368 FOSSIL INVERTEBRATES/Molluscs Overview feeding taxa, notably in some examples of the nuculoid bivalves and trochid gastropods In all of the living classes, except the Bivalvia, feeding is assisted by a tongue like apparatus (the radula) just within the mouth, which continuously produces rows of chitinous teeth It is generally accepted that organisms ancestral to the bivalves did have a radula Cephalopods had developed a horny beak (superficially parrot-like) which may also be preserved as a fossil, while certain opisthobrach gastropods developed chitinous gizzard plates which have also been preserved in Cenozoic sediments The soft tissues of the body of Mollusca are attached to the shell by muscles which in most Bivalvia, Amphineura, Monoplacophora, and Cephalopoda are arranged in a bilaterally symmetrical pattern In the Amphineura and the Monoplacophora this musculature is arranged in eight serially arranged blocks (metameres), giving the body a repetitive structural pattern, known as pseudometamery In the Amphineura the musculature of the anterior two metameres is attached to the head, while the poserior six metameres are attached to the more posterior part of the body (foot) In the tryblidioidean Monoplacophora, the head makes up the three anterior metameres This pseudometamery differs from true metamery of the Annellida in not having a discrete coelom associated with each metamere The attachment of the muscles to the shell is by way of a particular shell microstructure, low aragonitic prisms forming the myostracum which is virtually universal in the Conchifera but not obvious in the Amphineura Reproduction in the Mollusca occurs primitively by fertilization of gametes released into the sea The fertilised ovum develops into a trochophore larva similar to those of the annellids After a short period of time, sometimes two days, the larva settles and metamorphoses into a benthonic adult which develops a shell The larvae of many groups may extend their life by the development of a shell (secreted by a shell gland) invariably consisting of aragonite and organic material In the larvae many bivalves and gastropods, paired lateral processes (the velar lobes) help the growing animal to support itself in seawater and provide a certain amount of locomotion Where this stage occurs it is known as the veliger larva Any of these stages may be modified, often within the protection of an egg capsule, which is clearly of importance in the invasion of the land habitat Most larval shells are external, but in the scaphopods and nuculoid bivalves, the larval shell develops within the expanded skin of the trochophore, and is known as the test cell larva The history of larval development in the Mollusca is not firmly established During the nineteenth century larvae with a seven-part bilaterally symmetrical chitinous shell, resembling the early development of the Amphineura, were described for the Solenogasters These observations need to be confirmed, but if correct might indicate that the Solenogasters and Caudofoveata are secondarily without a shell This view is contrary to that of some modern workers that the non-shelled chitons are more primitive than those with a shell and are the living representatives of a ‘pre-shelled’ molluscan stock Living Cephalopoda emerge from their eggs as (free swimming) miniatures, resembling their adults In many bivalves and gastropods, extension of larval life is achieved by increasing the size of the larval shell by growth at the shell margins, which often produces daily growth lines This extension of larval life enables some achitectonicid gastropods to cross the Atlantic Ocean and may have allowed some Mesozoic inoceramid bivalves to exist in the plankton for more than sixty days, a feature coincident with the almost global distribution of some of their species The periostracum consists of proteinaceous material known as conchyolin and may have as many as three layers It may be smooth or hairy and can include calcareous spicules which are sometimes fossilised There are up to three layers of calcareous shell The primitive conchiferan shell is considered to be made up of an external layer of aragonite prisms together with an internal layer of nacre This shell microstructure is known to occur in the types of bivalves, gastropods, cephalopods, and monoplacophora which have other features thought to be primitive This view is supported by evidence of shell microstructures preserved from the Ordovician However, it has been suggested that some shells from the early Cambrian, whose inner surface structure has apparently been replicated by phosphatic material, may have consisted of calcite Cross-lamellar aragonite and complex cross-lamellar aragonite may develop in any of the layers and at least one of these microstructures occurs in some scaphopods, gastropods and bivalves ‘Amorphous aragonite’, probably very fine-grained cross-lamellar structure, is present in some bivalves Outer calcite layers have evolved in some epifaunal benthonic molluscs, including some bivalves (Ostreoida, Pterioida, etc.) and gastropods (Euomphalidae, Patellidae, Buccinidae) This development may be related to the greater resistance of calcite to solution in seawater In the oysters, bivalves in which all the shell layers have become calcitic, the muscle attachment layers (myostracum) alone remain as typical aragonitic prisms which are joined to the muscular tissue by microvilli on the basal cell membrane Amphineura have a dorsal integument of conchiolin, known as the girdle, which is sometimes pustulose