Crinoids, also known as sea lilies or feather stars, are marine animals that belong to a class of Echinoderms. They lived in both shallow and deep water. They are characterized by a mouth on the top surface that is surrounded by feeding arms. Crinoids usually have a stem used to attach themselves to a substrate, but many live attached only as juveniles and become free-swimming as adults. There are only a few hundred known modern forms, but crinoids were much more numerous both in species and numbers in the past. Some thick Limestone beds dating to the mid- to late-Paleozoic are entirely made up of disarticulated crinoid fragments.