HOLOSTEI. 154 HOLOTHURIAN, HOLOSTEI, lu>l-6s't6-i (XcoLat. nom. pi., fmiu (ik. o/"i> holos, entire + unriov, osteon, bone). A group of bony {,'unoiils, embmving the garpikcs nnd bowtinii (q.v.). HOLOTHU'RIAN (from Lat. hololluirium, Cik. u/itdnifWn', wati'r polyp ; probalily I'roiii o'lvr, holos. entire + Sm/xi/f, Ihourioi, ffni/mr, (houros, impetuous). An celiinoderm uf the ehiss llol- othuroidea, easily reeo^nizecl by the soft body, generally worm-like form, and eirele of tentacles about the mouth. The nuidreporie pinte is in- ternal. 'Sea-cneumber,' 'sea-s(piirt,' and 'sea-slug* are sonic of the popular names of these animals, whose dried bodies form the Orii'iital 'trepang* (q.v.). The class have not the covering of cal- careous |>lates eharacterislic of the more typical Echinodermata, but a tough leathery nuiseular integument, in some genera strengthened by minute or rather large calcareous plates or hooks, very irritable, and capable of great dis- tention and contraction. Some of them are al- most globose, some truly wormlike; but the same individual is often capable of extending itself to several times the length which it has in a state of repose. In locomotion the body is extended and contracted in the annelids, and in the apodous forms this is the only means of locomotion, except such aid as is given by the tentacles. In the pedate forms the principal or- gans of l<K'omotion, as in startishes and sea- urchins, are the ambulacral feet, of which there are sometimes five double rows, while sometimes they are distributed over the whole siirface of tho body; but some of the species have the feet de- veloped only on the ventral side, and the body then i)resents an iip- jier and an under surface. When the feet are funiislied with suckers and are very extensible they are called 'pedicels,' Init when they end in a blunt point and their function seems to be chiefly respira- tory, or to serve as organs of touch, they are called 'papilhr.' Sometimes all the feet are pedicels; sometimes there are pedicels on the ven- tral side and papillte 20 arc the usual numbers. They are very rarely frhort, simple, and unliranched; generally lliey are provided with branches which nniy be ar- ranged along the sides, clustered at the top, or irregularly scattered. The tentacles are very sensitive and are the most important sense-or- gans of the animal; upon them are occasionally found special sense-cups, sujiposed to be olfactory, or there may be at the base ])ignient e.ves or posi- tional organs. The nervous system is perfectly radiate in structure and very simple in organiza- tion. The mouth opens into an alimentary canal, usually much longer than the body, and therefore more or less looped or coiled. An a»so]>hagus, stomach, intestine, and cloaca can usually be distinguished. Arising from the cloa- ca, there are often two irregularly branched or- gans, known as the 'respiratory trees.' These are hollow, very thin-walled, and capable of contain- ing a great deal of water, which they receive ilirough the anus. They are excretory, and per- haps respiratory in function. In nuiny species (here arise from the wall of the cloaca, and extend into the body-cavity of the animal, numerous glandular tubules, known as Cuvier's organs. These can be ejected from the anus as long, sticky threads, and thus seem to be protective. The blood system is well dcvidoped in most hidothii- rians, especially ab>ng the intestine. In those forms which have a respiratory tree one branch of it is often closely bound up with a network of blood-vessels, arising from the principal vessel on the intestine. The sexes are generally sepa- rate, hermaphrodite forms being quite unusual; but there is no external dilTerence between male and female, save in a very few forms. The eggs are generally thrown out into the water, where they are fertilized, but .several viviparous forms are kncjwn, in which the eggs fall into the body- cavity of the mother and the young develo]) there. In such cases the development is direct, but in species whose eggs develop in the water the young assume a peculiar larval form, known as the auricularin, „ from which the ad- ults develop by a more or less com- plicated metamor- phosis. Several cases are known of female holothurians which brood their eggs and care for the yovmg. on the dorsal : rare- ly all the feet are papilla-. In two THB COMMON NORTH ATLAXTio families the feet (/>;;r;:,"fr"::;o«..) ?^<^ con,pieteiy want- ing. The radiate structure is apparent at the mouth, which is surrounded with tentacles, in nimilK'r frecpicntly a multiple of five, exhibiting consideral)le variety of form, and capable of being more or less retracted. These tentacles vary in number from 8 to 30, but 10, , 1.5, and 8TNAPTA. a, free-BnTliuniing young; b, adult ejnnpta. The holothurians are capable of the most extra- ordinary regeneration of parts, even of the most im])ortant organs. In direct relation with this is their cirious and noteworthy habit of eviscera- tion. Many species when alarmed or irritated ex- pel from the anal opening (or less commonly the mouth) the viscera, either wholly or in part. They thus lose their entire digestive, reproductive, respiratory, and excretory systems and a large part of the blood system; hut if not disturbed further, they will, in the course of a few weeks, replace all they lost with an entirely new set.
