1: Molluscs Flashcards
Are molluscs coelomates?
Yes, they have: a body cavity. gut inside, surrounded by endoderm, outside = ectoderm. 2 layers of mesoderm
What sort of cleavage do molluscs undergo?
Spiral
Describe the body plan of GENERAL mollusc
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
Describe the circulartory system of a GENERAL mollusc
Haemocoel (blood system and coelom)
Blood oxygenated in gills
Heart-like structure
How does the body plan of the Gastropod differ from the generalised mollusc
Development of head
Dorso-ventral elongation of body
Shell (shield → protective retreat)
Torsion
What is torsion in Gastropods?
rotation of visceral mass & mantle cavity by 180O
What are the advantages of torsion in Gastropods?
- 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
What are the disadvantages of torsion in Gastropods?
Nervous system twisted
Anus over head
Evolution of gastropods → modifications of mantle cavity to solve sanitation & water flow problem (caused by torsion)
What groups make up the gastropods?
snails, slugs, conchs, periwinkles and sea slugs
Some gastropods undergo de-torsion. Give an example and describe them
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
Describe the coiling in gastropods
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
Describe the Pulmonates
→ most specialised of the gastropods
Land snails and slugs
No longer have gills → modified
Vascularised mantle cavity → functions like a lung
Describe sequential hermaphroditism
→ 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)
What groups make up the Bivalves?
clams, mussels, oysters, scallops
Describe the body plan of the bivalves
- 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
How has the body plan of the Cephalopods changed from the generalised mollusc
- 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
Describe the nervous system of the Cephalopods
Nervous system well developed (most developed in inverts!):
Great cephalisation
Have largest brain of all inverts
Describe the circulatory system of Cephalopods
Closed circulatory system
Blood vessels that run throughout body
3 heart like structures
Gills
Describe the Nautiloids
(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
Describe the function of the siphuncle in the Nautilus?
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
What organisms are in the subclass Coleoidea?
cuttlefish, squid, octopus
How has the shell been modified in Cuttlefish?
internal shell (cuttlebone)
How has the shell been modified in Squid?
shell reduced to proteinaceous pen
How has the shell been modified in octopus?
shell vestigial/absent
What is a coelomate?
An organism with a coelom (fluid filled body cavity) that is fully surrounded by the mesoderm
What is the function of a coelom
Proctective cushion for internal organs, and provodes space fpr organs to move and function independently of the outer body wall
