1: Molluscs

Question 1

Are molluscs coelomates?

Answer 1

Yes, they have: a body cavity. gut inside, surrounded by endoderm, outside = ectoderm. 2 layers of mesoderm

Question 1

What sort of cleavage do molluscs undergo?

Answer 1

Spiral

Question 2

Describe the body plan of GENERAL mollusc

Answer 2

Made up of:
Foot
Head (radula → feeding apparatus)
Visceral mass (covered by mantle → secretes shell)

Features:
- Reduced coelom
- Metanephridia
- Mantle cavity → gills
- Gills covered in cilia → drive water over and remove particles
- Nerve ring around the oesophagus, go into foot and visceral mass

Question 3

Describe the circulartory system of a GENERAL mollusc

Answer 3

Haemocoel (blood system and coelom)
Blood oxygenated in gills
Heart-like structure

Question 4

How does the body plan of the Gastropod differ from the generalised mollusc

Answer 4

Development of head
Dorso-ventral elongation of body
Shell (shield → protective retreat)
Torsion

Question 5

What is torsion in Gastropods?

Answer 5

rotation of visceral mass & mantle cavity by 180O

Question 6

What are the advantages of torsion in Gastropods?

Answer 6

• Protection of veliger larva (stage when they undergo torsion)
• Protection of adult
• Mantle cavity allows them to withdraw into it
• Utilisation of oncoming water by gills

Question 7

What are the disadvantages of torsion in Gastropods?

Answer 7

Nervous system twisted
Anus over head

Evolution of gastropods → modifications of mantle cavity to solve sanitation & water flow problem (caused by torsion)

Question 8

What groups make up the gastropods?

Answer 8

snails, slugs, conchs, periwinkles and sea slugs

Question 9

Some gastropods undergo de-torsion. Give an example and describe them

Answer 9

Nudibranchs (sea slugs):
- No shell
- Nematocysts to protect them as they have no shell
- Rhinopohres → detect chemical queues
E.g sea hare → secrete opaline to ‘gum up’ specific predators antennae

Question 10

Describe the coiling in gastropods

Answer 10

Planospiral (symmetrical)
Most gastropods are conispiral → asymmetrical
Consequences of asymmetrical coiling → body is reduced on one side = effect internal structure

Most gastropods have right handed coils
Occasionally left handed though → left and right handers can’t mate!
Thought mutation causes left-handedness

Question 11

Describe the Pulmonates

Answer 11

→ most specialised of the gastropods
Land snails and slugs
No longer have gills → modified
Vascularised mantle cavity → functions like a lung

Question 12

Describe sequential hermaphroditism

Answer 12

→ can change sex
E.g slipper limpet
Older individuals are female → tend to be large (at the bottom)
Hermaphrodites → when female at bottom dies, they become functioning females
Males, younger and smaller (at the top)

Question 13

What groups make up the Bivalves?

Answer 13

clams, mussels, oysters, scallops

Question 14

Describe the body plan of the bivalves

Answer 14

• Reduced head and nervous system
• No radula
• Bivalve because shell is made up of 2 parts → held together by adductor muscles
• Can have simple/complex eyes on mantle edge

Question 15

How has the body plan of the Cephalopods changed from the generalised mollusc

Answer 15

• Orientation changed:Dorsal-ventral axis now
• Shell replaced/ lost (except in Nautilus)
• Cilia less important → rely on water being passed through the mantle by muscular contraction
• Modified radula → carnivores

Question 16

Describe the nervous system of the Cephalopods

Answer 16

Nervous system well developed (most developed in inverts!):
Great cephalisation
Have largest brain of all inverts

Question 17

Describe the circulatory system of Cephalopods

Answer 17

Closed circulatory system
Blood vessels that run throughout body
3 heart like structures
Gills

Question 18

Describe the Nautiloids

Answer 18

(Nautilus)
- Found in deep sea
- First appeared in Cambrian (ancient group)
- Simpler eye than other cephalopods
- Lives in final chamber of shell, other chambers aid buoyancy

Question 19

Describe the function of the siphuncle in the Nautilus?

Answer 19

Siphuncle → Regulates osmotic pressure to control buoyancy
- Draws in minerals etc. increases the osmotic pressure, water drawn in replaced by air in chambers = buoyancy
- If going down, salts etc. is excreted so water flows into chambers

Question 20

What organisms are in the subclass Coleoidea?

Answer 20

cuttlefish, squid, octopus

Question 21

How has the shell been modified in Cuttlefish?

Answer 21

internal shell (cuttlebone)

Question 22

How has the shell been modified in Squid?

Answer 22

shell reduced to proteinaceous pen

Question 23

How has the shell been modified in octopus?

Answer 23

shell vestigial/absent

Question 24

What is a coelomate?

Answer 24

An organism with a coelom (fluid filled body cavity) that is fully surrounded by the mesoderm

Diagram of coleom vs Pseydocoelom vs Acoelomate

Question 25

What is the function of a coelom

Answer 25

Proctective cushion for internal organs, and provodes space fpr organs to move and function independently of the outer body wall