Holothuroids fall into the phylum echinodermata along with 4 other orders: crinoidea, asteroidean, ophiuroidea, and echinoidea.  While the species within each of these orders fall neatly into clades, the phylogenetic relationships between the groups is highly debated.  In both most accepted hypotheses however, holothuroids are thought to have developed last (Figure)(Wray).

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       There are about 1100 species of Holothuroids, or sea cucumbers (Knight).  Holothuroids are divided into six Orders described below.
 

Apodida
Contains about 269 species in 32 genera and three families. Tentacles are digitate, pinnate or, in some small species, simple. Respiratory trees are absent. Tube feet are completely absent. The calcareous ring is without posterior projections. The body wall is very thin and often transparent. Found in both shallow and deep water.
Elasipodida
Contains about 141 species in 24 genera and five families. Tentacles are shield-shaped and used in shovelling sediment. Respiratory trees are present. The calcareous ring is without posterior projections. With the exception of Deimatidae, the body wall is soft to gelatinous. All forms live in deep water.
Aspidochirotida
There are about 340 species in 35 genera and three families. Tentacles are shield-shaped. Respiratory trees are present. The calcareous ring is without posterior projections. The body wall is generally soft and pliant. Most forms live in shallow water, though one family is restricted to the deep sea.
Molpadiida
There are about 95 species in 11 genera and four families. Tentacles are simple. Respiratory trees are present. The calcareous ring is without posterior projections. The body wall is generally soft and pliant. Most forms live in shallow water, though one family is restricted to the deep sea.
Dendrochirotida
Contains about 550 species in 90 genera and seven families. Tentacles are highly branched and extended to filter material from the water column. Respiratory trees are present. Some members with a calcareous ring composed of numerous small pieces or having long posterior extensions. Possess muscles for retracting the oral introvert. The body wall may be hardened from enlarged plate-like ossicles. Live either attached to hard bottoms or burrow in soft sediment. Most species live in shallow water.
Dactylochirotida
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. The body is usually "U" shaped. All members live burrowed in soft sediment. Most live in deep water.
(Kerr)