280 
Dr Grant on the Structure and 
homogeneous throughout; but, on being heated or dried, they lose 
their lustre, become less transparent, and of a greyish-white co- 
lour, and a distinct cavity is observed within them, extending 
from one point to the other, and occupying about half of their 
diameter. From the appearance of the sharp points at the ex- 
tremities of their axis, and from their bodies inflating and burst- 
ing by sudden heat, their internal cavity seems to be completely 
closed at both ends ; and from the homogeneous and solid ap- 
pearance of the spicula in their natural state, they seem to be 
then filled with a soft matter, decreasing in density from the cir- 
cumference to the axis, which may contribute to their strength 
and flexibility. When we place any object, measuring half a line 
in length, among these spicula under the microscope, we perceive 
that it requires four of them to extend the same length as that 
object ; thus shewing each spieulum to be the eighth of a line, or 
eightieth of an inch in length, and their diameter measured in 
the same way, is about the fourth of that of a human hair. As 
the spicula of this zoophyte are of a middle size, between the 
large and the minute, their dimensions might be assumed as 
unity in the measurement of other spicula ; and from the con- 
stancy of the forms, and dimensions of these elementary parts of 
the skeleton, their description would form an important charac- 
ter in the definition of every zoophyte possessed of spicula. 
Each longitudinal fasciculus, which appears to the naked eye 
as a single fibre, is composed of about ten spicula adhering close- 
ly together in a body, a like number being added to their extre- 
mities to an indefinite length. These spicula adhere to each other 
throughout their whole length, and are not easily separated by 
agitation, or by repeated maceration in hot-water ; but their con- 
necting matter is quickly dissolved in strong acids, which might 
lead us to believe, that it differs from the common gelatinous 
matter of the spongilla. The waving direction of these fasciculi 
is produced by the curves of one set of spicula being turned op- 
posite to the curves of the next adjoining, and so on in a conti- 
nued series. The single transverse spicula, which connect the 
longitudinal fibres together, generally pierce completely through 
these strong groups, to secure a firmer adhesion. The forms 
and nature of the ultimate spicula, and the general construction 
of the skeleton I have always found to be the same, whatever 
