|
Phylum MOLLUSCA
Make great contribution to America's food supply - 130 million lbs./yr
Cephalopods have the capability to learn
Nudibranchia (order) have lost their shells
Size range 1.0 mm to 50 ft. (15.24 m)
weight to 2 tons
giant clams 5 feet long, 500 pounds
Unsegmented (except Neopilina)
Bilateral symmetry - except gastropods
Body - Head - with tentacles and eyes, lost in pelecypods
foot - ventral and muscular
visceral mass - coiled in gastropods, internal organs concentrated in this mass
mantle - (pallium) a soft skin or sheet of tissue overgrowing visceral mass. Mantle also
secretes calcareous shell with organic matrix.
Mantle cavity - space at posterior end of visceral mass, contains paired gills (respiratory chamber),
anus, excretory and reproductive systems open into mantle cavity. Exhalent currents
remove waste products.
Gills - cilia on gills create water movement. In class Pelecypoda, gills also used as feeding structure.
Digestive tract - Mouth, jaws, radula, stomach. Pelecypods lack jaws and radula.
Organs - 3 chambered heart (ventricle and two auricles), capillaries in cephalopods
gills - one pair, except in class Amphineura (up to 80 pairs)
kidneys or paricardinal glands
sexes separate, hermaphroditism widespread, fertilization external.
Shell - calcareous (aragonite and calcite)
prismatic - twinning of calcite or aragonite
foliated - calcite - resembles x-bedded Ss, inner shell layers
nacreous - aragonite separated by thin leaves or organic material cross-laminar
homogeneous - no structure under plane light. Under X nicols, small xls seen.
Fossil Molluscs
Monoplacophora - molluscs with cap, spoon shaped or arched single shell, (mammilated
shell) (L.C. - Holocene). Some groups bilaterally symmetrical
Polyplacophora - Amphineura - class chitins (U. Cambrian - Holocene)
shore-living molluscs, radula
8 calcareous articulating plates
Scaphopoda - benthonic ("scaphe" = bowl, "podas" = foot)
neritic and bathyal realms
Modern forms appear in early K
Gastropoda - ("gastro = stomach) (L. Cambrian - Holocene)
head - distinct, fused or partially fused to foot, eyes and
tentacles unspecialized
foot - solelike, adapted for creeping, modified in pelagic forms
Radula - normally present
Nervous system - cerebral and pleural ganglia
Torsion -
Symmetry - varying degrees of bilateral asymmetry
Shell - single, univalve, calcareous (with aragonite), closed apically, lacks chambers
Classification of Gastropods
Prosobranchia
Opisthobranchia
Pulmonata
Subclass PROSOBRANCHIA: torsion, varishaped shells, conispiral,
planispiral, operculum. High-spired shells rare
visceral loop twisted into figure 8
mantle cavity opens to front
freshwater, terrestrial, marine
(L. Cambrian - Holocene)
Order ARCHAEOGASTROPODA
lacks siphon
male without prostate and penis
heart with two auricles
inner layer of shells often nacreous
(L. Cambrian - Holocene)
Limpets
Abalone
Bellerophons
Nerites
Order MESOGASTROPODA
varishaped, conispiral
siphonal notch
marine, freshwater, terrestrial
(Ord. - Holocene), not common until the Mesozoic/Cenozoic
Helmet shells
Sundails
Periwinkles
Cowries
Conchs
Wentletraps
Ceriths
Turritellas
Moon snails
Ecphora
Order NEOGASTROPODA
Conispiral, with siphonal notch or canal
not as diverse as Mesogastropods
(Cretaceous - Holocene)
Muricids (Murex)
Whelks
Volutes
Olives
Cones
Subclass OPISTHOBRANCHIA
gastropods display detorsion - straightening out of the nervous
system
shell reduced or absent
operculum commonly absent
exclusively marine
(Dev.? Miss. - Holocene)
Where shell present
aciculate - slender, tapered to a sharp point, low spired
miolute - last whorl envelopes earlier ones
convolute - last whorl completely embraces and conceals
earlier ones
heterostrophic - Apical whorls of embryonic shell (protoconch) coiled in
opposite direction to rest of shell
poorly represented as fossils, common today
Order PLEUROCOELA - (Tectibranchia)
shell dextral (right-handed)
involute or convolute
shell and mantle cavity become obsolete
(Miss. - Holocene)
Order Uncertain
Superfamily Pyramidallacea: living opisthobranchs
(Dev.? Miss. - Holocene)
Order PTEROPODA
Pelagic molluscs: naked or covered with thin, generally transparent varishaped
shell, often operculate
(Mesozoic - Holocene)
Subclass PULMONATA: Origin of pulmonates through family Ellobidae
Mantle cavity acts as lung
Nervous system in adults becomes symmetrical following torsion
well developed shell
radula with teeth reduced
Terrestrial and fresh water
hermophroditic
(Mesozoic to Holocene)
Order BASOMMATOPHORA
pulmonates with shell (spiral, cap, bowl-shape)
single pair noninvaginable tentacles with eyes at base
intertidal marine, coastal and inland terrestrial, freshwater
Order STYLOMMATOPHORA
pulmonates with helical shell
shell may be reduced to calcareous granules
two pairs of invaginable tentacles with eyes on tips of posterior pair - terrestrial
|
|
