This sicula, which had originally the shape of a hollow cone, is formed of two portions or regions - an upper and smaller (apical or embryonic) portion, marked by delicate longitudinal lines, and having a fine tabular thread (the nema) proceeding from its apex; and a lower (thecal or apertural) portion, marked by transverse lines of growth and widening in the direction of the mouth, the lip or apertural margin of which forms the broad end of the sicula.
Some of the thecal plates, infrabasals I B, basals B, and orals 0 are forming around the bodycavities r.pc and l.pc; p is the water-pore.
Beneath each food-groove was a radial water-vessel and probably a nerve and blood-vessel, all which structures passed either between certain regularly arranged thecal plates, or along a furrow floored by those plates, which were then in two alternating series.
These two types are, in the main, correlated with two gradual differentiations in the minute structure of the thecal plates.
Chirocrinus-alter, one of of the nervous, waterthe Rhomb i fera, showing the reduced vascular and blood-vas number and regular arrangement of the cular systems, testified thecal plates, and the concentration of to their Pelmatozoan ancestry.
How would you define thecal? Add your definition here.