Holothurians

Holothurians 

(sea cucumbers), a class of invertebrates (Holothurioidea) of phylum Echinodermata.

The skeleton of the holothurians is greatly reduced. The body is pentactinally symmetrical, which is characteristic of the phylum in general, but in the holothurians it is marked by the biliterally symmetrical arrangement of many of the organs. The largest holothurians reach 2 m in length. The body is vermiform, with a mouth at one end and an anal opening at the other. The mouth, surrounded by a crown of tentacles that serve to capture the food, leads into a long tubular gut. Respiration is accomplished by an ambulacral system and a so-called respiratory tree—branched sacs leading to the rectum. Holothurians develop by metamorphosis; free-swimming larvae hatch from the eggs. There are some 1,100 species.

The holothurians are exclusively marine, benthic, sluggish, creeping animals, feeding on sediment or tiny plankton. When greatly irritated they are capable of self-mutilation (au-totomy), but the lost organs readily regenerate. In China the sea cucumber is dried and used as food. The animals (particularly the trepang) are also of some commercial importance in the USSR, in the Far East.

REFERENCES

Dogel’, V. A. Zoologiia bespozvonochnykh, 5th ed. Moscow, 1959.
Bol’shoi praktikum po zoologii bespozvonochnykh, part 2. Moscow, 1946.
Zhizn’ zhivotnykh, vol. 2. Moscow, 1968.