Test: Internal skeletal composition of calcitic plates.
Peristome: Central section on underside of test where the mouth opens.
Abactinal (aboral/apical) side: Side opposite the peristome.
Actinal (oral) side: Side where the peristome is situated, usually facing the substrate.
Aboral plates: Section farthest from peristome.
Ambulacra: Five segments of test from peristome to apical plates, alternating with interambulacra. Designated with Roman numerals in Lovénian numbering system.
Interambulacra: Five plates alternating with ambulacra. Designated with Arabic numerals in Lovénian numbering system.
Apical plates: Plates arranged around the aboral terminus.
Aristotle’s lantern: Internal structure for mastication.
Food groove: Grooves mostly in the ambulacral region near the mouth (adoral) supplied with tube feet that aid in gathering and transporting food.
Genital plate: Apical plate with specialized pores (openings) for reproductive process.
Lunule: Opening or slot through the entire test, sometimes connecting to the exterior margin. Some may be found within an ambulacrum (ambulacral lunule) or below the peristome (anal lunule).
Petaloid: Specialized aboral ambulacra, made up of small holes where tube feet emerge for respiration.
Periproct: Small opening for anus, found below the peristome or on the ambitus (periphery edge).
Apex: Highest point on the test.
Eccentricity: Positioned away from the central location.
(Durham 1966)
