haemocoel, the fluid between the cells. It has since been found however that, in response to the presence of Symbiodinium, a tertiary series of tubules develop from the walls of the digestive system and ramify through the tissues. The algae are found in these and are separated from the blood cells in the haemolymph by a layer one cell thick. This is analogous to the tertiary tubular system found in Tridacna and first described by Mansour in 1946, but subsequently overlooked by other researchers. The algae enter the system through the mouth. The translucent shell and the bivalve's habit of lying on the surface both encourage photosynthesis in the algae. The mollusc benefits from the metabolites produced and the alga benefits from a safe environment in which to live. References Cardiidae Molluscs of the Indian Ocean Molluscs of the Pacific Ocean Molluscs described in 1758 Taxa named by Carl Linnaeus