across land
Thinner shells
De-torted to various degrees
Hermaphroditic, egg laying, no larval stage

Question 13

What are the key characteristics of the class Bivalva (clams, ship worms, etc.)?

Answer 13

Shell hinged, closed by muscles
Sessile filter feeders
Burrowing/cementing
Separate sexes, free spawning
No head except for mouth and labial palps
No radula: Feeding/respiration via enlarged ctenidia
Siphons (tubes of mantle tissues)
Sensory structures on mantle edge

Question 14

What are the key characteristics of the class Cephalapoda (squids, octopi, cuttlefish, nautilus)?

Answer 14

Active swimmers
Well-developed nervous system
Closed circulatory system
Internal shell