Contains about 35 species in seven genera and three families. Tentacles are simple or with a few small digits. Respiratory trees are present. The calcareous ring is without posterior projections. Possess muscles for retracting the oral introvert.
All members have a rigid body encased in enlarged flattened ossicles.
All members live burrowed in soft sediment. Most live in deep water. The evolutionary relationships of the major holothuroid lineages were, until quite recently, poorly understood. This was in part due to their lack of an integrated skeleton like that providing the extensive fossil record and numerous morphological characters of other groups of echinoderms. There have been numerous speculations about the relationships within Holothuroidea extending well back into the 19th century.