Filtration rates for Phallusia were computed from the rate of clearance of sus-pensions of colloidal graphite and of the flagellate Isochrysis. Rates varied from 825 ml./h to 5100 ml./h for animals between 8 and 128 g wet weight (40–336 ml./h/g wet weight; 88–570 ml./h/mg nitrogen). The greater part of this current is ciliary; less than 2 % is accounted for by squirting. Squirting thus plays but a minor role in maintaining the feeding current. Its probable function is perhaps more comparable with the expulsion of pseudofaeces in filter-feeding molluscs.