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CHAPTER 12: MOLLUSCA
Mollusca is an enormous taxon 2nd only to Arthropoda in number of living species Includes many familiar animals such as snails, clams, oysters, mussels, slugs, octopods and squids in seven lining classes Mollusca includes the taxa:
Aplacophora consists of small, wormlike molluscs w/ numerous calcareous spicules rather than a shell. Polyplacophorans or chitons have eight shell plates
Monoplacophorans have a low, conical, limpetlike shell and live in deep water. (The limpet shape appears often in molluscan evolution and includes an uncoiled, caplike, low-profile shell and a large creeping foot) Gastropods are the snails and slugs and have a one-piece, often coiled shell and are found in the sea, in fresh water, and on land Cephalopods include the squids and octopods and mostly have reduced, absent, or internalized shells. The bivalves are the clams and their relatives w/ a shell divided into 2 pieces Scaphopods are the tusk molluscs w/ a one-piece tubular shell. GENERALIZED MOLLUSC
A marine benthic animal, bilaterally symmetric, dorsoventrally depressed, and ovoid in outline The body is divided into:
Head –anterior, small, poorly defined
Visceral Mass –large dorsal
Foot –broad, flat, ventral
Shell –dorsal shieldlike; can be clamped against the substratum to protect the soft parts from predators. This animal is adapted for life on hard, rocky substratum, where it uses its rasplike radula to graze on the biofilm of microscopic algae and other small sessile organisms Attaches to the substratum by the muscular foot, w/c it uses to move slowly about in search for foot. MANTLE
Dorsally, the body wall over the visceral mass is elaborated to form the mantle or pallium w/c is characteristics of all molluscs Mantle epidermis secretes protein, calcium salts, and mucus and is also sensory. SHELL
Simple, low, conical cap covering the dorsum of the generalized mollusc Originated in the ancestral mollusk as a thick dorsal cuticle of chitin and protein, to w/c calcium salts were later added. The eumolluscan shell consists of 3 basic layers: one organic and 2 calcareous. Periostracum outermost layer, composed of the protein conchiolin. Ostracum calcareous & immediately under the periostracum. Hypostracum inside the ostracum
The ostracum & hypostracum are composed of calcium carbonate deposited over an organic matrix In the gastropods, cephalopods and bivalves the hypostracum may be nacreous (pearly) MANTLE CAVITY
Mantle skirt –dorsal peripheral outfold of the mantle
Mantle cavity –a pocket that is partially enclosed by the mantle skirt; continuous w/ the surrounding seawater. - Central & characteristic feature of molluscan body and one on w/c many important processes depend. - Contains the gills, nephridiopores, anus, gonopores & sensory osphradia - Involved in feeding and when anterior may provide space for the retracted head and foot. The outer epidermis of the mantle skirt underlies the shell - Secretes the calcium carbonate required for increasing shell thickness Inner epidermis lines the mantle cavity, secretes mucus and is often ciliated. GILLS
The generalized mollusks has several pairs of gills (ctenidia; ctene = comb), one pair of w/c is located in the posterior mantle cavity. Consists of a central axis, w/c is attached to the mantle along one edge. - Contains muscles, blood vessels, and nerves
Gill filaments –arise from opposite sides of the axis; two rows & leaflike. - Each filament has an upstream frontal margin on one side and a downstream abfrontal margin on the opposite side. Bipectinate gills –two rows of filaments
Monopectinate gills –filaments on only one side of the axis Interfilamentary water spaces –separates the filaments. The gills divide the mantle cavity into ventral inhalant (infrabranchial) and dorsal exhalant (suprabranchial) chambers The exposed gill surfaces are formed by the combined edges of the...
Mollusca is an enormous taxon 2nd only to Arthropoda in number of living species Includes many familiar animals such as snails, clams, oysters, mussels, slugs, octopods and squids in seven lining classes Mollusca includes the taxa:
Aplacophora consists of small, wormlike molluscs w/ numerous calcareous spicules rather than a shell. Polyplacophorans or chitons have eight shell plates
Monoplacophorans have a low, conical, limpetlike shell and live in deep water. (The limpet shape appears often in molluscan evolution and includes an uncoiled, caplike, low-profile shell and a large creeping foot) Gastropods are the snails and slugs and have a one-piece, often coiled shell and are found in the sea, in fresh water, and on land Cephalopods include the squids and octopods and mostly have reduced, absent, or internalized shells. The bivalves are the clams and their relatives w/ a shell divided into 2 pieces Scaphopods are the tusk molluscs w/ a one-piece tubular shell. GENERALIZED MOLLUSC
A marine benthic animal, bilaterally symmetric, dorsoventrally depressed, and ovoid in outline The body is divided into:
Head –anterior, small, poorly defined
Visceral Mass –large dorsal
Foot –broad, flat, ventral
Shell –dorsal shieldlike; can be clamped against the substratum to protect the soft parts from predators. This animal is adapted for life on hard, rocky substratum, where it uses its rasplike radula to graze on the biofilm of microscopic algae and other small sessile organisms Attaches to the substratum by the muscular foot, w/c it uses to move slowly about in search for foot. MANTLE
Dorsally, the body wall over the visceral mass is elaborated to form the mantle or pallium w/c is characteristics of all molluscs Mantle epidermis secretes protein, calcium salts, and mucus and is also sensory. SHELL
Simple, low, conical cap covering the dorsum of the generalized mollusc Originated in the ancestral mollusk as a thick dorsal cuticle of chitin and protein, to w/c calcium salts were later added. The eumolluscan shell consists of 3 basic layers: one organic and 2 calcareous. Periostracum outermost layer, composed of the protein conchiolin. Ostracum calcareous & immediately under the periostracum. Hypostracum inside the ostracum
The ostracum & hypostracum are composed of calcium carbonate deposited over an organic matrix In the gastropods, cephalopods and bivalves the hypostracum may be nacreous (pearly) MANTLE CAVITY
Mantle skirt –dorsal peripheral outfold of the mantle
Mantle cavity –a pocket that is partially enclosed by the mantle skirt; continuous w/ the surrounding seawater. - Central & characteristic feature of molluscan body and one on w/c many important processes depend. - Contains the gills, nephridiopores, anus, gonopores & sensory osphradia - Involved in feeding and when anterior may provide space for the retracted head and foot. The outer epidermis of the mantle skirt underlies the shell - Secretes the calcium carbonate required for increasing shell thickness Inner epidermis lines the mantle cavity, secretes mucus and is often ciliated. GILLS
The generalized mollusks has several pairs of gills (ctenidia; ctene = comb), one pair of w/c is located in the posterior mantle cavity. Consists of a central axis, w/c is attached to the mantle along one edge. - Contains muscles, blood vessels, and nerves
Gill filaments –arise from opposite sides of the axis; two rows & leaflike. - Each filament has an upstream frontal margin on one side and a downstream abfrontal margin on the opposite side. Bipectinate gills –two rows of filaments
Monopectinate gills –filaments on only one side of the axis Interfilamentary water spaces –separates the filaments. The gills divide the mantle cavity into ventral inhalant (infrabranchial) and dorsal exhalant (suprabranchial) chambers The exposed gill surfaces are formed by the combined edges of the...
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