Molluscs

Question 1

Molluscs symmetry and cleavage

Answer 1

Bilaterial

Protostomes - Spiral cleavage

Coelomate - triploblastic -(body cavity). Have a reduce coelom

Question 2

3 body sections

Answer 2

Head
Foot
Visceral mass

Question 3

What is the visceral mass?

Answer 3

Guts etc.

Contained within shell

covered by mantle

Mantle secretes shell

Question 4

Excretory system

Answer 4

Metanepridia

Question 5

what does the mantle cavity (gap) contain?

Answer 5

Gills

Covered in cilia to drive water over gills

Question 6

Circulatory system

Answer 6

• Open circulatory system
• Haemocoel blood system(combinds blood with coelom)
• Simple system
• Oxygen is taken in via gills → sent to heart which then pumps blood out via ventricles into the coelom

Question 7

Nerve Ring

Answer 7

Goes round the oesophagus

Nerves going to foot and visceral mass

Question 8

What is a radula ?

Answer 8

• Feeding apparatus
• Organ covered in teeth which move around (conveyer belt) and scrape food off the substrate
• Some radula modifications e.g. cone snails

Question 9

Generalised mollusc reproduction

Answer 9

Dioecious (separate sexes)

Some hermaphrodite

Trochophore larvae

Question 10

3 morphologically diverse groups of molluscs

Answer 10

Gastropods

Bivalves

Cephalopods

Question 11

How do gastropods differ from the generalised molluscs ?

Answer 11

Development of head (eyes, tentacles etc)

Dorso-ventral elongation of body (grow upward - can lead to coiling)

Shell (Shield - protective retreat)

Torsion

Question 12

What is torsion?

Answer 12

Rotation of the visceral mass and mantle cavity - 180°

Mantle cavity and visceral mass end up over the head

Nervous system ends up in a knot

Question 13

Why does torsion happen?

Answer 13

Still unsure as to why gastropods undergo torsion

• Theory 1- protection of veliger larva (where they undergo torsion) - no evidence for protection against predation
• Theory 2- protection of adult - cavity above head, can crawl in for protection
• Theory 3- utilisation of water by gills above head

Question 14

Disadvantages of torsion

Answer 14

Anus over head

Twisted nervous system

Some gastropods have evolved holes in shells to expel waste e.g. limpet

Question 15

De-torsion

Answer 15

• Some have undergone de-torsion
• Nudibranchs:
• De-torsion not quite back to original evolutionary torsion
• No shell
• Rhinophore - detect chemical queues

Question 16

Gastropod shell coiling

Answer 16

• Planispiral (symmetrical)
• Conispiral (asymmetrical)
• Left and right handed coiling
• Most coil right handed (clockwise)
• Left coiling is less common - not due to mutation - can only mate with same coil (left + left etc)

Question 17

Most specialised group of gastropods?

Answer 17

Pulmonates (land snails and slugs)

Question 18

why are Pulmonates specialised?

Answer 18

• No gills - terrestrial
• Vascularised mantle cavity
• Functions like a ‘lung’
• Air is drawn in and gaseous exchange takes place over mantle cavity

Question 19

Pulmonates reproduction

Answer 19

• Hermaphrodites
• complex mating ritual
• ‘Love darts’
• Sequential hermaphroditism

Question 20

Pulmonate sequential hermaphroditism

Answer 20

Can change sex

E.g. slipper limpet

Pile up - females at bottom and males at top - if bottom ones die top will become female

Question 22

How have Bivalves developed from classic molluscs?

Answer 22

• Have two shells
• Held together by abductor muscles
• Reduced head
• No radula
• Reduced nervous system
• Foot - either small or enlarged

Question 23

Bivalves feeding

Answer 23

Most bivalves are filter feeders

Enlarged gills used for feeding

Question 24

Bivalves eyes

Answer 24

Can have eyes on mantle edge - some simple and some more complex

Question 25

Bivalve - Byssal threads?

Answer 25

mussels use these to hold onto rocks (may be able to use these to protect against predators)

Question 26

Bivalve sex

Answer 26

Most are dioecious

Question 27

Cephalopods orientation of body

Answer 27

On a dorsal ventral axis

Reduced or lost shell (Nautilus only shelled form)

Question 28

Cephalopod modified foot

Answer 28

Series of tentacles

males uses tentacles for mating

Question 29

Cephalopod nervous system & eyes

Answer 29

Nervous tissue concentrated into brain.

most well developed nervous system in inverts

Developed eyes - similar to vertebrate eyes (can focus on objects)

Question 30

Why are cephalopods specialised?

Answer 30

Marine carnivores

Geared up for predation

Question 31

Cephalopods Radula modification

Answer 31

Modified to form a jaw

Question 32

Cephalopods colour change

Answer 32

Chromatophores

Muscle cells attached modify how much pigment are in the chromatophores

Used for mating and defence

Question 33

Nautiloids

Answer 33

Only group of cephalopods that has a fully formed shell

Very ancient group (since cambrian period)

Question 34

Nautiloids - siphuncle function

Answer 34

Shell has chambers

a central tube runs through these chambers and regulates salt content (SIPHUNCLE)

Water flows in/out by osmosis depending on salt content

Creates buoyancy

Question 35

Coleoidea - cepholapods

Answer 35

Cuttlefish

Squid

Octopus

Question 36

Coleoidea features

Answer 36

• Reduction in shell
• Cuttlefish have cuttlebone - used to maintain buoyancy - changes salt content
• Squid - shell reduced to a proteinaceous pen
• Octopuses - shell vestigial / absent

Question 37

Mimic octopus

Answer 37

Dynamic mimicry
Can alter body shape

Cuttlefish can also mimic