Class Crinoidea, the most primitive and oldest class, consists of the sea lilies. They consist of both deep sea lilies and tropical feather stars. These represent the earliest echinoderms, and are probably similar to a common ancestor of the entire phylum.

Class Crinoidea is in the Phylum Echinodermata. The species that make up this phylum do not show body segmentation, and are radially symmetrical when fully grown but bilaterally symmetrical in the larvae stage. Almost all of the species are marine, although a few can live in brackish water. The coelom of the animals in this phylum is made from the digestive tube, not from cell masses. Therefore, echinoderms are deuterosomes. Echinoderms have an endoskeleton, made of 95% calcium carbonate. Another hallmark of the echinoderms is hard, spiny skin. This is a common feature, but not always apparent in echinoderms. The uniting feature of echinoderms is a water-vascular system. This is a system of canals branching throughout the body that branch into many sections called tube feet. There are at least 2,000 tube feet, which can penetrate the body wall and skeleton in places called ambulacral grooves, in most echinoderms. These tube feet, and in many echinoderms arms and even organs, can be regenerated. There are five major classes in the phylum Echinodermata.

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