ABSTRACT
Echinocardium cordatum harbours in its intestinal caecum nodules built by filamentous sulfooxidizing bacteria growing around detrital particles. In order to determine where and how the formation of the nodules occurs, individuals of E. cordatum were forced to ingest detrital materials (e.g. cleaned ossicles of the asteroid Asterias rubens). The echinoids were dissected at regular time intervals, and the ossicles retrieved from the different parts of the gut were observed with a scanning electron microscope. The results showed that the intestinal caecum is the only site where symbiotic bacteria colonize detrital particles. The first stage of nodule formation is described.
