class of echinoderms including the sea cucumbers

A class of marine invertebrates in the phylum Echinodermata. These are elongate and bilaterally symmetrical animals with a leathery skin and a diffuse skeleton of calcite spicules, commonly called sea cucumbers. Holothurians are mainly benthic with only those of the order Pelagiothurida being planktonic. There are 650 or so species of these, with some living in abyssal regions. See the Introduction to the Holothuroidea at the U.C. Museum of Paleontology.