Coprolites of both fish and cephalopods have been found containing ossicles of various crinoids, such as the pelagic crinoid Saccocoma, from the Jurassic lagerstatten Solnhofen, Some have the snail situated over the anus, suggesting that Platyceras was a coprophagous commensal, while others have the animal directly situated over a borehole, suggesting a more pernicious relationship.
They have a U-shaped gut, and their anus is located next to the mouth.These make the calyx somewhat cup-shaped, and there are few, if any, ossicles in the oral (upper) surface.The upper surface, or tegmen, is divided into five ambulacral areas, including a deep groove from which the tube feet project, and five interambulacral areas between them.The crinoid nervous system is divided into three parts, with numerous connections between them.The uppermost portion is the only one homologous with the nervous systems of other echinoderms.There are only about 600 extant crinoid species, Crinoids comprise three basic sections; the stem, the calyx, and the arms.
The stem is composed of highly porous ossicles which are connected by ligamentary tissue.
Primitively, crinoids had only five arms, but in most living species these are divided into two, giving ten arms in total.
In most living species, especially the free-swimming feather stars, the arms branch several times, producing up to two hundred branches in total.
These various fluid-filled spaces, in addition to transporting nutrients around the body, also function as both a respiratory and an excretory system.
Oxygen is absorbed primarily through the tube feet, which are the most thin-walled parts of the body, while waste is collected by phagocytic coelomocytes.
The anus, unusually for echinoderms, is found on the same surface as the mouth, at the edge of the tegmen.