HOLOTHURIOIDEA 647 



canals which pass along the inner surface of the radii between or ex- 

 ternal to the longitudinal muscles, the ambulacral appendages, and the 

 stone canal. The ambulacral feet are provided with ampullae. Ten- 

 tacular canals branch off from the radial canals and pass to the tentacles, 

 which may also have ampullae. The stone canal in most holothurians 

 does not extend to the body wall but ends free in the body cavity, receiv- 

 ing through the madreporite the body fluid. In some deep-sea forms and 

 rarely in synaptids it reaches the body wall; in many holothurians more 

 than one canal is present. The superficial nervous system consists of a 

 nerve ring around the mouth and radial nerves which proceed along the 

 radial ambulacral canals. Special sense organs are better developed than 

 any other class of echinoderms and pigment eyes, otocysts, olfactory cups, 

 and taste papillae are all known. The tentacles also are important tactile 

 organs. The respiratory and excretoiy functions are performed by the 

 respiratoiy trees and the ambulacral appendages. 



The reproductive organs consist of a single, branched gonad which 

 opens to the outside on the dorsal surface either within the ring of oral 

 tentacles or back of it; this gonad is probably homologous to the axial 

 organ of other echinoderms. The sexes are usually separate, but certain 

 hermaphrodites occur. The larv^al form is called the auricularia. In a 

 number of species the young are hatched in the body cavity, where they 

 undergo development, ultimately emerging through a rupture of the 

 body wall. Several species carry the eggs and young in brood pouches. 

 Like all echinoderms, holothurians have great regenerative powers; 

 individuals which have voided the viscera may live and regenerate the 

 lost parts, and some species have been observed to break themselves 

 in two by transverse division, each half developing into a complete 

 animal. 



Distribution and Habits.— Holothurians occur in most seas. They 

 are found at all depths, some species moving over the bottom swallowing 

 sand or mud and catching minute organisms, some living among rocks, 

 corals, or seaweeds, while others bury themselves in the sand or mud. 

 They are used for food extensively in .the Far East, the trepang of 

 the Chinese being the dried bodies of Holothuria marmorata and other 

 species. 



History.— The name Holothuria originated with Aristotle although 

 much doubt exists as to what animals he applied it. Rondelet, however, 

 and other medieval authors gave it to undoubted holothurians. The first 

 anatomical discriptions of holothurians were given hj Bohadsch (1761) 

 and Pallas (1766). Linnaeus in 1758 gave the name to the Portuguese 

 man-of-war (Physalia), and bj' a strict interpretation of the law of 

 priority it should still be given to this genus. 0. F. Miiller, however. 



