The ultrastructure of the cystid of Crisia eburnea has been studied. The cystid wall comprises an outer periostracum, a calcified layer and one inner cell layer, the ectoderm. The membranous sac, which consists of an outer basement membrane, a series of very thin annular muscle cells and an inner layer of epithelial cells, is interpreted as the detached mesoderm of the cystid wall. Accordingly, the atrial sphincter and the generally eight branched, longitudinal muscle cells connecting the terminal membrane and the membranous sac are interpreted as ectoderm. The membranous sac is attached to the cystid wall with two lateral ligaments and at four abfrontal areas: 1) a wide distal area, 2) an area at the origin of the retractor muscles, 3) a small area between 1 and 2, and 4) a small basal area at the origin of a pair of small muscle cells attached to the lowermost part of the caecum.
We infer that the protrusion of the polypide is caused by a sequential contraction of the annular muscle cells of the membranous sac, starting basally and aided by the contraction of the longitudinal ectodermal muscle cells.