The crinoids are commonly called _sea-lilies_ because of their
flower-like appearance. The _calyx_ is composed of symmetrically
arranged calcareous plates, and most crinoids have a long stem. Other
crinoids are free-swimming in the adult stage and are attached only
during the earlier phases of their development.
The crinoid calyx is typically cup-shaped (fig. 23) and five grooves
radiate out from its center. These grooves continue outward along the
complexly segmented arms and are used as channels to convey food to the
mouth.
[Illustration: Fig. 23. Typical modern crinoid, or “sea lily,” showing
principal parts.]
Calyx
Arm
Plate
Stem
Columnal
Cirri
Holdfast (root)
The crinoid stem is attached to the base of the calyx and serves for
purposes of support and attachment. This stem consists of a relatively
long flexible stalk composed of numerous calcareous disk-shaped segments
called _columnals_ (fig. 23; Pl. 35), each of which contains a round or
star-shaped opening in its center. Many crinoids have very long stalks
(some are as much as 50 feet in length), and when the animal dies the
columnals become separated and are scattered about on the ocean floor.
Many Paleozoic limestones contain such great numbers of crinoid
columnals that they are referred to as _crinoidal limestones_ (fig. 8).
Crinoidal limestones occur in some of the Mississippian and
Pennsylvanian formations of central Texas and in the Pennsylvanian of
north-central and Trans-Pecos Texas.
The stalk is attached to the sea floor or some other object by means of
a root system called the _holdfast_ (fig. 23). This structure commonly
branches out into the surrounding sediments, and in this manner the
crinoid animal is firmly anchored to the bottom of the sea.
Crinoids, like most echinoderms, are gregarious animals—that is, they
commonly live together in large numbers, and for this reason great
numbers of crinoid remains are commonly found concentrated in relatively
small local areas. Most fossil crinoids are found as stem fragments
because the more fragile calyx and root system are less likely to be
preserved.
The earliest known crinoids have been found in rocks of Ordovician age,
and their remains are particularly abundant in Paleozoic rocks. Crinoids
are living today but most of them are stemless free-swimming forms
called “feather stars,” much less abundant than their Paleozoic
ancestors.