1. Invertebrates 4
Phylum Mollusca

2. Phylum Mollusca: The “soft-bodied” animals

3. Evolutionary relationships
and HOX genes

4. Molluscan body plan Hypothetical ancestral mollusc (HAM)
Shell
Mantle
Mantle cavity
Ctenidia
Visceral mass
Head and assocated sensory structures
Radula
Foot and epipodial tentacles

5. Molluscan body plan (HAM)
Digestive system
complete
note position of anus
Circulation/Respiration
ctenidium
open circulatory system
coelom=pericardial cavity.
True coelom, but reduced.
Water/ion balance and ammonia excretion
Has paired nephridia: kidney-like in function

6. Molluscan body plan (HAM)
Nervous system
Brain
Paired ventral nerve cords.
Ganglia at key locations
Several sensory structures (see description of HAM)

7. Major Molluscan Classes
LEARN!: How are the features of HAM modified for each of the major classes, and how are these modifications adaptive for each particular mollusc? ?

8. Class Polyplacophora: “many plates”
The chitons
Differences from HAM
8 shell plates
Mantle cavity expanded around foot
Multiple ctenidia
Mantle grows over shell, thickened (new functions?)
Reduced head without sensory structures

9. Phylum Mollusca Class Polyplacophora
Cryptochiton stelleri

10. Class Gastropoda: “stomach-foot”
Representative members: Snails, limpets, abalone, nudibranchs, sea and land slugs
Differences from HAM
Undergo torsion (sketch)
What is torsion?
Advantages of torsion
Disadvantages of torsion
Be sure you understand how torsion makes Gastropods different from HAM!

11. Phylum Mollusca Class Gastropoda
Limpet: Lottia gigantea

12. Phylum Mollusca Class Gastropoda
Dogwinkle: Nucella sp.

13. Phylum Mollusca Class Gastropoda
Opisthobranchs: “rear gill”
Discodoris (Dialula sandiegensis)

14. Phylum Mollusca Class Gastropoda
Opisthobranchs: “rear gill”
Hermissenda (Phidiana) crassicornis

15. Class Bivalvia: “two valves”
Representative members: Clams, mussels and scallops
Differences from HAM
Two shells
Mantle/sensory structures
Expanded mantle cavity
Enlarged ctenidia
Respiration AND feeding
Reduced head
Foot more elongate;
Minimal area for suction

16. Class Cephalopoda: “Head-footed”
Representative members: Nautilus, squid, octopus
Differences from HAM
Shell reduced (pen in squid)
Foot gives rise to arms and funnel
Head
Modified cephalic tentacles
Well-developed eyes
Beak
Mantle
Elongate
Thickened and protective body covering
Functions in movement
Chromatophores
Mantle cavity also elongate, functions in movement
Paired ctenidia as in HAM
Circulatory system closed!
Nervous system highly developed