Phylum Mollusca Part1 Reviewer

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...