ABSTRACT
The intestinal caecum of most individuals of Echinocardium cordatum harbours peculiar, rounded nodules. Each nodule has a well defined structure and consists of a thick external cortex surrounding a central core. This central core is made of shell or plant fragments, packed diatoms and/or other unidentified debris. The cortex is stratified into three superimposed layers which result from the activity of large sheathed bacteria. The inner cortical layer consists of densely packed empty sheaths. Similar sheaths, inhabited by bacteria, form the middle layer, while the outer layer is made of the same kind of bacteria but unsheathed. Such a cortical structure suggests that the bacteria migrate outside when growing and that they continuously leave the old sheath and build a new one. The symbiotic bacteria are seemingly always present in the intestinal caecum of E. cordatum. Their main characteristic is to attach themselves to debris around which they progressively grow to form irregular to rounded nodules.
