MOLLUSCS Flashcards
Mollusca lineage
TROCHOZOA
3 familiar classes
Cephalopoda (squid and octopuses)
Bivalvia (bivalve clams and mussels)
Gastropoda (slugs and snails)
5 rarer, more primitive classes
Chaetodermomorpha & Neomeniomorpha: Worm-like
burrowers, shell-less or with dermal spicules.
Scaphopoda: Tusk-shells. Filter feeders with a conical shell.
Monoplacophora (deep sea limpets) & Polyplacophora
(Chitons). Grazers whose bodies show serial repetition.
Molluscan characters
1) calcium carbonate shell that is secreted by the ectoderm
2) radula and through gut
HERBIVOROUS radula: many fine teeth
CARNIVOROUS radula: fewer but longer hooked teeth
pulled over a odontophore
3) primitive forms show serial repetition of gills, excretory nephridia, muscles, nerves but NOT true segmentation
4) ciliated Trocophore larvae
How were shells developed?
Why are layers separated by protein layers?
From small ectodermal plates of CaCO3 secreted by mantle/ ectoderm
- to stop cracks spreading
Shell diversity
- many separate spicules (Class Chaetodermomorpha)
- 2 separate plates (class Bivalvia)
- multiple separate plates (class Polyplacophora)
- a single shell - (class Gastropoda)
- A single, chambered shell - (class Cephalopoda
Why have a shell?
- physical protection
- physiological protection (eg. dessication)
- buoyancy (Nautlius)
- brooding of young (Argonaut)
- egg shells
Why does secondary shell loss or reduction occur?
Seen in gastropods and internalisation in cephalopods.
1) reduce weight in free living forms
2) increase manoeuvrability
3) to allow other forms of defence to evolve:
a) crypsis in Nudibranch sea slugs
b) recycling nematocysts
c) toxins in aposematic Nudibranch
d) mimicry of sea slugs by polyclad flatworms
4) allows translucency for symbiosis to occur (light can reach zooxanthellae)
5) calcium carbonate is rare for terrestrial slugs
Describe the pulmonate lung structure
The gills are lost, and the roof of the mantle cavity is richly
supplied with blood vessels.
The opening to the mantle cavity becomes the pneumostome, which can be opened and closed.
The lung can be ventillated by muscle action.
Key features of Gastropods and their land take over
1) pulmonate lung
2) sealing the shell using mucus or operculum
3) excreting uric acid not ammonia
4) loss of shell if CaCO3 is rare
5) aestivation
6) suppression of ciliated larval stages
2 possible cephalopod life histories
1) live long and slowly (Nautilus)
2) live fast die young (squids)
Key cephalopod characteristics
1) internalised, gas filled shell controlled by siphuncle
2) beak like jaws
3) jet propulsion through mantle cavity and muscular chambers
4) highly advanced sense organs
5) foot is modified into a set of many tentacles
How do cephalopods use colour
- courtship and rivalry
- warning colouration
Describe the 3 cephalopod lineages
1) Nautiloids
- containing nautilus which elaborately controls buoyancy using ion exchange, siphuncle and osmosis
- more septa, more rapid buoyancy control
2) Ammonites (EXTINCT)
- Ammonites have far more complex
shells, with
• More septa
• More intricate suture forms
Both differences are thought to have allowed better , more rapid buoyancy
control in ammonites, and greater resistance to crushing.
4) Coeloids
- are the squids, cuttlefish and the octopi
- Extremely abundant predators in many marine habitats
Why are squids dominant ? Similar to key cephalopod characteristics.
1) loss of external shell
2) improved senses (eg. lensed eye rather than pinhole eye)
3) larger brain
4) live fast die young strategy
5) improved physiology (partly closed circulation, separate systemic + respiratory circulation)
