in the Gulf of Mexico. In the North Atlantic also, has been found another Apiocrinoid, which has been described under the name of Bathycrinus gracilis. Several other Crinoids have been found inhabiting the depths of the ocean in the Expedition of the Challenger. The Asteroidea, Ophiuroidea, and Echinoidea are represented in almost all seas, whether in tropical or temperate zones, extending their range even into high northern and southern latitudes. They have also a wide bathymetrical range, extending from between tide-marks to almost the greatest depths which have yet been explored by the dredge. Some of the SeaUrchins (such as Strongylocentrotus lividus) have the peculiar habit of hollowing out cavities for themselves in the solid rock, in which they spend their existence. The Holothuroidea enjoy a nearly world-wide distribution; but they have their metropolis in the Pacific Ocean, occurring abundantly on the coralreefs of the Polynesian Archipelago. One species (Holothuria argus) is collected in large numbers, and is exported to China, where it is regarded as a great delicacy. DISTRIBUTION OF ECHINODERMATA IN TIME. Numerous remains of Echinodermata occur in most sedimentary rocks, beginning with the Upper Cambrian Rocks, and extending up to the recent period. The two orders Cystoidea and Blastoidea, which are the most lowly organised of the entire class, are exclusively Paleozoic; and the Crinoidea are mostly referable to the same epoch. The more highly organised Asteroidea and Ophiuroidea commenced to be represented in the Silurian period; but the Echinoidea, with few exceptions, have no representatives earlier than the Carboniferous Rocks. The following exhibits the geological distribution of the different orders of the Echinodermata in somewhat greater detail:— I. CRINOIDEA.*—The Crinoidea attained their maximum in the Paleozoic period, from which time they have gradually diminished down to the present day. As has already been described, the Paleozoic Crinoidea differ in some important As regards the calyx of the Crinoidea, the following terms are employed to designate its different parts. The base of the cup, or calyx, is termed the "pelvis," and it is made up of from two to five plates, which are termed the "basals." To the "basals" succeed two or three rows of plates, which are termed respectively the "primary radials" (or "parabasals”), “secondary radials," and "tertiary radials," according to their distance from the basals. The axillary radials, which are the furthest removed, give origin to the arms, and are occasionally called the "scapula" (for this reason), whilst the primary and secondary radials are called the "costa." There may also be several orders of plates developed between the “radials,” and known as the "inter-radials. particulars from those which succeeded them. The order is well represented in the Silurian, Devonian, and Carboniferous Rocks, but especially in the latter; many Carboniferous limestones (crinoidal limestones and entrochal marbles) being almost entirely made up of the columns and separate joints of Crinoids. In the Secondary Rocks Crinoids are still abundant. In the Trias the beautiful "Stone-lily" (Encrinus liliiformis) is peculiar to its middle division (Muschelkalk). In the Jurassic period occur many species of Apiocrinus (Pearencrinite), Pentacrinus, and Extracrinus. The Chalk also abounds in Crinoids, amongst which is a remarkable unattached form (the Tortoise-encrinite or Marsupites). Of the non-pediculate Crinoidea, which are a decided advance upon the stalked forms, there are comparatively few traces; but remains of forms (such as Saccosoma and Solanocrinus) allied to the recent Comatulæ have been found in the Jurassic and Cretaceous deposits. 2. BLASTOIDEA.-The Blastoidea, or Pentremites, are entirely Palæozoic, and attain their maximum in the Carboniferous Rocks, some beds of which in America are known as the Pentremite Limestone, from the abundance of these organisms. They are, however, also found in the Silurian and Devonian Rocks. 3. CYSTOIDEA.-These, like the preceding, are entirely Palæozoic; but they are, as far as is yet known, exclusively confined to the Upper Cambrian and Silurian Rocks, being especially characteristic of the horizon of the Bala Limestone. Forms supposed to be Cystideans have been described from the Devonian Rocks, but their true nature is doubtful. The oldest known Echinoderms are two extremely simple Cystideans (Trochocystites and Eocystites) which have been discovered in the primordial zone of North America. 4. ASTEROIDEA.-These have a very long range in time, extending from the Lower Silurian period up to the present day. In the Silurian Rocks the genera Palæaster, Stenaster, Palæodiscus, and Petraster are among the more important, the greater number of forms being Upper Silurian. The next period in which star-fishes more especially abound is the Oolitic (Mesozoic); the more important genera being Uraster, Luidia, Astropecten, Plumaster, and Goniaster, some of which have survived to the present day. Many star-fishes occur, also, in the Cretaceous Rocks, the genera Oreaster, Goniodiscus, and Astrogonium being among the more noticeable. In the Tertiary Rocks few star-fishes are known to occur, but Goniaster and Astropecten are represented in the London Clay (Eocene). 5. OPHIUROIDEA.-The "brittle-stars are represented in the Silurian Rocks by the single genus Protaster. In the Triassic, Oolitic, Cretaceous, and Tertiary Rocks several genera of Ophiuroidea are known; some being extinct, whilst others (such as Ophioderma, Ophiolepis, and Ophiocoma) still survive at the present day. 6. ECHINOIDEA.-This order is represented in the Paleozoic Rocks by a single aberrant family; but it is numerously represented in the Mesozoic and Kainozoic periods. Of the Paleozoic Urchins or Perischoëchinidæ, the two most abundant genera are Archæocidaris and Palachinus, both of which are principally Carboniferous, though the latter occurs in the Upper Silurian. Melonites and Oligoporus are exclusively Carboniferous; and Lepidechinus and Eocidaris are principally so, though both commence their existence in the Devonian. The Secondary and Tertiary Echinoidea resemble those now living in being composed of not more than twenty rows of calcareous plates. The Oolitic and Cretaceous Rocks are especially rich in forms belonging to this order, many genera being peculiar; but the number of forms is too great to permit of any selection. It may be mentioned, however, that the singular genus Echinothuria, with its flexible test, the predecessor of the living Calveria and Phormosoma, is found in the Chalk. - 7. HOLOTHUROIDEA. This order, comprising, as it does, soft-bodied animals, has left but few traces of its existence in past time. The calcareous integumentary plates and spicules of Sea-Cucumbers are, however, by no means absolutely unknown in the fossil condition, ranging from the Carboniferous onwards. The shield of Psolus has also been detected in PostTertiary deposits in Bute. LITERATURE. GENERAL WORKS. 1. "Klassen und Ordnungen des Thier - Reichs," vol. ii. 'Strahlenthiere.' Bronn. 1860. 2. 3. 4. "A History of British Star-fishes and other Animals of the Class "Grundriss der Vergleichenden Anatomie.' Gegenbaur. 1874- 5. Article "Echinodermata.' Sharpey. Todd's 'Cyclopædia of Anatomy and Physiology.' 1839. 6. "Anatomie der Rohren-Holothurie," &c. Tiedemann. 1820. 7. "Prodrome d'une Monographie des Radiaires. Louis Agassiz. 'Mem. Soc. d'Hist. Nat. de Neufchatel. 1835. ECHINOIDEA. 8. "Monographies d'Echinodermes vivants et fossiles." Louis Agassiz. 1838-41. 9. "Catalogus systematicus ectyporum Echinodermatum fossilium," &c. Louis Agassiz. 1840. IO. Catalogue raisonné des familles, des genres, et des espèces de la classe des Echinodermes." Louis Agassiz and Desor. 1847. II. "Revision of the Echini." Alexander Agassiz. 'Memoirs of the Museum of Comparative Zoology at Harvard,' vol. iii. 1872-74. 12. "Monograph of the British Fossil Echinodermata." Thomas Wright. 'Palæontographical Society.' 13. "Monograph of the Echinodermata of the British Tertiaries." Edward Forbes. Palæontographical Society.' 14. "Paléontologie Française, Terrains Crétacés," tom. vi. and vii. 'Echinodermes.' A. D'Orbigny and G. Cotteau. 1856-57 and 1861-67. 15. "Synopsis des Echinides fossiles." E. Desor. 1855-59. ASTEROIDEA AND OPHIURoidea. 16. " System der Asteriaden." Müller and Troschel. 1843. "British Fossil Echinodermata. 'The Asteroidea.' Wright. 'Palæontographical Society.' 17. 19. Thomas As 18. "Ophiuride and Astrophytidæ." Theodore Lyman. 'Illustrated Catalogue of the Museum of Comparative Zoology at Harvard.' 1865. "Ueber die Larven und die Metamorphosen der Ophiuren." J. Müller. 'Abhandl. Berlin Akad. 1846. (See also several of the works mentioned under the head of "General works." regards the development of the Asteroids and Ophiuroids, the English student may consult Professor Huxley's Report upon the Researches of Professor Müller into the Anatomy and Development of Echinoderms," in the Annals of Natural History,' ser. 2, vol. viii. 1851; or Packard's 'Life-Histories of Animals.') 20. 21. CRINOIDEA. 66 A Natural History of the Crinoidea," &c. J. S. Miller. 1821. Memoir on the Pentacrinus Europæus.' J. V. Thompson. 1827. 22. Histoire naturelle des Crinoides vivants et fossiles." A. D'Orbigny, 46 1840. (Unfinished.) 23. Ueber den Pentacrinus Caput-Medusa." J. Müller. 1843. "Recherches sur les Crinoides du terrain carbonifère de la Belgique." De Koninck and Le Hon. 1854. 24. 25. "Development of Comatula." Sir Wyville Thomson. Phil. Trans., 1865. 26. "The Depths of the Sea." Sir Wyville Thomson. 27. 44 1873. Notice of new living Crinoids belonging to the Apiocrinidae." Sir 28. "On the Structure, Physiology, and Development of Antedon 30. "The Crinoidea of the Lower Silurian Rocks of Canada." E. Billings. Figures and Descriptions of Canadian Organic Remains,' Decade iv. 1859. CYSTOIDEA. 31. "On the Cystidea of the Lower Silurian Rocks of Canada." E. Billings. Figures and Descriptions of Canadian Organic Remains,' Decade iii. 1858. 32. "Notes on the Structure of the Crinoidea, Cystidea, and Blastoidea.” E. Billings. Paleozoic Fossils,' vol. ii. 1874. 33. "Ueber Cystideen. Von Buch, 1845. (Translated in 'Quart. Journ. Geol. Soc. Lond.' 1845.) BLASTOIDEA. 34. "Monographie der fossilen Krinoideen - Familie der Blastoideen." Ferdinand Romer. 1851. 35. 36. 37. (See also the works by Billings quoted above.) HOLOTHUROIDEA. "List of British Radiata, with Synonyma." J. E. Gray. 1848. "Reisen im Archipel der Philippinen." C. Semper. CHAPTER XXIV. SCOLECIDA. CLASS II. SCOLECIDA.-This class was proposed by Professor Huxley for the reception of the remaining members of the Annuloida, comprising the Rotifera, the Turbellaria, the Trematoda, the Taniada, the Nematoidea, the Acanthocephala, and the Gordiacea. Of these the Rotifera stand alone; whilst the Turbellaria, Trematoda, and Taniada constitute the old division of the Platyelmia (Flat Worms); and the Nematoidea, Acanthocephala, and Gordiacea make up the old Nematelmia (Round Worms or Thread-worms). For some purposes these old divisions are sufficiently convenient to be retained, though they are of little scientific value. The term Entozoa has acquired such a general currency that it is necessarily employed occasionally, but it has been used in such widely different senses by different writers, that it would be almost better to discard it altogether. It certainly cannot be used as synonymous with Scolecida, many of these not being parasitic at all. It will therefore be employed here, in a restricted sense, to designate those orders of the Scolecida which are internal parasites, comprising the Trematoda, Taniada, Nematoidea (in part), Acanthocephala, and Gordiacea. The Turbellaria and |