A treatise on zoology . THE STELLEROIDEA the union of the bases of the arms, but is sharply marked oiFfrom them, and they are attached to it as appendages; thereis not, moreover, in the Ophiuroids the ventral groove of the star-fish. These characters, however, are not absolutely to be reliedon; thus in some species of Asfroschema there is no sharp separa-tion between the arms and the disc ; while in Ophioteresis () the radial ambulacral vessels and nerve-trunks lie in shallowgrooves on the ventral surface of the arms. An idea of the Structure of a typical Ophiuroid may probablybe best
A treatise on zoology . THE STELLEROIDEA the union of the bases of the arms, but is sharply marked oiFfrom them, and they are attached to it as appendages; thereis not, moreover, in the Ophiuroids the ventral groove of the star-fish. These characters, however, are not absolutely to be reliedon; thus in some species of Asfroschema there is no sharp separa-tion between the arms and the disc ; while in Ophioteresis () the radial ambulacral vessels and nerve-trunks lie in shallowgrooves on the ventral surface of the arms. An idea of the Structure of a typical Ophiuroid may probablybe best obtained by the careful examination of a representativespecies, for which purpose the commonest English brittle star{Ophiura ciUcnis, Linn, sp.) is a convenient type. This Ophiuroid consists of a round, flat, scale-covered disc, fromMhich radiate five long, tapering arms. The Arms are composedof a series of jointed segments, each containing six plates. Twoof these are fused together into a single vertebral ossicle, and. Kic. XIV. Ophiolere-iif (after IJoll). Aboral surface of an arm ossic e ; (/, tlie doubh dorsal anu-plates ;n, articular cavities ; /, lateral arin-platos. a series of these forms the axis of the arm. The remaining fourplates form an external tube round the vertebral ossicle. OneI)air occurs at the sides, and is known as the lateral arm-plates orshields. Another of the four plates lies above the vertebralossicle, and is accordingly known as the dorsal arm-plate (ordorsal shield); the fourth lies on the lower surface of the arm,and is accordingly known as the ventral arm-plate (or ventralshield). Each lateral arm-})late bears seven short spines. The plates forming the central chain of the arm arc knownas verte)>ral ossicles, because, in typical Opliinroids such usOphiura ciliaris, they articulate l>y a series of knobs and socketslike the bones of a vertebral column. In a ly])ital vertebralossicle the two articular surfaces are very dillercnt ; in the proxim
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