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 that they or their ancestors had acquired even the Streptophiuran type of vertebra. Sollas has separated them as an order Ophiocistia.

Class III. —Eleutherozoa with a test of roughly circular, subpentagonal or elliptical outline, spheroidal, domed or flattened, of primary pentameric symmetry affecting all systems of organs except the gut. The radial water-vessels lie within the test through which their podia pass (fig. 12, D); the ambulacra thus formed are continuous from the peristome to the apical system of plates; the hydropore is connected with a definite plate of that system, and thus marks a secondary bilateral symmetry. An anus is present either within the apical system (endocyclic, fig. 3, A and B), or outside it in an interradius (exocyclic, fig. 19, 7), thus initiating yet another bilateral symmetry. Skeletal appendages are spines (radioles), pedicellariae, and, in some forms, minute sense-organs called sphaeridia.

The echinoids or sea-archins (see ) may be grouped under the following orders, here named in the sequence of their appearance in the rocks.

Order 1. Bothriocidaroida.—Ambulacrals simple, each with two pores vertically superposed, 2 columns to each ambulacrum; interambulacrals multi-tuberculate, in 1 column, none passing on to or resorbed by the peristome; mouth central, jaws unknown, no external gills or sphaeridia; anus aboral, endocyclic. Sole genus Bothriocidaris (fig. 5), Ordovician.

Order 2. Melonitoida.—Ambulacrals simple, each with two pores horizontally juxtaposed, in 2 to 18 columns; interambulacrals granulate with occasional tubercles, in 3 to 11 columns, not more than one row passing on to the peristome; mouth central, with jaws, no external gills or sphaeridia; anus aboral, endocyclic. Families: Palechinidae (fig. 19, 1), Melonitidae and Lepidesthidae, Silurian to Carboniferous.

Order 3. Cystocidaroida.—Ambulacrals simple, each with one or two pores, which sometimes pass between rather than through the plates, in 2 columns; interambulacrals, uni- or multi-tuberculate, in numerous (say 10 or more) columns, none passing on to peristome; mouth central with jaws, no external gills or sphaeridia; position of anus doubtful, acyclic, i.e. no apical system so far as known. Include only Echinocystis, Palaeodiscus and (?) Myriastiches, all Upper Silurian.

Order 4. Cidaroida.—Ambulacrals simple, each with two pores horizontally juxtaposed, in 2 columns; interambulacrals unituberculate, in 2 to 11 columns, some rows may pass on to the peristome; mouth central, with jaws, no external gills or sphaeridia; anus aboral, endocyclic. Families: Lepidocentridae and Archaeocidaridae (fig. 19, 2), Devonian and Carboniferous; Cidaridae (fig. 19, 3, 4). Permian to present; Diplocidaridae and Tiarechinidae, Mesozoic.

Order 5. Diademoida.—Ambulacrals generally compound, with two pores obliquely juxtaposed, in 2 columns as in all subsequent orders; interambulacrals usually with large radioles surrounded by smaller ones, as in Cidaroida, in 2 columns as in all subsequent orders, only one plate resorbed; mouth central, with jaws and external gills, sphaeridia present; anus aboral endocyclic. J. W. Gregory divides this into four suborders, each representing a distinct evolutionary series; i. Calycina, Saleniidae (fig. 19, 5) and Acrosaleniidae; ii. Arbacina, Hemicidaridae and Arbaciidae; iii. Diademina, Orthopsidae, Diadematidae, Diplopodiidae, Pedinidae, Cyphosomatidae, and Echinothuridae; iv. Echinina, Temnopleuridae, Triplechinidae, Strongylocentrotidae and Echinometridae. The order is Triassic to Recent.

Order 6. Holectypoida.—Ambulacrals sometimes compound, with one or two pores to a plate, some dorsal podia begin to assume respiratory function; interambulacrals multi-tuberculate, none resorbed; mouth central, with jaws weak or wanting, with external gills and sphaeridia; anus exocyclic. Families: Pygasteridae, Discoidiidae, Galeritidae, Conoclypeidae; Jurassic to Recent.

Order 7. Spatangoida.—Ambulacrals simple, with two pores juxtaposed, dorsal podia respiratory; interambulacrals bearing numerous small spines, none resorbed; mouth central or shifted forwards, with no jaws or external gills, sphaeridia numerous; anus exocyclic. As the mouth moves forward and the anus downward, the posterior interambulacrals between them are enlarged and strengthened so as to form a sternum. The order may therefore be divided into: (i.) Asternata, Famm. Echinoneidae, Nucleolitidae and Cassidulidae (fig. 19, 8); (ii.) Sternata, Famm. Collyritidae (fig. 19, 6), Echinocorytidae, Spatangidae (fig. 19, 7), Palaeostomidae, and Pourtalesiidae; Jurassic to Recent.

Order 8. Clypeastroida.—Ambulacrals simple or compound, with two pores juxtaposed, dorsal podia respiratory; interambulacrals multi-tuberculate, none resorbed; mouth central with flattened unequal jaws, reduced external gills, and few sphaeridia; anus exocyclic. Families: Fibulariidae, Laganidae, Scutellidae, Clypeastridae; Cretaceous to Recent.



The probable relationship of these orders is shown in the annexed table. Here the Cystocidaroida occupy an isolated position. It is, however, quite possible that Echinocystis may some day be referred to the Cidaroida, and Palaeodiscus to the Melonitoida. This would leave the Echinoid scheme remarkably simple, with the Melonitoida and Cidaroida as divergent branches from an ancestor like Bothriocidaris; but while the former branch soon decayed, the latter continues to flourish at the present day. To take the Echinoidea now living, and to divide them into Endocyclica and Exocyclica, Branchiate and Abranchiate, Gnathostomata and Atelostomata, is easy and convenient; or again to distinguish as Palechinoidea those pre-Jurassic genera which do not conform to the fixed type of twenty vertical columns found in the later Euechinoidea, is to express an interesting fact; but all such divisions obscure the true relationships, and the corresponding terms should be recognized as descriptive rather than classificatory.

—In addition to the works referred to at the beginning of the article, the following deal with the general subject: Bather, Gregory and Goodrich, “Echinoderma,” in Lankester’s Treatise on Zoology (London, 1900); F. J. Bell, Catalogue of the British Echinoderms in the British Museum (London, 1892); P. H. Carpenter, “Notes on Echinoderm Morphology,” ''Quart. Journ. Micr. Sci., 1878–1887; Y. Delage and E. Hérouard, Traité de zoologie concrète, iii., Échinodermes (Paris, 1904); A. Lang, Text-Book of Comparative Anatomy'', transl., part ii. (London, 1896); Ludwig and Hamann, “Echinodermen,” in Bronn’s Klassen und Ordnungen des Tierreichs (Leipzig, 1889), in progress; M. Neumayr, Die Stämme des Tierreiches (Wien, 1889); P. B. and C. F. Sarasin, “Über die Anatomie der Echinothuriden und die Phylogenie der Echinodermen,” ''Ergebnisse naturw. Forsch. auf Ceylon'', Bd. i Heft 3 (Wiesbaden, 1888); R. Semon, “Die Homologien innerhalb des Echinodermenstammes,” ''Morph. Jahrb. (1889); W. P. Sladen, “Homologies of the Primary Larval Plates in the Test of Brachiate Echinoderms,” Quart. Journ. Micr. Sci., 1884; K. A. v. Zittel, Handbuch der Paläozoologie, i. pp. 308-560 (München, 1879); also Grundzüge, translated and revised by C. R. Eastman as Text-Book'' 