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Phylum Mollusca
• Second largest animal phylum (> 100,000 species)
• Oysters, snails, clams, octopus, squid
• Two unifying characteristics– External calcium carbonate shell– Muscular foot for locomotion
Generalized Molluscan Shell
• Outer periostracum (organic layer – conchiolin
• Inner layers largely calcium carbonate with some organic material in a matrix
Class Monoplacophora
• Originally known from Cambrian to Devonian fossils
• Living representatives found 1952• Since, 20 more species found (1800 – 7000 m deep)• Single bilaterally symmetrical shell• Probably share common ancestor with other
Mollusca except Polyplacophora• Members probably actually gave rise to the other
molluscan groups (bivalves, gastropods, cephalopods)
• Small – 3 mm to 3 cm long
Class Polyplacophora
• Chitons
• Retain many features of generalized mollusc
• Some adaptation for predominant lifestyle (intertidal zone)
• Shell structuring (layering) different from other molluscs (Conchifera)
Class Gastropoda
• Most diverse group (~60,000 species)
• >15,000 described fossil species
• Most extensive adaptive radiation of any mollusc group
Class Gastropoda
• Three “groups” – phylogeny revision• Prosobranchs – most common members
when think of snails– Terrestrial, freshwater, and marine*– Common feature – operculum
• Opisthobranchs– Sea slugs, sea hares– Many members lost shell
• Pulmonates– Many terrestrial species, also freshwater, a few
marine
Major Changes from Generalized Mollusc
• Development of head
• Dorsoventral elongation
• Shell – from shield to retreat
• Torsion
• Conispiral coiling and asymmetry
Torsion
• Weight of shell over head, mantle cavity posterior
• Torsion – 180o counterclockwise rotation of visceral mass, shell, mantle, mantle cavity
• Occurs in larvae not adult
• First gastropods
• Detorsion
Costs of Conispiral Shell
• Loss of a gill, nephridium, atrium
• Mantle cavity (anus and nephridiopore) now anterior and near mouth
• Compensation - changes in water flow or shell structure
• See Figure 12-20 (mantle cavity evolution) and 12-21A (abalone)
Shell
• Apex, whorl, columnella, aperature, siphonal canal
• Spire, body whorl, outer lip, inner lip
• operculum
Locomotion
• Most move using foot
• Most have ciliated sole and secretory glands (mucus producing)
• Hard-bottom dwelling and terrestrial, and large soft-bottom snails - undulating wave of muscle contractions (Figure 12-30)
Feeding
• Most often thought of as algal scrapers (radula)
• Deposit feeders
• Suspension feeders
• Scavengers
• Predators
• Parasites
Class Bivalvia
• Oysters, clams, mussels ~8,000 species (1,300 freshwater, rest marine)
• Benthic filter-feeders (a few exceptions)– No radula– Enlarged gills
• Compressed body• Shell
– Two valves– Hinged dorsally– Completely encloses body
Class Cephalopoda
• ~700 living species, 10,000 fossil species
• Highly specialized
• Pelagic (octopus secondary)
• Shell – coiled, internalized, reduced, or lost
• Closed circulatory system
• Visual eye