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ECHINODERMATA

skeletal substance (“ stereom ”) by Theel (in Fcstskriftfor Lilljeborg, manifest the true genealogy of each class, and reconstruct 1896). Knowledge of the development has been enormously ex- its ancestral forms by proof instead of conjecture. The tended by numerous embryologists, e.g., Ludwig (op. cit.), MacBride problem of the interrelations of the classes- will thus be (“ Asterina gibbosa,” Quart. Journ. Micr. Sci., 1896), Bury {Quart. reduced to its simplest terms, and even questions as to the Journ. Micr. Sci., 1889, 1895), Seeliger (on “ Antedon,” Zool. nature of the primitive Echinoderm and its affinity to the Jahrb. 1893), Goto (“Asterias pallida,” Journ. Coll. Sci. Japan, 1896) Grave (“ Ophiura,” Mem. Johns Hopkins Univ., 1899), Theel ancestors of other phyla may become more than exercises ((* Echinocyamus, ” JSTov. Jcl. Soc. Sci. l/psala, 1892), Semon for the ingenuity of youth. Work has been and is being (“ Synapta,” Jena. Zeitschr., 1888), and Loven (opp. citt.) and done by the laborious methods here alluded to, and though though the theories based thereon may have been fantastic and the diversity of opinion as to the broader groupings of contradictory, we are now near the time when the results can be co-ordinated and some agreement reached. But the scattered classification is still restricted only by the number of details of comparative anatomy are capable of manifold arrange- writers, we can point to an ever-increasing body of assured ment, while the palimpsest of individual development is not knowledge on which all are agreed. Unfortunately merely fragmentary, but often has the fragments misplaced. The such allusion to these disconnected certainties as alone morphologist may propose classifications, and the embryologist may erect3 genealogical trees, but all schemes which do not agree might be introduced here would be too brief for comwith the direct evidence of fossils must be abandoned ; and it is prehension, and we are forced to select a few of the this evidence, above all, that gained enormously in volume and broader hypotheses for a treatment that may seem dogin value during the last quarter of the 19th century. The matic and prejudiced. Silurian crinoids and cystids of Sweden have been illustrated in Angelin’s Iconographia Crinoideorum, 1878 ; the Palaeozoic crinoids Calycinal Theory.-—The theory which had most influence on the ancf cystids of Bohemia are dealt with in Barrande’s Systemc conceptions Echinoderms in the two concluding decades ot the Silurien, 1887 and 1899 ; P. H. Carpenter published important 19th centuryofwas that of Loven, elaborated by P. H. Carpenter, papers on fossil crinoids in .the Journal of the Geological Sladen, and others. This, which may be called the calycinal Society, on Cystidea in that of the Linnean Society, 1891, and, theory, will be appreciated comparing the structure of a simple together with R. Etheridge, jun., compiled the large Catalogue crinoid with that of some by other types. A crinoid reduced to its ofBlastoidea in the British Museum, 1886 ; O. Jaekel, in addielements consists of three principal portions —(i.) a theca tion to valuable studies on crinoids and cystids appearing in the simplest test enclosing the viscera; (ii.) five arms stretching upwards Zeitschrift of the German Geological Society, has published the or or outwards from the theca, sometimes single, sometimes branchfirst volume of Die Stammesgeschichte der Pelmatozoen (Berlin, ing; (iii.) a stem stretching downwards from the theca and at1899), a richly suggestive work ; the Mesozoic Echinoderms of taching it to the sea-floor (see Fig. 1). That part of the theca France, Switzerland, and Portugal have been made known by P. de Loriol, G. H. Cotteau, J. Lambert, Y. Gauthier, and others (see Paleontologie Fran chaise, Mem. Soc. Paleontol. de la Suisse, Trabalhos Comm. Geol. Portugal, kc.)-, a beautiful and interesting Devonian fauna from Bundenbach has been described bv Follmann, Jaekel, and especially Stiirtz (see Verhandl. nat. Herein preuss. Bheinlande, Palaeont. Z bhandl., and Pcdasontographica); while the multitude of North American palaeozoic crinoids has been attacked by AVachsmuth and Springer in the Proceedings of the Philadelphia Academy and the Memoirs of the Harvard Museum (1897). The vast mass of material made known by these and many other distinguished writers has to be included in our classification, and that classification itself must be controlled by the story it reveals. Thus it is that a change, characteristic of modern systematic zoology, is affecting the subdivisions of the classes. It is not long since the main lines of division corresponded roughly to gaps in geological history : the orders were Palseocrinoidea and Heocrinoidea, Palechinoidea and Euechinoidea, Palaeasteroidea and Euasteroidea, and so forth. Or divisions were based upon certain modifications of structure which, as we now see, affected assemblages of diverse affinity. thus both Blastoidea and Euechinoidea were divided into Eegularia and Irregularia; the Holothuroidea into Pneumophora and Apneumona; and Crinoids were discussed under the heads “ stalked ” and “ unstalked.” _ The barriers between these groups may be regarded as horizontal planes cutting across the branches of the ascending tree of life at levels determined chiefly by our ignorance; as knowledge increases, and as the conception of a genealogical )elow the origins of the free arms is called the ‘‘dorsal cup” ; the classification gains acceptance, they are being replaced by rented part above the origins of the arms, serving as cover to the •up, is known as the “ tegmen.” All these parts are he support d by vertical partitions which separate branch from branch. dates or ossicles of crystalline carbonate of lime. I cuP» * The changes may be appreciated by comparing the sys- amplest form, consists of two circlets of five plates. Each p tematic synopses in the Treatise on Zoology, edited by Pay if the upper circlet supports an arm and is called a ad ’[ d Lankester, with the classification adopted in the article in dates of the lower circlet, the “basals,” rest on the stem ana Ency. Brit., 9th ed., vol. vii., or in any zoological text-book dternate with those of the upper circlet, i.e are interradm! Some crinoids have yet another circlet Mow, these, contemporary therewith. In the present stage of our know- position. constituent plates of which are called “.^fratesa^ and ar ledge these minor divisions are the really important ones. situated radially. The tegmen m most primitive foims, asj^ For, whereas to one brilliant suggestion of far-reaching is in the embryonic stages of the living Antedon ( o- i d homology another can always be opposed, by the detailed if five large triangular plates, alternating with the rad a s ailed “orals,” because they roof over the mouth, biaddito o comparison of individual growth-stages in carefully selected these three or four circlets of plates, two other ele^nts wem on series of fossils, and by the minute application to these of supposed 'essential to the ideal crinoid: the dorso-cental and the principle that individual history repeats race history, oro-central. The former term was applied to a «attenea p it actually is possible to unfold lines of descent that do observed in the embryonic stage of a single gemis (^« to not admit of doubt. The gradual linking up of these will that end of the stem attached to the sea-floor, and comparab