Phylum Mollusca Aplacophora Class Polyplacophora Class Monoplacophora Class

Phylum. Mollusca Aplacophora √ Class. Polyplacophora √ Class. Monoplacophora Class. Gastropoda Class. Cephalopoda Class. Bivalvia Class. Scaphopoda Class.

po alo ph Ce Ga str op od a da ra op ho lac on op Polyplacophora M Aplacophora Bivalvia Scaphopoda

Monoplacophora Mouth Nerve cord Nephridium Ctenidium Gonads Heart atria Pedal retractor muscle Anus

Monoplacophora • • Extant spp discovered in 1952 Only 20 spp, all marine, deep water zones (1800 -7000 m). Poorly studied Likely ancestor of gastropods, cephalopods and bivalves, and the bivalvia and scaphopods • Monoplacophorans and Polyplacophorans evolved shells independently from a shell-less ancestor. Evidence: shells differ in internal layer structure • Superficially similar to gastropod limpets

Gastropoda

Gastropoda • Most diverse taxon of mollusca • Estimates range from 40, 000 -100, 000 spp (probably 60, 000 extant, 15, 000 extinct spp) • Three major groups: – Prosobranchs – benthic marine spp – Opisthobranchs – secondary loss of the shell – Pulmonates – air breathers

Torsion is unique to gastropods Most gastropods are dextral Pretorsion Post torsion

Prosobranch Opisthobranch Pulmonata

Prosobranchs • • Mantle cavity anterior, due to torsion Most common, typical “snail” Mostly marine, some freshwater, terrestrial Most primitive group of gastropods

Opisthobranchs • Mantle cavity lateral or posterior, due to detorsion or loss of shell • ca 2000 spp. e. g. nudibranchs (sea hares, sea slugs) • Ctendia often lost. Gas exchange via cerata

Pulmonata • Highly vascularized mantle for gas exchange (lung) • 17, 000 spp: slugs, pond snails

Gastropoda • More active than mono and polyplacophorans – Highly cephalized: tentacles, eyes • Gonochoristic (dioecious) • Veliger larva (an advanced version of the trochophore larva)

Veliger larva Metanephridium Velum Shell Stomach Digestive cecum Foot Esophagus

Cephalopoda

Cephalopoda • • Swift, agile carnivores Closed circulatory system, 2 hearts Separate sexes Foot modified to form arms, tentacles, siphon Brain, cranium, complex image-forming eye 700 extant spp, 10, 000 extinct spp Arose from limpet-like monoplacophorans Ergo, ventral became functional anterior, etc

Cephalopoda Posterior surface Right Ventral Dorsal Left

Cephalopod eye Iris Retina Optic nerves Lens Cornea

eye Optic lobe Buccal ganglia Cerebral ganglion statocyst esophagus Brachial nerves Brain is surrounded by a cranium

Cephalopoda • Ectocochleate cephalopods – Have external shell with internally subdivisions used for buoyancy control – This ancestral group is almost completely extinct – E. g. Nautilus

Cephalopoda • Endocochleate cephalopds – Reduced internal shell, or shell absent – Squids, cuttlefish, octopi

Tentacle Arm Funnel (siphon) Collar Eye Fin

Shell (Pen) Systemic heart Branchial heart Ctenidium Funnel Hectocotylus (sperm-bearing arm in males) Reproduction: trochophore and veliger are bypassed and hatch into planktonic juveniles

Nautilus is the only cephalopod with an external shell and lacking chromatophores Chromatophores (color cells) Iridocytes (reflective cells) - Millions of these allow rapid changes in color, polarized signals - Also have photophores for bioluminescence Cephalopods except Nautilus have ink sac

Bivalvia

Bivalvia (Pelecypoda) • • 8000 extant spp (1300 fw, 6700 marine) Specialized for infaunal habitat Sessile, little cephalization Filter feeders, using gills – 3 major groups of bivalves based on gill shape – Protobranchs (deposit feeders, most primitive) – Lammelibranchs (suspension feeders, most common) – Septibranchs (carnivores, most derived)

Protobranchs • Gills for gas exchange only • Tend to live in deeper waters (>1000 m)

Lamellibranchs • Gills: gas exchange + filter feeding • Incurrent siphon, excurrent siphon Cut-away of gill structure Hinge Blood vessel mouth Ctenidium Excurrent siphon Foot Incurrent siphon

Locomotion

Glochidia glochidium Glochidia on gills Freshwater mussels

Septibranch • Ctenidia lack filaments • Feed on polychaetes, crustaceans • Weird side group

Scaphopoda • Shared (extinct) common ancestor with bivalves • 300 -400 spp • Lack ctenidia, heart • Burrowers • Have 100 -200 captacula (tentacles) with which to catch food