UNUSUAL TYPES OF NEPHRIDIA IN NEMERTEANS

Abstract

The 3 types of excretory systems found in the nemerteans show great diversity in histological structure. The simplest are the protonephridia of the terrestrial species, consisting of several thousand small groups of flame cells, each group with its own efferent duct to the exterior of the body. The vast majority of species have multiple protonephridia, with a pair of longitudinal collecting tubules each with one or more efferent ducts to the exterior. The third and most complicated type comprises the metanephridia which have been found as yet only in species of Cephalothrix. Here each multinucleate terminal organ opens into a chamber in close contact with a blood vessel and the efferent duct is provided with a thick-walled convoluted tubule, lined with specialized excretory cells. The protonephridia are com- parable with the larval excretory organs of Annelids, while the metanephridia are in many respects similar to those of adult Annelids.