In any case the cavity of the prostomium is single, and not formed, as is the cavity of the segments of the body, by paired coelomic chambers.
Thus in the Chaetopoda the perivisceral cavity is coelomic; in this respect the group contrasts with the Arthropoda and Molluscs, where the perivisceral cavity is, mainly at least, part of the vascular or haemal system, and agrees with the Vertebrata.
Among the Oligochaeta the dorsal vessel in Dinodrilus and Megascolides is enclosed in a separate coelomic chamber which may or may not communicate with the main coelomic cavity.
The nephridia, in fact, on this view, are ectodermic ingrowths, the coelomoducts coelomic outgrowths.
Rarely the nephridium does not communicate with the coelom; in such cases the nephridium ends in a single cell, like the "flame cell" of a Platyhelminth worm, in which there is a lumen blocked at the coelomic end by a tuft of fine cilia projecting into the lumen.