Among these the abyssal starfish and holothurians described by W.P. Sladen and H. Théel respectively, in the Report of the "Challenger" Expedition, are most notable. The sea-urchins, ophiuroids and crinoids also have yielded many important novelties to A. Agassiz ("Challenger," "Blake," and "Albatross" Expeditions), T. Lyman ("Challenger"), Sladen ("Astrophiura," _Ann. Mag. Nat. Hist._, 1879), F.J. Bell (numerous papers in _Ann. Mag. Nat. Hist._ and in _Proc. Zool. Soc._), E. Perrier ("Travailleur" and "Talisman," Cape Horn and Monaco Expeditions), P.H. Carpenter "Challenger" Reports), and others. The anatomical researches of these authors, as well as those of S. Lovén ("On Pourtalesia" and "Echinologica," published by the Swedish Academy of Science), H. Ludwig (_Morphologische Studien_, Leipzig, 1877-1882), O. Hamann (_Histologie der Echinodermen_, Jena, 1883-1889), L. Cuénot ("Études morphologiques," _Arch. Biol._, 1891, and papers therein referred to), P.M. Duncan ("Revision of the Echinoidea," _Journ. Linn. Soc._, 1890), H. Prouho ("Sur Dorocidaris," _Arch. Zool. Exper._, 1888), and many more, need only be mentioned to recall the great advance that has been made. In physiology may be instanced W.B. Carpenter's proof of the nervous nature of the chambered organ and axial cords of crinoids (_Proc. Roy. Soc._, 1884), the researches of H. Durham (_Quart. Journ. Micr. Sci._, 1891) and others into the wandering cells of the body-cavity, and the study of the deposition of the skeletal substance ("stereom") by Théel (in _Festskrift för Lilljeborg_, 1896). Knowledge of the development has been enormously extended by numerous embryologists, e.g. Ludwig (_op. cit._), E.W. MacBride ("Asterina gibbosa," _Quart. Journ. Micr. Sci._, 1896), H. Bury (_Quart. Journ. Micr. Sci._, 1889, 1895), Seeliger (on "Antedon," _Zool. Jahrb._, 1893), S. Goto ("Asterias pallida," _Journ. Coll. Sci. Japan_, 1896), C. Grave ("Ophiura," _Mem. Johns Hopkins Univ._, 1899), Théel ("Echinocyamus," _Nov. Act. Soc. Sci. Upsala_, 1892), R. Semon ("Synapta," _Jena. Zeitschr._, 1888), and Lovén (_opp. citt._); and though the theories based thereon may have been fantastic and contradictory, we are now near the time when the results can be co-ordinated and some agreement reached. But the scattered details of comparative anatomy are capable of manifold arrangement, while the palimpsest of individual development is not merely fragmentary, but often has the fragments misplaced. The morphologist may propose classifications, and the embryologist may erect genealogical trees, but all schemes which do not agree with the direct evidence of fossils must be abandoned; and it is this evidence, above all, that gained enormously in volume and in value during the last quarter of the 19th century. The Silurian crinoids and cystids of Sweden have been illustrated in N.P.