consists usually of five interradial, and five notched or per forated radial plates united by muscles the homologues of the auriculae of the Echinidea. A circlet of plates, in certain species, surrounds the anus. The fluid filling the various canals of the ambulacral system contains nucleated cells. The ambulacral circular vessel lies behind the calcareous ring ; it gives off from one to five or more Polian vesicles, also one or more madreporic canals. Calcareous spiculoe are contained in the wall of the madreporic canal, and often it is terminated at the free end by a cribriform plate similar to a madreporite. By means of the madreporic canal the ambulacral vessels communicate with the peritoneal cavity. From the ambulacral ring there proceed five, sometimes more tentacular canals, which supply ctecal branches to the ten tacles, and usually, also, five ambulacral canals, which give rise to as many rows of pedicels situated in most cases radially, and forming a dorsal bivium and ventral trivium. Sometimes, as in Psolits, the pedicels are irregularly dis persed over the whole perisome, or they may be met with only on the lower surface of the body, where they subserve locomotion. In Molpadia and its allies there are ambula cral canals, but no pedicels ; in the Synaptidce the canals are wanting. The sexual organs are one or two groups of branched tubuli, which open either on the dorsal surface or between the two dorsal tentacles. Except in the Synaptidae, and apparently also the LiodermatidoB, the sexes are dis tinct. The nervous system consists of a circular cord, lying above the ambulacral ring, and giving off five apparently hollow branches, which pass through holes or notches in the radial plates of the calcareous ring, to pro ceed down the centre of the longitudinal muscular bands of
the body.
Development is direct in Holotlmria tremula and Pcntacta, doli- oluin; and a species of Synnpia is viviparous. The majority of the Holothuridea undergo metamorphoses, during which, however, no portion of the echinopredium is discarded. By invagination of the morula an intestinal cavity is produced, into which an ecto- dermic invagination opens, forming the upper portion of the alimentary canal. The cilia that at first clothe the body commonly become limited to a doubly bent band, the larva developing into an auricularia, which has sac-like processes, and occasionally spicules, but never a skeleton. Growing less transparent, the auricularia loses its lateral processes, the mouth disappears, and the larva reaches the "pupa-stage," in which the body is surrounded by five ciliated hoops. A new mouth with tentacles is now developed, the ciliated bands are lost, and the animal attains the same shape as the adult. The peritoneal cavity and the ambulacral system both originate in a coecal process of the archenteron or primitive endo- dermal sac, which, separating from the latter, forms what has been termed the vctso-pcritoneal vesicle. This gives off a process which opens on the dorsal surface, and which eventually furnishes the madreporic canal. A portion of the vesicle remains in con nection with this process, and is transformed into the ambulacral vessels ; the other portion becomes two sausage-shaped vesicles at the sides of the alimentary canal, above and below which they grow together, and form a continuous space, the future peritoneal cavity. The inner and outer wall of the chamber thus resulting become attached respectively to the parietes of the body and to the alimentary canal, and mainly contribute to the production of their peritoneal and muscular layers.
The Holothuridea are creeping in habit ; some, however, of the Synaptidee are able to swim. The apodal forms move themselves by contractions of the body, and by mr-ans of their tentacles. The Holothuridea derive their nourishment from the sand which they swallow, and from Diatomace?e, Foramiuifera, and other minute mirine organisms. Among the internal parasites of the Holo thuridea are small fishes of the genus Fierasfcr, embryos of the gasteropod Entoconcha miralnlis, and Copepod crustaceans (Pin- notJieres). Certain species of Holothuria are much esteemed in China as food, and constitute an important article of commerce with that country.
The Holothuridea may be grouped as follows:—
I. APNEUMONA. Respiratory trees and Cuvierian organs absent ; mouth and anus at opposite ends of the body ; ambulacral canals five ; hermaphrodite.
(1.) Synaptidre. Pedicels absent. Ex. Synapta, Cldrodota.
(2.) Oncinoldbidie. Pedicels present. Ex. Ecldnosoma.
II. TETRAPNEUMONA. Respiratory trees four ; body flask-like ; mouth and anus at the same end of it, the former surrounded by ten tentacles and ten calcareous plates, the latter by as many papillee and plates ; five ambulacra diverge from the anal, and five from the oral region of the body ; pedicels in two rows. Ex. PJiopalodina laycniformis only = the class Diplostomidca of Semper.
III. DIPNEUMONA. Respiratory trees two ; Cuvierian organs present ; mouth and anus polar ; pedicels single-ranked.
(1.) Liodermatidce. Pedicels absent ; tentacles shield-like, cylin drical, or branched. Ex. Liosoma, Ilaplodadyla, Molpadia.
(2.) Dendrochirotie. Tentacles branched. Ex. Thycnc, Pliyllo- plwrus, Ochnus, Psolus.
(3.) Aspidochirofcc. Tentacles shield - like. Ex. Aspidocliir, Stichopus, Sporadijnis, Holot.huria.
Mr C. Moore, E.G.S. (J{<:p. lint. Aasoc., 1872, p. 117), has de scribed wheel-like spiculae of four species of Clnrodota, one from the Inferior Oolite, one from the Upper Lias, and two from the Middle Lias. "They are formed of a number of minute wheel-spokes, varying from 5 to 13, which start from a central axis, and are sur rounded on the outside by a wheel-tire; on the inner edge of some species are a scries of very minute tocth, extending over the contra! cavity." Mr R. Etheridge, jun. (in the Memoirs of the Gcol. Survey of Scotland, Explanation of Sheet 23, 1873), has called ^attention to the discovery by Mr J. Bonnie, survey-collector, of similar organ isms in the Lower Carboniferous Limestone group of E. Ivilbride, and in shales of the Upper Limestone group of Williamwood, near Glasgow.
Bibliography.—The following arc some of the more important treatises on the Echinodermata : Tiedemann. A natomie der Rohrenholothwrie, <tc.. Heidi Ib. 1820 ; Miller, A Natural History of Crinoida, 1821; J. V. Thompson, "On Ptntacrinus turopceus," Fdinb A eic J /ii/.Jotirn..^K.,1S"6; l l .Agassiz,Monog.d"Echinodermes, <tc., Neuchatel, 1838-42, see also Compt. rend., xxiii., 1846, xxv., 1847; L. Agassiz and Desor, " Catalogue Raisonne"," <tc., Ann. sc. not., 3d series, vi., vii., viii., 1846-47 ; E. Forlics, A History of British Starfishes, Ac.. 1841; see also if,m. of the Geolof/. Sun-ey of Great Britain, ii., 1848; J/Orbigny, Hist.des Crinoidtn, 1841; J. Mtiller, "lumber den Ban von Pmtacriniii caput-Medusse" Abhandl. der Jierl. Akad., 1841; Id., " Uebev die Larven und die Entwickelung dor Echiuo- dermen," ib.. 184fi, 1848-52, 1804 (see Huxley in Ann. of Nat. Hist., 1851) ; Id., " Ueber die Gaining Comatula," ib., 1847; Id., Ueber Synapta digitata, &c.. lk:ilin, 1852; J. JIuller and Troschel. System der Asteriden, Brunsw. 1842; De QuntR-fages, " Me"m. sur. la Synapte de Duvernoy,"^nn. sc. nat 1S42; Austin, A Monog. on Recent and Fossil Crinoida, 1844; Von Jjucli, " Ueber Cystideen." Abluindl. tier Rerl. Akad., 1844; Sars, "M<?m. sur le developpeniclit des Aste rics," Ann. sc. nat., 3d ser., ii., 1844; Id., Ofversigt af Norges Echinodermer, Christian):!, 1861; Id., Mem. pour scrvir a la connaissance des Crinoidesvivants,Clm&t.l8GR; Id., Remarkable Forms of Life, 1875; Gaudry, Mem. sur les pieces solides cliez les Stellerides," Ann. des sc. nat., 1851; Romer, "THonojr. der . . . Blastoideen," Arcfnvf. Naturgesch., 1851; Gray, "A Description of Uhojialodina," Ann. of Nat Hist., 1853; Desor, Synopsis des Kchinides fossiles, Paris and Weisbaden, 1855-58; Wright, Monog. of British fossil Echinodermata from the Oolite, 1855-00; British Oolitic Echinodermata, 1862-66; Monoy. of the Cretaceous Echinodermata, 1S64; Liitken, Overtigt over Griinland s Eehinodermata, Copenli. 1857; Id.. Bidrug til Kundskabom Echinodeime," Vidensk. Meddeltls. Kjobenharn , 1863; Id., "Om Ves- tindions Pentacriner," &c., Naturh. Foreninrts Meddelels. Kjijbenh., 1864; Id., "Ophiuiidarum . . . dcscriptionc s nonnullaV Ore rsigtover d. K. D. V. Kelsk. For- /iand., 1872 ; De Koninck and Le Hon. "Crim. idcs du terrain carbonifere de la Bc-1- gique," A ouv. Mem. Acad. Selg., 1858; Cailliaud. " Sur les Oursins perforant s de Bretagne." Revue et Hag. de Zool, 1856, and (. finpt. rend^ xlv, 1857, see also Deshayes and Lory in Hull. Soc. Ge ol., 1856; Wilson, "The Nervous System of the Asterida." Trans. LinneanSoc., 1862; A. Agassiz, "Embryology of Echinoderms," Mem. of the Americ. Acad., 1864, and Ann. *r. nat., 5th ser., i., 1865; Id., "Embryo- logy of the Starfish," Contrib. to the Nat. Hist, of the Vnited States, v.. 1864 ; Id., Revision of the Echini, Cambr. U.S., 1872-73; A. Bam, Beitriige zur Naturgesch. der Synapta digitata, Dresd. 1864, Jena, 1865 ; Sir Wyville Thomson, " On the Embryo logy of Antedon rosaceus," Phil. Trans., 1865; Id.. Depths of the Sf.a, 1872; V. I?. Cai-penter, " On the Structure ... of Antedon rosaceus," J hil. Trans., 1866 ; see also / roc. Roy. Soc., 1876; Sclmlze, Monog. der Echinodermrn der Eifeler Kalkfs, Vienna, 1866; Selenka, " Beitrage zui 1 Anat. und Systematik der Holothurlen," Zeitsch. f.Wiss. ZooL, 1867-68; Semper, Reisen ini Archipel der Phil ippinen, I.eipsic, 2 vols. 1868-76; Id., "Ueber Comattitit," Tl iirzburg Arbeiten, 1874; Metschnikoff, "Studienti. die Entwickelung dtrEeiiinoderiiicn und Nemeitinen." J/e m.^1 corf., S< Petersbourg, 1869; Terrier, " Sur les jiddiceHaircs et les nmbulaen-s des As- tdries et des Oursins," Ann. sc. nat., 1869 and 1870; Id., "Sur I Ariatomie de la Comatula rosaeea," Arch, de Zoolog. ej-per., 1873; Id.. "Sur rApiuireil cir- culatoirc des Oursins," ib., 1875; Id.. Reri*ion de la collection de Stellerides du Museum d Jfist. Nat. de. Paris, with bibliography, 1875-76; Bandelet, "Con trib. a 1 histoire du systeme nerveux des Kcliinodermes," in Hull, de la Soc. d Hist. Nitt. de Strasbourg, 1870. and Arch. d< Zool. expe r., ii.. 1872; Greef, "Ueber den Ban der Echinodennen;" Marb. Sitzungsber., 1871-74 ; Hoffmann, " /ur Anat. der Echinen und Spatangen," A ifderl. Archiv f. Zoo!., 1871 and 1872; "/ur Anat. der Asteriden," ib.. 1875; Laube, Die F.chinoiden der oesterreMiisch-vngari- schen oberen Tertiarablagerungen, Vienna, 1871; I.ove n, "On the Structure of the Echinoidea" (trans, by Dallas), Ann. and Mag. of Nat. Hist., 4th ser., x., 1872; Id., "Etudes sur les Eehinnidees," Kongi-1. Svensl a Vetenskaps Acad. ffandlingar, 1875; 0. Ilertwig, "Beitiiige zur Kcnntniss der Bildung . . . der tbieriscben Eies," G<>genbaw s morphol. Jahrb. 1875; P. II. Carpenter, "On the Anat. nf the Anns of the Crinoids," Journ. of Anat. and J hysiol., 1876 : Teuscher, "liiitiage zur Anat. der Echinodennen," Jenaische, Ziitsch.. 1876; Simrpck, "Anat. und Schizogonie der Ophiactis virens," Zeitsch. f. Wiss. Zool , 1876. See also the papers and larger works of Allman, Hillings, De Blainville, Claus, Costa. Daniell- sen, Desmoulins, Diiben, Dujardin. Fremonvillo, Geg< nbaur.Grny, Grube. Haeckel, Ilenson. Huxley, King. Koren, Krohn, Liitken, Lyman, M Coy, Von Martens, Pictet, Pourtales, A. Schneider, Verrill, and others.
mountain nymphs. The word denotes mere sound ; and the stories told of her are so transparent that they can scarcely be said to belong to the class of fully-developed myths. As Selene to the Greek was clearly the moon, so Echo was the being who could not speak until she was spoken to. and then could only repent the last words of the
speaker. This penalty is said to have been inflicted upon
