Various interpretations of the holothurian system and phylogeny are critically reviewed and the main characters that form the basis of the existing systematics of this group are analyzed. A system of holothurians based on thorough analysis of their morphology and anatomy is proposed. Four subclasses are recognized in the class Holothuroidea: Arthrochirotacea, Synaptacea, Elpidiacea, and Holothuriacea. The subclass Arthrochirotacea includes the extinct Paleozoic order Arthrochirotida. The subclass Synaptacea includes the order Synaptida with two suborders and three families. The subclass Elpidiacea includes the order Elasipodida with four families. The subclass Holothuriacea includes four orders: Aspidochirotida with five families; Dendrochirotida with 15 families (14 extant and one extinct); Molpadiida with three families; Gephyrothuriida with one family and two genera Gephyrothuria and Hadalothuria. The order Gephyrothuriida is re-established. The order Dactylochirotida Pawson et Fell, 1965 is synonymized under the order Dendrochirotida. A new suborder Cucumariina and new family Mesothuriidae are described. The family Vaneyellidae is synonymized under the family Cucumariidae. Four subfamilies are classified as families: Cladolabidae, Sclerothyonidae, Monilipsolidae, and Thyonidiidae.
