All species have at least one vesicle wall. Most species have a vesicle composed of two wall layers, however, one or three layers have also been reported. The two layers can be of equal thickness, but in most cases the inner wall is very thin and the outer layer homogeneous, massive, and thick. It is more or less a rule that species with long and big vesicles and lacking in spinose ornamentation have a very thick and heavy outer wall layer and a thin membranous inner layer.
This inner layer seals the oral part of the basal process. This, combined with the prosome, perfectly isolated the vesicle cavity from sea-water. There are also granules on the interior of the neck of the vesicle wall which served to anchor the prosome in a fixed position.