The Crinoid Stem
The stems of crinoids are a stack of interlocking
disks or columnals of a variety of shapes. Often the
columnals are arranged in a repeating series of
smaller disks periodically interupted by larger cirri bearing columnals.
Cirri are hair-like protrusions constructed much like
a stem but with smaller columnals. Each columnal is centrally perforated
creating a stem lumen leading into the
coelomic cavities housed in the calyx.
The stems of crinoids end in a variety attachment structures.
Some have a root-like system of cirri while others utilize a cementing
disk to attach to hard substrates. There are crinoids that have an
anchor-like holdfast while others produced bulbs.