372 FOSSIL INVERTEBRATES/Bivalves Figure Scanning electron micrographs of four of the most common bivalve shell microstructures (A) Aragonitic nacre (from the inner shell layer of Pinna nobilis) (B) Aragonitic crossed lamellar structure (from the outer layers of Corbula gibba) (C) Foliated calcite (from Ostrea edulis) (D) Calcitic prisms (from the outer shell layer of Pinna nobilis) Scale bars for (A) (C) represent 10 mm and for (D) represents 100 mm The mantle cavity is spanned by one or two adductor muscles that attach to the shell and act antagonistically with the ligament to close the valves on contraction (Figure 4C) A significant part of the mantle cavity is occupied by a pair of gills (the ctenidia) lying on either side of the rest of the viscera In most bivalves, the gills are involved with both respiration and ciliary suspension feeding (filtering small particles out of the water which are then transferred to the mouth by a pair of labial palps) Recent bivalves show a number of different gill morphologies depending largely on the feeding process employed Deposit feeders, e.g., Nucula, have less well-developed (protobranch) gills, whilst members of the Lucinidae augment their filter feeding by energy gained from the activities of sulphide-oxidizing chemosymbiotic bacteria living within the modified gills The carnivorous septibranch bivalves (e.g., Cuspidaria, Poromya) suck in small prey (such as amphipods) using their modified siphons These extraordinary bivalves have ‘lost’ their gills and respire over the inner surface of the mantle Other significant organs within the mantle cavity include the gut, heart, circulatory system, and the foot The gut runs between the anteriorly positioned mouth and the posterior anus, and includes a complex stomach which, again, has a number of configurations depending on the feeding biology of the animal Blood is circulated throughout the animal by a three-chambered heart A muscular foot is present in all juvenile and most adult bivalves and occupies the centre of the mantle cavity Naturally, the soft part anatomy of bivalves is very seldom preserved, although preservation of gill and muscle material has been reported in exceptional circumstances Various details, however, can be inferred from the study of the internal surface of the shells Apart from adductor muscle scars, the practised eye may pick out the attachment points of more minor muscles and impressions of radial muscles and blood vessels within the mantle Ecology Modern bivalves exploit a wide range of life habits Many burrow to varying depths within soft sediments,