First formed plate in series immediately adjacent to the peristome. This plate is commoly resorbed in regular echinoids but usually preserved in irregular echinoids.
bidentate (pedicellaria)
A pedicellaria with a head composed of just two valves.
bigeminate (plating)
A compound plate composed of two elements.
biserial
Arrangement of pore-pairs (tube-feet) forming a double series in each half-ambulacrum
boss
The elevated, mound-like zone on a tubercle leading up to the mamelon. This is where the collagenous catch apparatus, binding the spine to the tubercle, attaches.
bourrelet
A primordial plate which is swollen forming a tooth-like or knob-like projection away from the test surface. The five bourrelets often form a star-like opening to the peristome, and support a grill of spines that cover the opening.
buccal membrane
Another name for the peristomial membrane.
buccal notches
Indentations of the peristomial margin marking the place where pharyngeal expansion sacs emerge.
buccal plates
Ovate ambulacral plates that lie in the peristomial membrane and support tube-feet. There are 10 buccal plates in most regular echinoids, one pair in each ambulacral zone. The insert shows a SEM image of one buccal plate.
buccal pore/tube-foot
An enlarged pore (bp) and tube-foot that lies immediately adjacent to the peristome and is the most adoral pore in a phyllode.
buccal sacs
Soft-tissue organs that are external extensions of the pharyngeal coelom. They allow coelomic fluids to move in and out of the test to compensate for movement of the lantern in and out of the test during feeding. They were originally thought to have had a role in respiration, hence their older name "gills".
buttresses
Internal calcite pillars that connect upper and lower surfaces of the test (present in certain clypeasteroids).
