Abstract
Bullia digitalis is a sandy-beach whelk which displays a well-defined series of reactions to most pollutants. A marked sub-lethal response is cessation of burrowing, the animals either refusing to burrow or emerging from the sand if already buried. The concentrations of a series of organic substances eliciting the latter response are reported and a correlation is demonstrated between these concentrations and the first-order molecular valence connectivity indices of the substances concerned.