ABSTRACTThe association of the limpetTectura testudinalis(Müller 1776) and the crustose coralline red algaClathromorphum circumscriptum(Strömfelt) Foslie (Melobesioideae) is considered a species‐specific mutualism between grazer and prey. Anatomical features ofC. circumscriptum, especially those associated with epithallial cells, have been viewed as being unusual and directly related to the mutualism. Transmission and scanning electron microscopic methods were used to investigate the structure and development of epithallial cells inC. circumscriptum.Contrary to prior reports, terminal cells of the epithallial filaments ofC. circumscriptumundergo senescence and shedding much like epithallial cells in other members of the Corallinales; hence, multicellular epithallial filaments did not evolve in this alga simply because epithallial cells were unable to slough. Although epithallial cells in many coralline algae are lightly calcified, those inC. circumscriptumare heavily mineralized, more so than the underlying initial cells and cells comprising the cortex. As in other coralline algae, starch is absent from the epithallial cells ofC. circumscriptum.The feature of having abundant chloroplasts in the epithallial cells of C. circumscriptum is shared by other species with multi‐cellular epithallial filaments and transiently by species with single epithallial cells. Except for having a thick epithallus, a feature found in other genera and in other species ofClathromorphum, the anatomy ofC. circumscriptumis unexcept