The annelid excretory system consists of nephridia; within
most body segments, ciliated paired tubules—nephridia—draw
in wastes from coelomic fluid and then discharge dissolved
waste through external pores called nephridiopores. Additional
waste moves into nephridia from the nephridial blood vessels.
Gametes also exit through nephridiopores in many annelids and
through the mouth in a few species. Nephridia also regulate the
water content of the coelomic fluid. Castings of intertidal polychaetes
are sand, cleaned of organic matter as it passed through
the gut and out of the anus.