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& other Molluscs |
From Eldredge (1991)
Molluscs are greatly varied in both
morphology and life habits. They include familiar living
forms such as clams, snails, the octopus, and squids.
Several groups have great economic importance such as
oysters and the blue mussels, and some groups such as garden
slugs and the zebra mussel can be bothersome pests. In this
laboratory you will become familiar with several of the
molluscan groups, of which the cephalopods and gastropods
are geologically the most important. Most molluscs are mobile marine
creatures, although some have invaded terrestrial
environments. Molluscs have an external shell (single or
bivalved) enclosing the mantle and visceral mass, a muscular
foot which aids in attachment and/or locomotion, and a
radula. The radula is not known from the bivalves (a group
covered in the next lab), but is presumed to have been
primitive in the bivalve ancestor. CLASSIFICATION
& GEOLOGIC RANGES Phylum
Mollusca (Precambrian-Recent) Class
Monoplacophora (Cambrian-Recent) Class
Amphineura (U. Cambrian-Recent) Class
Schaphopoda (Ordovician-Recent) Class
Gastropoda (Cambrian-Recent) Subclass
Prosobranchia (Cambrian-Recent) Order
Archaeogastropoda
(Cambrian-Recent) Order
Mesogastropoda
(Ordovician-Recent) Order
Neogastropoda (Cretaceous-Recent) Subclass
Opisthobranchia
(Carboniferous-Recent) Subclass
Pulmonata (Carboniferous-Recent) Class
Cephalopoda (Cambrian-Recent) Subclass
Coleoidea (Devonian-Recent) Subclass
Nautiloidea (Cambrian-Recent) Subclass
Ammonoidea (Devonian-Cretaceous)
Class
MONOPLACOPHORA The organ system of
monoplacophorans are arranged in a series and are segmented
(or pseudosegmented) leaving paired muscle scars on the
shell interior. Some investigators suggest that a Paleozoic
and early Mesozoic group called the Some fossils typically
classed as bellerophons (here treated as gastropods), may
have an untorted body plan. This has been deduced from the
arrangement of muscle scars on the interior of the univalve
bellerophons. Most other bellerophons, however, do exhibit
torsion and should rightly be classified as true gastropods.
Apart from the bellerophons (no muscle scars can be observed
in our specimens) we have no monoplacophoran
specimens.
Class
AMPHINEURA Amphineura (Polyplacophora of some
texts and commonly referred to as the Chitons) are
characterized by an elongated body with a head and bilateral
symmetry. Polyplacophorans possess a radula. The soft bodies
of polyplacophorans are surrounded by a muscular mantle
girdle (cuticle) which has aragonitic spicules. Above the
mantle cuticle, polyplacophorans have a dorsal shell made up
of eight articulated plates or valves. Some recent examples
of polyplacophorans are provided here. Class
SCHAPHOPODA Scaphopods (commonly called tusk
shells for obvious reasons) are a relatively minor class of
marine molluscs. They can actively trap food particles
within the sediment by use of their specialized tentacles.
Scaphopods are characterized by a small univalved shell that
is open at both ends. The larger anterior end is permanently
embedded in the sediment. The smaller posterior end is opens
near the sediment-water interface. See the examples of
Recent and fossil Dentalium.
