Crinoids (cry'-noids) are called "sea lilies," but they are animals rather than plants. They look like plants, however, because the body skeleton or calyx generally is on the end of a stem made of button-like discs and held on the sea floor by either a stony anchor or root-like arms. The mouth, on top of the body, is surrounded by arms that sweep food into it. The body is made of calcareous plates that fit together like irregular bricks.
When the animal dies, the plates and discs tend to fall apart and sink to the sea floor. Crinoid stems or stem discs are common throughout most of Illinois, and their popular names include "Indian beads" or "fish bones." Many of the limestone beds in Illinois are composed mostly of crinoid plates and discs. The complete calyx is a highly prized fossil. Well-preserved specimens are found in the limestone cliffs along the Mississippi River between Burlington and Alton.
The oldest crinoids come from Ordovician rocks. Some crinoids live today, mainly in deep parts of the ocean, but they are not nearly as common as in the past.