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the side or direction away from the mouth. Also referred to as adapical
accessory pores
Microscopic isopores (B) (less than 100 microns) found in Clypeasteroida often forming dense fields (A, C). Each connects a tube-foot to the interior of the test.
acetabulum
The basal surface of a spine that rests directly on its associated tubercle.
acrosaleniid plate compounding
A compound ambulacral plate composed of three elements of which only two are united by an overlying tubercle: the third element is simple. This is also known as a semicompound plate.
adapical
at or towards the apical disc (direction). Also known as aboral.
adradial
at or towards the boundary betwseen ambulacral and interambulacral zones (direction)
adradius
The line separating ambulacral and interambulacral columns of plates.
ambital
At or towards the ambitus
ambitus
The point of widest circumference on the test
ambulacra
pleural of ambulacrum
ambulacrum (-a)
The zone of plates associated with the water vascular system and tube-feet and usually biserially arranged. There are five ambulacral zones in the corona of extant echinoids.
amphiplacous
An interambulacrum in which the first (primordial) plate (red) is in contact with the succeeding plates (purple) from both columns.
amphisternous
Referring to echinoids with an amphiplacous plastron.
ampulla
Internal bulbose or leaf-shaped organ that is part of the water vascular system and directly underlies tube-feet. It acts as a fluid resevoir for expansion and constraction of the associated tube-foot.
anal (fasciole)
A fasciole band that runs vertically from the subanal fasciole passing on either side of the periproct (ppt).
anal lunule
A lunule that pierces interambulacrum 5 and which, on the adoral face, is closely associated with the periproct.
anal sulcus
A groove on the aboral surface running towards the posterior of the test from the periproct.
anal tuft
A single or double cone of spines that projects from the rear of the test beneath the anus. These spines are used in constructing the subanal funnel and are surrounded by a subanal fasciole.
anisopore
The connection between tube-foot and internal water vessel where there are a pair of pores of markedly unequal size.
apical
at or towards the apical disc (direction)
apical disc
The small area of plates forming the aboral surface of the test at the apex of the ambulacral and interambulacral zones. Composed of ocular and genital plates, which (in regulars) surround the periproct.
apophysis
Interambulacral element of the perignathic girdle. An internal flange (ap) that originates from the most adoral interambulacral plates. Lantern protractor muscles attach to these. [Picture shows the interior of the oral region.]
arbaciid plate compounding
A compound ambulacral plate composed of three (rarely four) elements of which the middle is always the largest. Upper and lower elements are always small and do not reach the perradius.
areole
A flat platform of fine stereom surrounding the boss and marking the attachment site of spine muscle on a tubercle. Can be depressed in cidaroids
Aristotle's lantern
The complex jaw apparatus found in regular echinoids.
auricle
Ambulacral element of the perignathic girdle. An internal flange (au) originating from the most adoral ambulacral plates. Lantern retractor muscles attach to these. [Picture shows the interior of the oral region.]
