A treatise on zoology . e chambers just mentioned. The axial nerve-cord of the arm does not, as all the organs yetdealt with, pass to the oral centre, but enters the theca overthe radial. If there is a separate axial canal, it may be continuedthrough the radial facet into the substance of the thecal cords ultimately pass into the epithelial covering of thechambered organ, but their passage is not a direct one (Figs. XII.). Each cord is really a double structure, connectedat intervals by chiasmas, and so soon as it enters the radial itdivides into two branches, one of which pr
A treatise on zoology . e chambers just mentioned. The axial nerve-cord of the arm does not, as all the organs yetdealt with, pass to the oral centre, but enters the theca overthe radial. If there is a separate axial canal, it may be continuedthrough the radial facet into the substance of the thecal cords ultimately pass into the epithelial covering of thechambered organ, but their passage is not a direct one (Figs. XII.). Each cord is really a double structure, connectedat intervals by chiasmas, and so soon as it enters the radial itdivides into two branches, one of which proceeds to the basal onthe right, the other to that on the left. In addition the branchesare connected with each other and with those of the other radii bya series of commissures that form rings all round the cup. One I04 THE CRINOIDEA such ring is at the level of the radials. If the crinoid have amonocyclic base, the cords that pass to the basals join one anotherin a ring immediately surrounding the chambered organ, the. Fi<!. XI. Courst! of axiul iKiivH-cords in Isocrinus. Diagrammatised from sections tij^iuttl by P. , li, basal; A, radial; «x, axial or<,aii ; ch, five chambers of chambered organ; 7il,nerve-cord from /• to /;; «2, cord passing down B,- n3, cord from /; to radially plact^d lobes ofchambered organ, lobes of which in this case correspond with the basals, areinterradial. If the base be dicyclic, the ring forms a commissureat the level of the centres of the basals; and from these points
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Keywords: ., bookcentury1900, bookdecade1900, bookpublishe, booksubjectzoology
