27 
“On the Structure of the spines of Echini/’ by H. A. 
Hurst, Esq. 
Notwithstanding the general appreciation by microscopists 
of the spines of Echini, the author had been unable to find 
any satisfactory account of their structure, and he attributed 
this to the fact that the examination of these objects had 
been chiefly confined to their sections, mounted in Canada 
balsam, which frequently has the effect of making transparent 
objects too transparent. He recommended for this purpose, 
however, the use of Smith and Beck’s semi-paraboloid lieber- 
khun, together with transmitted light — cutting off either 
means of illumination by a slight motion of the hand, or 
using both together ; but the readiest means of ascertaining 
their real structure he found to be the examination of un- 
mounted and splintered ends of broken spines by incident 
light, a method bringing out details and showing the con- 
nection of parts in a manner superior to any other. It was 
with diffidence he dissented from Dr. Carpenter’s views in 
the last edition of his work on the microscope; but he 
begged to propose the following as more in accordance with 
the appearances of structure presented by these spines under 
the microscope. They are composed of two substances in 
outward appearance, though chemically perhaps the same, 
one so perfectly homogeneous and transparent when viewed 
by transmitted or polarised light that it cannot be distin- 
guished from the blank field of the microscope — yet, under 
incident light, so dark and opaque as to appear black. This 
substance is frequently traversed by winding anastomising 
channels, which, though only containing air, seem opaque 
and show as solid by transmitted light, the substance they 
