A treatise on zoology . us. 5, thesame theca from below, with posterior interradius uppermost. (Both from Brit. Mus. E8013).xf. 6, slightly restored section across part of a radius, anib, so-called ambulacrum orpseudambulacrum, with food-groove; As, aperture for anus ; i>, basal; br, brachiole ; ,covering-plates ; h, hydrospire-folds ; L, lancet-plate ; 0, moutli-aperture ; R, radial; , side-plate ; A, deltoid. structure of the test. The marked pentamerous symmetry of the thecalplates (with the apparent exception of BB) and of the hydrospires isdisturbed only by the anus, which makes
A treatise on zoology . us. 5, thesame theca from below, with posterior interradius uppermost. (Both from Brit. Mus. E8013).xf. 6, slightly restored section across part of a radius, anib, so-called ambulacrum orpseudambulacrum, with food-groove; As, aperture for anus ; i>, basal; br, brachiole ; ,covering-plates ; h, hydrospire-folds ; L, lancet-plate ; 0, moutli-aperture ; R, radial; , side-plate ; A, deltoid. structure of the test. The marked pentamerous symmetry of the thecalplates (with the apparent exception of BB) and of the hydrospires isdisturbed only by the anus, which makes an opening between the jdos-terior A and the adjacent radial processes. From this IE, hydrospire-foldsare said to be absent (but one or two may be seen in some specimens,Fig. V. 2). The anal o^^ening was closed by small plates. The essentialstructures of Codasfer are all to be found in Protoblastoidea ; the absenceof interambulacrals, in Blastoidocrinus; the ambulacral structures, in 84 THE BLASTOIDEA ] Fig. VI. Asteroblastus and Blastoidocrinus ; the position of the anus, in Asterohlastus;the hydrospires, though far less developed, in Blastoidocrinus. So muchis this the case that Codaster has been referred to the Cystidea by severalwriters of eminence. Further arguments for such action are found in thefact that Codaster has no spiracles at the proximal ends of the ambu-lacra, and no hydrospire-pores along their sides,structures which are often considered character-istic of the Eublastoidea. The gradual evolutionof these structures may, however, be tracedwithin the order. P/iaenoscTiYswa, Etheridge & Carpenter (1882,f.^.--4--yS^§a J^m -86), Devonian and Carboniferous, Europe and N. America, differs from Codaster mainly in the factthat the hydrospire-folds become more concen-trated, and pushed in under, or overgrown by,the side-plates and part of the lancet-plate, sothat as a rule only their ends are visible ( 2) ; they are also well developed in thean
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