Mineral Deposit in the Rhizopods and Sponges. 73 
development the spicula [of Sponges] appear to consist of a 
double membrane, between which the first layer of silex is 
secreted ; and in this condition they present an internal cavity 
approaching very nearly to the size of the external diameter.” 
And again: “the deposit of silex is not continuous and homo- 
geneous, but produced in concentric layers, which, it would ap- 
pear, are, at least for a period, equally secreted by the inner 
surface of the outer membrane and the outer surface of the 
imner one.” 
Now, although all truly spicular sponge-growths are formed, as 
here laid down, by concentric layers of silex secreted from two 
distinct surfaces, and, in their earliest condition, occasionally 
“approach very nearly to the size of the external diameter,” 
unless I am much mistaken in my interpretation of the appear- 
ances, they are neither “secreted equally ” from these two sur- 
faces after the deposit of the first layer evolved by each, nor are 
they formed within membranous cavities. 
In order to render the process intelligible, it is desirable to 
take as an illustration the simplest type of siliceous spicule, 
—namely, the common elongate cylinder, without reference 
to the shape of its extremities, or the closure of its tubule 
either at one or both ends. But first with regard to the mem- 
brane here spoken of by Dr. Bowerbank as occurring among the 
Spongiadee, and asserted by Professor Schultze to be present in 
the Foraminifera*. : 
In the living or fresh sarcode, whether of the Sponges or 
Foraminifera, there are no membranous cavities from the sur- 
faces of which mineral deposit takes placet. There are cavities, 
and these doubtless present a definite outline, but not more 
definite than that of the vacuole which, in the sarcode of both 
classes, appears and disappears without leaving the slightest 
trace behind. It is also true that in spirit-specimens, and 
under the action of acids, an amount of “ hardening” is produced 
which causes the external layer of sarcode to assume the appear- 
ance of membrane. But, as I have endeavoured to show with 
regard to Ameba t, we are by no means warranted in taking it 
for granted that characters, manifest only under the action of a 
chemical reagent, have necessarily existed prior to its employ- 
ment ; and, in further confirmation of this view, I may state 
* See paper above referred to, p. 413, where the following passage oc- 
a : ae : : = 
curs: ‘The organic substance remaining after the treatment of specimens 
of Polytrema preserved in spirits consists of an external membrane and a 
tenacious brownish-red substance.” + See same paper, p. 418. 
{ “On the Value of the Distinctive Characters in Amba,” ‘ Annals’ 
for August 1863, p. 128. 
