Heart urchins (Echinoidea: Spatangoida) are considered infaunal, deposit feeding sea urchins that utilize the surrounding sediment as a source of nutrients. Sediment occupies most of the digestive tract lumen but never enters the gastric caecum, a prominent structure that is filled with a transparent fluid. The aim of this study was to shed light on the nature of the fluid found inside the gastric caecum of a well-studied spatangoid species, Echinocardium cordatum. Our conclusions are based on a three-step-approach: firstly, by following the movement of dyed seawater from the mouth up to the caecal lumen; secondly, by comparing the osmolarity of various body fluids; and thirdly, by describing the particulate content of the gastric caecum. In addition, we employed magnetic resonance imaging (MRI) to reveal the absence of sediment within the gastric caecum. Our osmolarity measurements show that the coelomic fluid is significantly more concentrated than the caecal fluid, which in turn has an osmolarity similar to seawater. MRI reveals that the gastric caecum, in contrast to the rest of the digestive tract, is always devoid of sediment. Light and electron microscopy observations reveal the presence of a variety of detrital particles suspended in the caecal fluid that are identical to those occurring in seawater sampled over the seafloor. We argue that the fluid filling the gastric caecum must be predominantly seawater, and we propose a scenario that explains seawater circulation in E. cordatum. In this context, the gastric caecum could act as an internal trap for suspended particulate organic matter. We hypothesize that spatangoid sea urchins could have adopted internal suspension feeding as a secondary feeding mode in addition to deposit feeding.