. The microscope and its revelations. FK;. C7i5.—Transverse section of spine of Echinometra. (fig. 673). The number of these layers is extremely variable, de-pending not merely upon the age of the spine, but (as will presently appear) upon the part ofits length from which thesection happens to betaken. The centre isusually occupied by avery open network (fig.(572) ; and this is boundedby a row of transparentspaces (like those at a «,b b. r c, &c., fig, ).which on a cursory in-fection might be sup-posed to lie \ did, but are Ki,.. (J74.— On, o\ bhe segments of the calcareous found on close
. The microscope and its revelations. FK;. C7i5.—Transverse section of spine of Echinometra. (fig. 673). The number of these layers is extremely variable, de-pending not merely upon the age of the spine, but (as will presently appear) upon the part ofits length from which thesection happens to betaken. The centre isusually occupied by avery open network (fig.(572) ; and this is boundedby a row of transparentspaces (like those at a «,b b. r c, &c., fig, ).which on a cursory in-fection might be sup-posed to lie \ did, but are Ki,.. (J74.— On, o\ bhe segments of the calcareous found on closer exnmina-skeleton of an ambulacral disc of. tion to be the sections ofsolid ribs or pillars, which run in the direction of the length of the spine, and form the exteriorol every layer. Their solidity becomes very obvious when we SPINES OF ECHINOIDS 887 either examine ;i section of a spine whose substance is pervaded (asoften happens) with a colouring matter of some depth, or when welook at a very thin section by black-ground illumination. Aroundthe innermost circle of these solid pillars there is another layer ofthe calcareous network, which again is surrounded by another circleof solid pillars ; and this arrangement may be repeated many times,as shown in fig. 675, the outermost row of pillars forming theprojecting ribs that are commonly to be distinguished on the surfaceof the spine. Around the cup-shaped base of the spine is a membranewhich is continuous with that covering the surface of the shell, andserves not merely to hold down the cup upon the tubercle over whichit works, but also by its contractility to move the spine i
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Keywords: ., bookcentury1900, bookdecade1900, booksubjectmicrosc, bookyear1901
