Practice question

Nemertean excretion and osmoregulation rely on protonephridial system superficially similar to flatworm protonephridia but uniquely associated with circulatory vessels enabling filtration of blood. System comprises terminal flame bulbs with ultrafiltration weir leading to ciliated tubules converging to paired ducts opening via nephridiopores laterally in anterior region typically one to three pairs. Tubules intimately associated with lateral blood vessels facilitating ultrafiltration from blood into lumen driven by pressure. Number variable some highly branched others reduced. Unlike metanephridia no true nephrostome present, function eliminates ammonia and maintains ionic balance in marine osmoconforming animals adapting to varying salinities.