Phylum Mollusca

• > 100,000 extant species

• At least 45,000 extinct species

• Nice fossil history based on shells– Fossils from Pre-Cambrian

• Importance?– Shells - collectors, jewelry– food

Mollusca characteristics:

• 1. Foot

• 2. Mantle

• 3. Secretes shell

Shell: 3 layers

Periostracum: horny protein, conchiolin in some

Prismatic layer: calcite crystals w/membranes

between

Nacreous layer: CaCo3

Mollusca characteristics:

• 1. Foot

• 2. Mantle

• 3. Secretes shell

• 4. External surfaces - ciliated epidermis w/ mucous glands– Food capture, feeding, locomotion,

cleaning body surfaces

Cilia move mucous and create water flow

• Gas exchange + bring food in

• Sorting surfaces separate food particles by size

Cilia over gill surface

• Water movement opposite of blood flow

5. Coelom is reduced

• Only pericardial cavity

6. Open circulatory system

• Blood sinuses (no capillaries)

• Heart = one or two auricles – collecting chambers

• one ventricle – pumping chamber

More circ. system

• Hemocyanin pigment in blood (copper)– Blood w/ O2 = blue

– Blood w/o O2 = colorless

• Pulmonate gastropods have hemoglobin

• Cephalopods have closed circulatory system

7. Digestive system

• Sclerotized buccal cavity

• Tubular esophagus

• Cone-shaped stomach

• Long, coiled intestine

Radula

• Chitin-toothed

• Rasping organ for scraping algae

Stomach • Contains style sac, rotates contents

– Pulls strands of mucous from esophagus– Mucous viscosity decreases w/ low pH– Stomach wall is chitinized

• Crystalline rod = hyaline mucoprotein

• Style has hydrolase digestive enzymes

Stomach, cont.

• Sort food particles by size

• Intracellular digestion in digestive gland walls

• Some extracellular dig. in stomach

• Carnivores have no style

Sorting in stomach

Intestine

• Fecal compaction

• Anus opens into mantle cavity

8. Nitrogenous waste

• Pair of coelomoducts– Open to pericardial cavity

• Discharge into mantle cavity via nephridiopores– Probably not homologous to annelid

metanephridia (annelid origin = mesoderm; mollusk origin = ectoderm)

Waste product?

• Ammonia in aquatic molluscs

• Uric acid in terrestrial molluscs

9. Nervous system - varied• Polyplacophora (chitons) - decentralized, no

ganglia

• Cephalopods - as developed as in vert’s

• Primitive gastropods:– Nerve ring around esophagus, 2 pair of major nerve

cords

Reproduction and development

– Pair of gonads in coelom– Eggs + sperm into pericardial cavity,

outside via coelomoducts– Fert external in sea water– Molluscs mostly dioecious, some

gastropods hermaphroditic

Most gastropods, all cephalopods:

• Sperm transferred to female’s mantle cavity

• Internal fertilization

• Hermaphroditic gastropods do reciprocal cross-fertilization

Development

• Trochophore larvae = free-swimming

stomach

prototrochciliated band

mouth

protonephridiumanus

intestine

eye

Trochophore larvae

• Archaeogastropoda

• Polyplacophora

• Aplacophora

• Most marine bivalves

• Develops into veliger larvae– Foot, shell, other structures appear

Phylogenetic significance of trochophore larvae

• Hatschik (1878)

• Present in molluscs, annelids, other phyla

• Promotes ctenophora - trochophore theory of bilateral animals from radial ancestors– body shape, apical sense organs,

statocysts, nervous systems

Problem with ctenophora-trochophore connection

• Flatworms don’t fit– Degenerate annelids?

7 mollusca classes

• Polyplacophora

• Aplacophora

• Monoplacophora

• Gastropoda

• Scaphopoda

• Bivalvia

• Cephalopoda

Class Polyplacophora

• Chitons and oval-flattened beasts - mostly in rocky intertidal zones

• All marine, ~ 800 spp.

• Mostly 2 - 12 cm

• Largest (30 cm)is Cryptochiton stelleri from N. Pacific coast of N. America

= Pacific gumshoe chiton

Chiton characteristics:

• Most feed on algae and micro-organisms on rock surfaces

• Few are predators on small inverts

• 1. Rudimentary head– No tentacles or eyes

Characters

• 2. Mantle covers dorsal surface– Secretes 8-piece shell

• 3. Broad, ventral foot

• 4. Many paired gills in mantle cavity

• 5. Anterior mouth with radula

Repro:

• 6. Dioecious– trochophore larvae, no veliger– external fert. in sea water

mouthGills in mantle cavity

mantle

foot

Classification of Polyplacophora - 2 orders

• Order Lepidopleurida: few genera, Hanleya NE coast

• Order Chitonida - most chitons– Chaetopleura (New England - Fl)– Chiton (gulf coast)– Katherina (N. Pacific coast)– Cryptochiton (N. Pacific coast)– Mopalia– Ishnochiton

Class Aplacophora

• Solenogasters are worm-like molluscs 0.5 - 30 cm long– Largest is Epimenia verrucova; 30 cm

• All marine• Mostly deep waters, 20 - 9000 m• Some crawl and feed on hydroids and

corals • Poorly known, seldom seen, ~ 250 spp.

Characteristics:

• 1. Worm-like body shape

• 2. No shell, mantle, or foot

• 3. Cuticle w/layers of imbedded calcareous spicules

• 4. Ventral surface has longitudinal pedal groove

• 5. Hermaphroditic

• 6. Radula well-developed

Pedal groove cloaca

Class Monoplacophora

• Originally known only from fossils

• Living Neopalina from 3600 m in Pacific Ocean coast of Costa Rica (1952)

• Two genera – Neopalina (7 spp.) and Vema

Characteristics:

• 1. Dorsal surface covered by flat conical shell.

• 2. Ventral surface with mantle, paired gills and foot.

• 3. Multiple paired gills, coelomoducts, heart chambers, gonads, and retractor muscles.

Neopalina:

Ventral

Dorsal

Dissection: bivalve

anterior

umbo

Remove valve