MOLLUSCS

1. MOLLUSCS

2. Phylum Mollusca Subphylum Amphineura Class Polyplacophora – chitins Subphylum Cyrtosoma Class Monoplacophora – almost extinct Class Gastropoda – snails Class Cephalopoda – squid, nautilus Subphylum Diasoma Class Rostroconchia – Camb-Perm Class Bivalvia – clams, oysters Class Scaphopods – tusk shells All major groups (ex scaphopods) appear in Cambrian, became important part of Mesozoic/Cenozoic faunas

3. Common features unsegmented body – bilaterally symmetrical head (most) - contains sensory organs radula - with primarily organic teeth foot – crawling, burrowing, or grasping mantle – fleshy covering which secretes shell shell (when present) periostracum/calcareous prismatic layer/nacreous layer gills – in chamber in mantle Phylogeny poorly understood

4. H.A.M. Hypothetical Ancestral Mollusc

5. Diasoma “through body” Cyrtosoma “curved body” rostroconchs

6. Polyplacophora body covered by 8 overlapping plates – poor fossil record on eyes or tentacles, marine Monoplacophora faintly segmentedbody covered by cap-shaped shell Cambrian-Devonian, Recent (1952)

7. Veliger larva Trochophore larva Gastropods prosobranchs – (“forward gills”) marine opisthobranchs – (“rear gills”) reduced shells pulmonates – (“lungs”) air breathers Habitats/niches scape algae with radula scavengers/ detritus feeders predators (drills/harpoons) suspension feeders

8. Torsion • Nerve cord in fig-8 • Left/right gills reversed in cavity over head • Anus above head – fouling problems

9. Water circulation

10. Reasons for torsion • Larval retraction: larvae could retract head into mantle – no real advantage. • Larval settling: aids larva to balance shell over head – evidence contradicts • Veliger swimming: balance body mass for swimming - ? • Adapted adult: good for adult to have everything up front – primitive ones don’t • Adult retraction: allowed retraction into mantle cavity, closed by operculum – doesn’t work for primitive • Helicospiral coiling: better balance asymmetrical shell – doesn’t explain limpets

12. GASTROPODS Hyperstrophic dextral – right-handed, whorls to left, carried diagonal, spire to front Orthostrophic dextral – whorls to right, carried diagonal, spire to back

13. PROSOBRANCHS Archaeogastropods - Primitive forms in which there are two auricles, two kidneys and two gills present.  Nerve system is never concentrated.  Shell is either coiled or secondarily symmetrical as in the limpets.  Largely marine but there are a few that inhabit brackish water, freshwater or even terrestrial habitats.

14. Mesogastropods possess one gill, one auricle and one kidney. An operculum may be present. Mostly marine but a few do inhabit freshwater. This is the largest order of gastropods and contains many common species. Appeared in Late Paleozoic – unipectinate gills, sometimes small siphonal notch (burrowing)

15. Neogastropods possess a concentrated nervous system and usually a shell with a siphonal canal. They are a carnivorous species having a radula containing two or three large teeth in each row.  Some possess a poison gland. Nearly all have an operculum. All are marine inhabitants. Appeared in Jurassic and became dominant group. Possess long siphon

16. Fossil record of Opisthobranchs and Pulmonates not as good. Opisthobranchs appear in Mississippian Pulmonates known from Pennsylvanian fresh water