The Tridacnidae are a family of the Cardiacea in which the byssal apparatus has been retained and hypertrophied in connection with obligate life on the surface of Indo-West Pacific coral reefs. The greatly enlarged siphons occupy the entire upper surface, their inner marginal folds housing enormous populations of dinoflagellate symbionts (Symbiodinium microadriaticum Freudenthal) exposing them to high light intensities. The umbones are displaced on to the under side alongside the byssal gape. The least specialized species (T. maxima and T. squamosa) retain byssal attachment throughout life. On the under side intimate contact is maintained with the irregular substrate by adventitious secretion of shell around the byssal gape and by a grinding action probably assisted by chemical activity by way of the enlarged middle folds of the mantle margins. This penetration is further developed in the smaller T. crocea which bores into coral rock, umbonal side foremost, by this probable combination of mechanical and chemical means. In the "giant" species, T. gigas and T. derasa, the byssal apparatus atrophies after a certain size is attained, the byssal gape closing with reduction of the mantle folds. Subsequently the unattached animals maintain themselves solely by their great weight. Adaptation here involves increase in size with the much greater number of algae that can be maintained. Hippopus differs in the more globular and smoother adult shell and by retention of the siphons within the valve margins. The final habitat is on the lee of reefs, frequently on sand, with initial attachment probably on the seaward side, then early freedom and subsequent rolling over the reef surface. The globular shell represents a self righting mechanism. Knowledge about the significance of the zooxanthellae—certainly the major food source—is reviewed and the probable course of evolution in the Tridacnidae with acquisition of the symbionts, possibly from hermatypic corals, surveyed. The Tridacnidae appear to have separated from the other Cardiacea about the beginning of the Caenozoic, possibly filling the niche left vacant when the bivalve rudists (Hippuritacea) became extinct.