Bell Pettigrew Museum
of Natural History
Interpretative Panels
Text: Dr Iain Matthews
Design: Steve Smart & Cavan Convery
A University of St Andrews Development Fund Project
School of Biology
http://biology.st-andrews.ac.uk
7:1 Mollusca
P hy l u m
Body Plan:
• Bilaterally symmetrical
• Triploblastic
• Acoelomate,
with haemocoelic spaces
• Doral protective shell
over visceral mass,
ventral foot
• Chitonous toothed
radulla
Mollusca
S u b p hy l u m
Super class
Class
The molluscs are a large and diverse phylum displaying
arguably the greatest range of structural and life-style
diversity found in the animal kingdom. However
some generalisations on molluscan structure can be
made; they are soft-bodied three-layered animals,
lacking a true body cavity but with a complex throughgut and a basic body shape that is essentially squat
and roughly conical.
The phylum is characterised by the external, dorsal
shell which is secreted by the epidermis of the mantle,
however in some species the shell has been reduced
or lost. The mantle cavity is often invaginated and
houses the ctenidial gills. The majority of species are
herbivorous but diet has extensively radiated and the
mouth houses a toothed chitonous radula that is
used for rasping and boring. The molluscan blood
system is open with extensive haemocoelic blood
sinuses, and the heart is contained in a mesodermal
pericardium.
The 100,000 species that have so far been described
include marine, freshwater, terrestrial and parasitic
forms. Molluscs also display enormous lifestyle diversity
including worm-like shell-less burrowers (e.g. Chaetoderma,
Neomenia), flat or conical rock surface grazers (e.g.
limpets
and chitons ), coiled shelled snails (e.g.
Littorina , Planorbis , and Helix ), secondarily
shell-less slugs (e.g. Doris
and Arion ), bivalved
filter-feeding, benthic shellfish (e.g. Mytilus
and
Margaretifera ), tubular burrowing tusk shells (e.g
Dentalium ), pelagic floating predators with
Gut
Endoderm
Body Cavity
Mesoderm
Ectoderm
Exoskeleton
chambered shells (e.g. Nautilus ), fast swimming
marine predators, with internal shells (e.g. squid
and cuttlefish) and benthic, grasping or pouncing
marine predators, with no shell or a secondary papery
shell (e.g. Octopus
and Argonauta ).
Classification
within
Mollusca
Class:
Chaetodermomorpha
Class: Bivalvia
Order: Caudofoveata
Subclass: Protobranchia
Class: Neomeniomorpha
Superorder: Ctenidobranchia
Order: Aplotegmentaria
Order: Nuculida
Order: Pachytegmentaria
Superorder: Palaeobranchia
Class: Monoplacophora
Order: Solemyida
Order: Tryblidiida
Subclass: Lamellibranchia
Class: Polyplacophora
Superorder: Pteriomorpha
Order: Lepidopleurida
Order: Arcida
Order: Ischnochitonida
Order: Mytilida
Order: Acanthochitonida
Order: Pteriida
Class: Gastropoda
Superorder: Palaeoheterodonta
Subclass: Prosobranchia
Order: Trigoniida
Order: Docoglossida
Order: Unioniida
Order: Pleurotomariida
Superorder: Heterodonta
Order: Anisobranchida
Order: Venerida
Order: Cocculiniformia
Order: Myida
Order: Neritida
Superorder: Anomalodesmata
Order: Architaenioglossa
Order: Pholadomyida
Order: Ecotbrachida
Order: Poromyida
Order: Neotaenioglossa
Order: Heteroglossa
Class: Scaphopoda
Order: Stenoglossa
Order: Dentalida
Subclass: Heterobranchia
Order: Siphonodentalida
Superorder: Pulmonata
Order: Archaeopulmonata Class: Cephalopoda
Order: Basommatophora
Subclass: Nautiloidea
Order: Stlyommatophora
Order: Nautilida
Superorder: Gymnomorpha
Subclass: Coleoidea
Order: Onchidiida
Order: Sepiida
Order: Soleolifera
Order: Teuthida
Order: Rhodopida
Order: Ocotpoda
Superorder: Opisthobranchia
Order: Vampyromorpha
Order: Cephalaspida
Order: Anaspida
Order: Saccoglossa
Order: Nudibranchia
Order: Pleurobranchomorpha
Order: Umbraculomorpha
Superorder: Allogastropoda
See specimen.
Order: Pyramidellomorpha
Molluscs
The molluscs are a hugely diverse group of animals. They
include the two-shelled oysters and mussels , the oneshelled snails
and limpets
and some species that
have either reduced or lost their shells like squid
and
sea slugs .
The golden cowrie , from Fiji and The Solomon Islands is
a symbol of power and rank. Tribal chiefs adorn themselves
with the rare shell, known as buli kula, hung from their necks
on a cord.
One of the molluscs holds the record as the world’s largest
invertebrate. The Giant squid can reach lengths of up to
18 m (59 ft) and can weigh up to 900 kg (1,980 lb, nearly
1 ton).
Some bivalve molluscs can produce pearls. This ability to
coat particles of sand and grit is not restricted to oysters
and pearls can be found in both marine and freshwater
musssls .