1. Phagocytosis in the perivisceral fluid of Arbacia is carried on by the leucocytes alone. These cells possess membranous processes similar to those of the leucocytes of Asterias and are the smallest and, most numerous of the amibocytes. As phagocytes the leucocytes have a long period of activity. 2. The leucocytes tend to agglutinate even in freshly drawn perivisceral fluid. In a drop of perivisceral fluid on a plane surface, the leucocytes lose their characteristic membranous processes upon contact with the surface and become elongate. Long filamentous processes are produced by the shrinkage of the membranous processes. These filamentous processes adhere to those of neighboring leucocytes forming a delicate mesh to which the pigmented and colorless amibocytes adhere. 3. The leucocytes show an alkaline reaction when treated with neutral red in sea-water, but are not affected by weak solutions of intravitam stains introduced into the perivisceral cavity. 4. If a piece of the peristomial membrane or the test is resected, the leucocytes form a clot which closes the wound.