HOLOPHYTES, 11816-fits. See FUNGI.
HOLOSTEI, 116-16s'te-i, a group of fishes, the bony ganoids, largely fossil, represented by the garpikes. See ICHTHYOLOGY.
HOLOTHURIA, 1161-6-thferi-a, echino derms (q.v.) of the class Holothuroidea, popu larly called useacucumbers," from their resem blance in shape and rough skin to that vege table, in which the body is long, cylindrical, somewhat wormlike, less obviously radiated than other echinoderms with a thick muscular body-wall of longitudinal and transverse muscles. The skin is usually thick, tough and imbedded in it are in certain forms calcareous plates, wheels and anchors. The mouth is sur rounded with a circle of 10 branched tentacles, adapted both for respiration and for seizing the food, which consists mainly of foraminifera. The intestine is suspended by mesenteries, is very long and slender, thus in Thyme briareus, which lives in mud and sand on the coast south of Cape Cod, the intestine in an individual three or four inches long is nearly seven feet in length; it opens at the end of the body, and connects with the ((respiratory tree," by which the water is introduced into the interior of the body. Also opening into the cloaca or terminal part of the intestine are the so-called Cuvierian organs, secreting slime on their peritoneal sur face. The holothurians possess the power of eviscerating themselves through the cloaca, and these organs then serve as means of snaring an enemy. The destroyed viscera are then regen erated. Unlike other echinoderms the so-called madreporic body is internal. Holothurians move by tubes or ambulacra which are filled with water, and when distended act as suckers to drag the animal over the bottom. These suckers are either arranged in five rows or with three rows on the ventral surface, and two above, the latter serving the function of sense organs, or they are scattered irregularly over the surface of the body, while in Caudina arenata of the New England coast there are no suckers. A tendency to bilateral symmetry is seen in a form Psolets, which has a creeping disk and three rows of suckers on the flattened disc-like under side. The nervous system re sembles that of the sea-urchins, with its five radial nerve cords prolonged into the radia, and terminating in sensory patches of epithelium. Pigment spots, however, are lacking. There is a ccelomic nervous system. The blood system consists of a ring around the oesophagus, with branches along the stone canal and above and below the gut. The genital organs are located on the left of the dorsal mesentery, and the genital duct opens below and a little behind the feelers.
The holothurians undergo a metamorphosis, somewhat like that of the starfish; but the transparent larva called uauriculariab is barrel shaped; what corresponds to the hoops of the barrel being bands of cilia, while the ear-like projections in certain forms give it the name auricularia. Before the larva is fully grown, the body of the young holothurian begins to bud out from near the side of the larval stom ach, the calcareous cross-like plates are depos ited, and the tentacles begin to grow out. Fi nally after the larval body is absorbed the young holothurian sinks to the bottom. The degree of metamorphosis is less marked than in other echinoderms, while in two forms development is direct, the young growing in a marsupium or broodpouch. A form (Clado dactyla crocea) living in the south seas at the Falkland Islands, carries its young in a sort of nursery where they are densely packed in two continuous fringes adhering to the dorsal tubes. Holothurians are remarkable from the fact that when captured they eject their intestine, a new one in time being regenerated. The large forms lying about on the coral reefs are known to harbor a small slender fish (Fierasifer) which lodges in their cloaca or in the branchial tree. Many of the species are very large, being nearly two feet in length. A common species on the Florida keys and reefs is Holothstria floridana; it lives in water only a few inches deep and can be picked up in large numbers; it is fully 15 inches in length and lives on foram inifera. It has been collected, dried and a ship load exported to China, but the trepang or beche-de-mer of commerce is either of two species (H. edulis, and H. trentula) inhabiting the Pacific Ocean. (See TREPANG). A Cali fornia species is also dried and exported by the local Chinese.
The class of Holothuroidea is divided by Ludwig into two orders: (1) Actinopoda, with radial canals, represented by Holothuria, Cues maria, Thyone, Psolus, etc.; and (2) Paractitt opoda, without radial canals, of which Synopta is an example, the common form living in sand at low water on the New England coast being Leptosynapta girardii. MacBride, on die other hand, divides the holothurians into six orders. A few forms inhabit great depths. Remains of holothurians have been found fossil; certain calcareous plates attributed to them occurring in the Carboniferous, Lias, Jura and Cretaceous strata. Minute calcareous bodies referable to Synapta, etc., have been detected in the Paris Eocene limestones. Consult (Cambridge Natu ral History' (Vol. I, London 1909).