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:
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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
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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
eye
stomach
prototroch
ciliated band
mouth
intestine
protonephridium
anus
Trochophore larvae
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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 ctenophoratrochophore connection
• Flatworms don’t fit
– Degenerate annelids?
7 mollusca classes
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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 microorganisms 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
mouth
Gills 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:
Dorsal
Ventral
Dissection: bivalve
umbo
anterior
Remove valve