This is continuous with the general body-cavity in the Phylactolaemata, while in the Gymnolaemata it develops in the bud as a part of the body-cavity, from which it becomes completely separated.
In a few Gymnolaemata the ectocyst is merely chitinous, although in most cases the four vertical walls and the basal wall of the zooecium are calcareous.
The bodywall is greatly simplified in the Gymnolaemata, in correlation with the functional importance of the skeletal part of the wall.
In some Gymnolaemata, polypides which develop an ovary possess a flask-shaped "intertentacular organ," situated between two of the tentacles, and affording a direct passage into the introvert for the eggs or even the spermatozoa developed in the same zooecium.
In the Gymnolaemata protrusion is effected by the contraction of the parietal muscles, which pass freely across the body-cavity from one part of the body-wall to another.