HOLOTHURIA, in natural history, a genus of the Vermes Mollusca class and order. Body detached, cylindrical, thick, naked, and open at the extremity ; mouth surrounded by fleshy branched tentacula or feelers. These are all inhabitants of the sea, and expand or contract them selves at pleaiure ; the anterior aperture serves them both las a mouth and vent, and from the hinder one they reject wa ters which had been previously drawn in ; the tentacula are retractile. There are twenty-three species. H. pentactes, or five-rowed Holothuria is noticed by Pen nant. It has an incurvated cylindric body, marked with longitudinal rows of papilla ; out of the centre of each issue at plea sure, slender feelers like the horns of snails; the upper extremity retractile ; when exerted it assumes a cordated form, surrounded at the apex with eight tenta cula, elegantly ramified, of a yellow and silver colour. It is found on the shores near Penzance. H. tremula is a foot long, inhabits the Mediterranean and Adriatic seas; the body is cylindrical when extend ed, and oblong when contracted ; it is va rious in colour, but generally of a beauti ful mixture of red and white ; the cylin drical tubes beneath the body act as so many suckers, by which the animal fixes itself firmly to the bottom of the sea.
Another curious species noticed by Gme lin is H. denudata, is oblong, with inter rupted lateral lines, and without a crest or tail, inhabits the American ocean. It is three or four inches long, with a body slowly tapering at both ends, transparent, of a firm gelatinous consistence and hol low, opening by a small triangular aper ture next the crest, and a narrow round one at the other extremity ; they have a spiral milky line down the back, under this another larger opaque one, and on each side below these another smaller purple one. They are sometimes found single, and frequently sticking length ways together. The word holothuria is used by Pliny and Aristotle ; but Mr. Pennant supposes they both intended, un der this name, to describe those marine bodies now denominated zoophyta. Aris totle, however, seems to have admitted that they possessed animal life, a circum stance that has in modern times been completely ascertained.