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Biol Bull 145: 509-531. (December 1973)
© 1973 Marine Biological Laboratory
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THE BIOLOGY AND FUNCTIONAL MORPHOLOGY OF LATERNULA TRUNCATA (LAMARCK 1818) (BIVALVIA: ANOMALODESMATA: PANDORACEA)

BRIAN MORTON 1

1 Department of Zoology, The University of Hong Kong, Pokfulam Road, Hong Kong

Laternula truncata (Lamarck) is a highly specialized member of a somewhat diverse assemblage of bivalves, the Anomalodesmata. It is found buried in coarse sand on the seaward side of mangrove formations in South East Asia. (1) The shell is thin and possesses a lithodesma, which effectively prevents the opening of the shell in the typical bivalve manner. Movement of the shell is achieved by the bending of the dorsal margin of the shell at a dorso-ventral crack in each shell valve and by the flexibility of the shell itself. Ventral orbital muscles are probably as important in valve movements as are the reduced adductors. A strengthening strut or clavicale is present on each valve. (2) Mantle fusions are of type B ventrally, and type C dorsally. There is a small pedal gape, but no fourth pallial aperture. (3) The siphons which are of type C, are covered in sand grains. At their tips they possess tactile tentacles and optic tentacles; the structure of the complex eyes which the latter possess are described. The tactile tentacles undergo unusual flicking motions when a shadow falls over the siphons. This may be a defensive and a cleansing response and also play a role in feeding. (4) The ctenidia are plicate and heterorhabdic and have ciliary currents of type E (Atkins, 1937a). The outer demibranch is reduced, and the ctenidial axis possesses an aperture on each side of the body which unite posteriorly to form a Y-shaped cavity. (5) The ctenidial/ labial junction is of type 3 (Stasek, 1963), and the labial palps are large. (6) The stomach is of type 4 (Purchon, 1958). The structure of the style sac and the digestive diverticula are described. (7) The structure of the kidney is described. (8) The reproductive system is described. L. truncata is hermaphrodite and the oviduct, vas deferens and urinary duct open, unusually, via a common urinogenital aperture. Many of the morphological adaptations of L. truncata foreshadow the highly specialized condition in the Clavagellacea.




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