They are very thin-walled membranes, very poor in blood-vessels, formed by the bulged-out pleural or peritoneal covering of the lungs, through the parabronchial tubes of which they are filled with air.
The body wall of the Chaetopoda consists of a "dermo-muscular" tube which is separated from the gut by the coelom and its peritoneal walls, except in most leeches.
It is further noticeable that in Rhynchelmis the covering of vesicular cells which clothes the drainpipe cells of the adult nephridium is cut off from the nephridial cells themselves and is not a peritoneal layer surrounding the nephridium.
This coelom is lined by peritoneal cells and is divided into a series of metameres by septa which correspond to the segmentation of the FIG 15.
The adult stage, for example, has been found in the nasal passages of sheep, goats, horses and even of man, and the larval stage in the pleural and peritoneal cavities of dogs and cats.