94: PROCEEDINGS OF SOCIETIES. 
growth, gradually coalesce, and, at a certain distance from the 
centre, consolidate into a rib or pillar, which runs from the root 
to the apex of the spine, forming a longitudinally furrowed exte- 
rior, caused by the centre of each rib projecting slightly. Once 
this consolidation of tubes into ribs effected, the growth changes, 
the ribs extending eccentrically into plates radiating from the 
centre, and separated from each other by a mass of tissue similar 
to that at the centre. At the close of the second period of growth 
these plates thicken concentrically so as almost to touch each other, 
which, however, they do not, leaving a furrow of separation. 
They then continue their radiating extension till the end of 
another year’s growth, when the concentric thickening into an 
outer rib again takes place, and so on insuccessive years. These 
radiating plates are horizontally perforated by circular apertures 
bearing a singular resemblance to rivet-holes in boiler-plates, and 
appear to have rivets passing through them of the opaque sub- 
stance in a fibrous state. Altogether the structure of the spine 
may be compared to that of aniron tubular bridge. The thickened 
exteriors of these plates are highly coloured, and traversed by 
winding, anastomizing channels containing air; in some portions 
of the spine, the opaque substance either grows into, or is de- 
stroyed and replaced by, the transparent substance, which then 
forms a solid mass, perforated by the rivet-holes, now changed into 
winding, anastomizing passages. Forms intermediate between the 
extremes of complexity and simplicity were those of Echinus sphera 
and lividus ; Echinus sphera being chiefly composed of the pith- 
like substance, with twenty-five or thirty radiating glassy plates of 
a whitish colour; while Zchinus lividus was more solid, the pith- 
like substance passing into the solid glassy radiating plates through 
portions consisting of this glassy matter, perforated by anasto- 
mizing channels.—Mr. Hurst was not able to say whether the 
pithy and glassy substances are distinct or not ; but while the 
cellular matter leaves no trace after the prolonged action of 
vinegar, the transparent glassy substance, as well as the exterior 
of the spine, appears to be enveloped by a membrane, resisting 
the action of vinegar, which curiously converts this solid, opaque, 
hard, and brittle spine into a transparent, flexible body, retaining 
its original form. By using direct sunlight and a semi-paraboloid 
condenser, the glassy matter could be distinctly seen through, 
even when viewed as an opaque object, and the arrangement of 
the cellular matter ascertained. It is this transparent substance 
which is tinged with the beautiful purple hue so well known to 
microscopists. Mr. Hurst expressed his disappointment that the 
use of polarized light in these observations had led to no result, 
and thought its value had been oyer-estimated. 
~ ~ be lee Sere ~ 
