Perforation (0.1 - 0.6 micron hexagonal holes) through valve with internal or external sieve membrane. If the hole goes straight through it is called a POROID AREOLA, if constricted at one side it is called LOCULATE Areolae are thought to be necessary for the cell to obtain nutrients and dissolved gasses such as CO2 from the seawater, since a solid silica sheet would be impermeable to molecules and ions. Many tests also have sub-areolar structure visible only in electron microscopes.