These secrete waste into the haemolymph, prior to further filtration in the nephridium. The excretory system includes a nephridium and one or two pairs of Malpighian tubules. The primary organ of excretion is a nephridium. Pulmonates lack these glands, so that the nephridium is the only major organ of excretion. From his research of amphioxus, the anatomical terms- "Hatschek's pit" and "Hatschek's nephridium" are derived. The nephridium projects into the main venous sinus in the animal's foot.