Giant clams (family Tridacnidae) are special in that they contain large numbers of symbiotic dinoflagellates, Symbiodinium sp., commonly called zooxanthellae which live in the clam's siphonal mantle (hypertrophied siphonal tissues) (1) and are important in its nutrition (2, 3). In 1946, Mansour (4) partially described a tubular system arising from the clam stomach, extending into the mantle and containing zooxanthellae. However, the eminent scientist Sir Maurice Yonge (5, 6) disputed its existence. Subsequently, Yonge's views appear to have suppressed further investigations of Mansour's observations. The zooxanthellae have been universally regarded as living in the hemal spaces of the mantle (2, 5, 7, 8, 9). This study, however, has conhrmed the presence of the tubular system indicated by Mansour and has shown that the zooxanthellae live within a branched, tubular structure that has no direct connection with the hemolymph. The existence of this tubular system has important implications for our understanding of the symbiosis between tridacnids and their symbiotic algae.