Mineral Deposit in the Uhizopods 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 

 inner 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 

 Spongiadse, 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 placef. 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 Amoeba J, 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- 

 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." f See same paper, p. 418. 



X " On the Value of the Distinctive Characters in Amaba," ' Annals ' 

 for August 1863, p. 128. 



