sea cucumber, (class Holothuroidea), any of 1,200 species of marine invertebrates that constitute a class within the phylum Echinodermata.The soft cylindrical body, 2 to 200 cm (about 0.75 inch to 6.5 feet) long and 1 to 20 cm (0.4–8 inches) thick, is usually a dull, dark colour and often warty, thus resembling a cucumber.The internal skeleton is reduced to numerous distinctively shaped ...