The Echinoid Directory

The peristome is the opening in the test associated with the mouth. In life it is covered in a flexible connective tissue membrane with embedded platelets. In cidaroids (above left) and echinothurioids the ambulacral plates continue over the peristome as a series of imbricate plates (a), each one perforated by a pore-pair and bearing a tube-foot in life. There may also be a single or double series of non-ambulacral plates (n) in the interradial position. In all other groups the peristomial membrane has just five pairs of buccal plates, which are oval ambulacral plates embedded in the membrane and bearing a single tube-foot each. The rest of the membrane is usually rather lightly skeletalised, with small platelets only.

Echinoids other than cidaroids have soft-tissue sacs that are external extensions of the internal coelom. These are needed to compensate for volume change inside the rigid test as the lantern is moved in and out during feeding. These buccal sacs emerge at the outer rim of the peristomial opening immediately adjacent to the adradial sutures of interambulacral zones. Their position is marked by distinct notches in the rim of the peristome (right-hand photograph, arrowed). These are the buccal notches.