Mineral Deposit in the Rhizopods and Sponges. 79 
&c., in the Number of the ‘ Annals’ for June last (PI. X. fig. 13) 
and in those forms of Arcel/a in which a number of spine-like 
processes are formed, and the original chitinous test becomes 
covered with sandy particles, it is probably through apertures 
in these that the sarcode escapes. 
Lastly, it is a most interesting fact, that, if we turn to the Pro- 
tophytes, as, for example, the Diatoms and Desmidians, a layer 
of protoplasm, as is well known, envelopes the harder portions 
exteriorly. This layer is homologous with the chitosare of the 
Rhizopods. 
Now one of the strongest corroborations of the view here ad- 
vanced is to be found in the fact that in an abnormal condition 
of the oldest Globigerine shells, in which the larger foramina 
become almost wholly obliterated by calcareous deposit (and 
it would appear that elective affinity exercises greater power 
* than the inherent secretory faculty of the sarcode), a secondary 
free layer of shell-substance becomes deposited within the 
primary layer; and from this the delicate caleareous spines pre- 
sent in some of the heaviest of the free-floating surface Globi- 
gerinz of tropical seas seem to be projected. At first sight 
these spines look like pseudopodia ; they are undoubtedly cal- 
careous, however, and, as before stated, never tubular. 
In the diagram a second layer is represented as having been 
already added on the external surface of the primary chamber, 
p c,—this layer extending, however, only over the external 
area of the primordial chamber, and not over that portion of it 
which is covered by the second chamber, sc. The latter is repre- 
sented as seen prior to the deposition of this outer or secondary 
layer—the spreading out of the stolons, so as to form the chi- 
tosare, having yet to take place before any external addition to the 
shell can be brought about. Thus it has been shown that the 
deposit of mineral matter, of which the Foraminiferous shell is 
composed, takes place only in one direction at a time, and not 
within a cavity, but either upon or within a surface of sarcode. 
The same remark applies to the spinous projections which 
are occasionally present. These may be traversed by a canal- 
system, but they never exhibit a tubule originally occupied by a 
stolon from which they have been partly secreted. On the con- 
trary, they are built up of consecutive additions of calcareous 
matter, laid on, as it were, at right angles to their axes, and 
extending in one direction only, that is to say, from the axis of 
the chamber outwards ; whilst the secreting surface constitutes a 
progressive mould into which the mineral matter is poured out, 
the base of the secreting cavity being, at the commencement of 
the operation, closed by the calcareous area upon which the 
spinous process is projected. Here it is evident that the in- 
