Order 2. _Zygophiurae._--Rays simple and prevented from coiling by processes on the vertebral joints (fig. 18); dorsal, ventral and lateral arm-plates present. [Illustration: FIG. 18.--A vertebral arm-ossicle (fused ambulacrals) of a Zygophiuran, _Ophiolepis_. A, Proximal joint-face. B, Distal joint-face. c, Ventral groove, where lies the water-vessel, from which branches pass through the ossicle, emerging as podia at e and e.] Sub-order i. _Brachyophiurae._--Spines short, simple, pointing towards the end of the arm. Families: Pectinuridae (= Ophiodermatidae), Ophiolepididae. Sub-order ii. _Nectophiurae._--Spines may be variously elaborated and are set more at right angles to the arm-axis. Families: Amphiuridae, Ophiacanthidae, Ophiocomidae, Ophiothrichidae. Order 3. _Cladophiurae_ (= Euryalae). Rays simple or branched, capable of coiling, since the vertebrae articulate by surfaces of hour-glass shape; ventral arm-plates, and often the others, much reduced; spines reduced or absent. Families: Euryalidae, Gorgonocephalidae, Astrochelidae, Astroschemidae, Astronycidae. The Silurian genera _Eucladia_ and _Euthemon_ have the rays greatly reduced and merged in the disk, so that the ambulacrals are unseen. There are a few large dorsal, lateral and ventral arm-plates, and at the angles of the latter emerge huge podia with a granular or plated skin. There are five prominent mouth-shields and a separate madreporite on the ventral surface. These genera attained the Colophiuran grade in respect of external plating, but it is unlikely that they or their ancestors had acquired even the Streptophiuran type of vertebra. Sollas has separated them as an order _Ophiocistia_. Class III. ECHINOIDEA.--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 SEA-URCHIN) may be grouped under the following orders, here named in the sequence of their appearance in the rocks.