The burrows ofFungiacavain the skeleton of living fungid corals are initially superficial and peripheral, later they become deep; they always open into the coelenteron. Boring is exclusively chemical, possibly by secretion of a calcium‐chelating mucus by the epithelium of the pallial envelope, which surrounds the excessively delicate shell, and of the siphons. These epithelia also secrete aragonite with which all cavities in the corralum are filled and the presence of which reveals the track of the bivalve through the corallum. By the dual operation of resorption and secretion,Fungiacavaalways precisely fills the cavity it occupies. There is no reaction by the coral to the presence of the symbiont.Summary:The burrows ofFungiacava eilatensiswithin the skeleton of livingFungia scutariaare described.Burrows of young individuals are superficial, those of older ones are deep. They open respectively into peripheral and central regions of the coelenteron.Burrows change in shape from an ovoid to a dorso‐ventrally flattened heart‐shape with alteration in form of the bivalve.In view of the extreme delicacy of the shell and its complete enclosure within a pallial envelope, burrowing must be exclusively chemical, possibly by secretion of a calcium‐chelating mucus.The pallial envelope is ciliated and covered with a dendritic system of shallow grooves converging on the pallial gape. This may possibly assist in either application of secretion or removal of the products of resorption, or both.The burrow is enlarged but also altered in shape by selective resorption and secretion of aragonite—which fills in all trabecular spaces in the coral skeleton penetrated by the burrows—by the outer epithelia of the pallial envelope and the siphons.Movement ofFungiacavathrough the coral skeleton is clearly revealed by the presence of homogeneous aragonite deposits.There is no trace of reaction by the coral to the presence of the bivalve.Without knowledge of the life history, it is impossible to determine whether the initial superficial chambers are due to a possible initial tube secretion by the bivalve or to a reaction at this stage of the coral to the presence of the newly se