SummaryTwo species of invertebrates, Parachaeraps bicarinatus, a freshwater crayfish, and Helix pomatia, a garden snail, were examined for the presence of cells which could phagocytose particulate material introduced into their vascular systems. By means of blood clearance studies and histological techniques, it was demonstrated that there existed a highly active system of such cells, mainly associated with the liver‐like digestive diverticula of both species. The rate of clearance of particles from the blood was slower in snails, although these animals exhibited a much more widespread system of cells which apparently contributed in a minor way to the uptake of particles introduced by a vascular route.