Molluscs

Question 1

Characteristics of all Molluscs

Answer 1

bilaterally symmetrical

triploblastic

coelomate

unsegmented

Question 2

Qualities of mollusc coelom

Answer 2

the coelom is reduced
- the main body cavity is a haemocoel
- therefore the blood system is ‘open’: no capillaries (just major blood vessels):
- useful for hydrostatics but - circulation of blood (haemolymph) is slow (OK for snails and clams but not for squid!)

Question 3

Reproductive characteristics

Answer 3

Reproduction is always sexual

Molluscs are monoecious or dioecious (or sequential hermaphrodites - e.g. many oysters)

Primitively with a trochophore larva and a veliger larva

Question 4

reproduction and larvae

Answer 4

veliger larvae have flattened, ciliated arms - the velum – used for locomotion and feeding

Question 5

Summary of molluscan characteristics

Answer 5

triploblastic; bilaterally symmetrical unsegmented
coelomate; haemocoel (open blood system)
shell, mantle and mantle cavity
ctenidia (ciliated gills)
radula
head, foot and visceral mass
simple, ganglionated nervous system
trochophore and veliger larvae

Question 6

Class Polyplacophora

Answer 6

8 shell valves
limpet-like
marine
benthic herbivores
many gills in the mantle

Question 7

Bivalvia (Lamellibranchiata):
mussels, clams, etc

Answer 7

Two valves
filter-feeders
always aquatic
mostly burrowers

Question 8

bivalves: ciliary feeding current

Answer 8

• cilia generate a water current
• food particles move along gills, sorted by palps
• digestion in digestive gland
• no radula needed
• no head needed
• ‘pseudofaeces’ extruded in exhalent current

Question 9

Reproduction in bivalves

Answer 9

• broadcast spawning
• thus dioecious
• trochophore & veliger larvae (except in fresh water)

Question 10

Bivalves: summary

Answer 10

-two valves
-foot reduced, often used for burrowing
-no radula, no head, but ctenidia enlarged, used for filter feeding (as well as respiration)
-dioecious with broadcast spawning
-trochophore and veliger larvae

Question 11

Class Gastropoda
(snails, limpets, abalone, etc.)

Answer 11

• a single shell (sometimes none)
• shell often coiled into a spiral
• aquatic
• dioecious (♂ & ♀)
• with trochophore & veliger larvae

Question 12

Disadvantages:

Answer 12

• energy-wasteful:
  - depends on chance meeting
  - larvae minute & so very vulnerable
  - finding the right place to settle
  - no parental care

Question 13

Advantages of broadcasting:

Answer 13

• simple repro. systems
• no need for mating
• larvae can be planktotrophic can disperse can exploit different foods (reduce competition)

Question 14

Subclass Prosobranchia:
limpets

Answer 14

• mostly aquatic
• benthic herbivores or carnivores
• most live on hard substrata

Question 14

Subclass Pulmonata:
Garden snail

Answer 14

• freshwater or terrestrial
• mantle cavity vascular (lung)
• mostly herbivorous
• monoecious
• cleidoic egg if terrestrial (desiccation-resistant egg with hard shell)

Question 15

Class Cephalopoda: octopus, squid & ammonites

Answer 15

• gills are not ctenidia (respiratory current is generated by muscular pumping)
  -jet propulsion (muscular mantle)
  -highly developed nervous system, including eyes

Question 16

Class Cephalopoda: reproduction

Answer 16

-no free larvae: large, yolky, shelled eggs
-eggs protected by ♀(dies after eggs hatch)
-hatch as juveniles similar to adults