The gut material of Pyura praeputialis consists of two components: exogenous seston and endogenous mucus in which the seston particles are embedded. It has not heretofore been possible to separate these components; it follows that quantification of each component had not been possible.

A method is described for separating the endogenous and exogenous components. It allows an estimate of how much mucus is produced and what proportion is resorbed. It also allows an estimate of how much seston is ingested and what proportion is digested.