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The first, unpaired plate in interambulacrum 5 (red, 1) in atelostomates. This forms the anterior part of the plastron.
labrum
The labral plate
labyrinthic (stereom)
A stereom arrangement which results in calcite struts and pore spaces are haphazardly arranged.
lantern
See Aristotle's lantern.
latero-anal (fasciole)
A fasciole band that runs from just behind the anterior paired petals and passes beneath the periproct. At its anterior it connects to the peripetalous fasciole.
levator muscles
Muscles in the suckered disc of tube-feet used in adhesion.
Loven's system
The standard reference system by which the various ambulacral (amb) and interambulacral (Iamb) zones are numbered. The key tie point is that the madreporic plate lies in Iamb 2.
lumen (spine)
A central space running up the length of the shaft of a spine (cross-section of spine illustrated).
lunule
An ovate to slit-like perforation through the test, either interradial or perradial in position. These perforations are lined with tubercles and spines and are used to sieve sediment for its organis content and as a pressure release channel. Confined to Clypeasteroida. There are two types of lunule; cross-linked lunules formed by resoption, with upper and lower series of plates vertically stacked; and festooned lunules, formed by secondary closure of notches and with plate sutures highly oblique
