Chalcedony occurs as a secondary mineral in volcanic rocks, representing usually the silica set free by the decomposition of various silicates, and deposited in cracks, forming veins, or in vesicular hollows, forming amygdales.
The former consist of one pair or more of vesicular testes communicating by fine ducts with a vesicula seminalis.
Respiration is conducted by the general surface, by the branchial lamina (external branch) of the feet, and the vesicular appendage (when present) at the base of this branch.
Tabulae and vesicular endotheca present.
Orifice concealed at the bottom of a vestibular shaft, surrounded by a solid or vesicular calcareous deposit.
Minute vesicular cavities are not infrequently present, sometimes as negative cubes, and these may contain saline solutions or carbon dioxide or gaseous hydrocarbons.
In the Graptoloidea certain lateral and vesicular appendages of the polypary in the Lasiograptidae have been looked upon as connected with the reproductive system; and in the umbrella-shaped synrhabdosomes already referred to, the common centre is surrounded by a ring of what have been regarded as ovarian capsules.
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.
The tail may be a simple hollow muscular process or provided with stiff bristles set in transverse rows, or divided into two equally long processes, or finally it may form a large vesicular structure.