The brachia rise from a corresponding number of thecal plates, “radials (RR).”
At the same time the thecal plates degenerated into spicules.
These two types are, in the main, correlated with two gradual differentiations in the minute structure of the thecal plates.
thecal the·cal (thē'kəl)
Of or relating to a sheath, especially a tendon sheath.
theca the·ca (thē'kə)
n. pl. the·cae (-sē', -kē')
A case, covering, or sheath, such as the outer covering of the cocoon of certain insects.