Mineral Deposit in the Rhizopods and Sponges. 77 
ber—a”’ being the principal aperture, through which the sarcode- 
mass is seen to bulge outwards. 
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The difference between these two chambers deserves special 
attention. As already stated, the first layer of shell is deposited 
from the immediate surface of the sarcode-mass within, being 
only interrupted at the main aperture and those points through 
which the stolons and pseudopodia make their escape. In the 
normal condition of the organism, no further deposit takes place 
within. Every subsequent addition to the thickness of the shell- 
wall is made from without, and is brought about by a special 
layer of sarcode which spreads over the entire external surface 
from the stolons*, and thus seems to secrete the shell-substance 
by its gradual retrocession outwards, as in the case of the wall 
of the cavity in which the sponge-spicule is formed. This outer 
layer of sarcode may very readily be seen in mature Globigerine, 
and it is probably present in all Foraminifera, although visible 
with difficulty in some, too subtle to be appreciable in others, 
and perhaps taking its origin, in the imperforate genera, by a 
distinct reflexion of the sarcode-substance through the main 
* These stolons, unlike those which take part in the secretion of silex 
in the case of the sponge-spicule, seem designed actually to prevent the 
deposit of calcareous matter wherever they occur; since otherwise no 
apertures would remain for communication between the internal sarcode 
of the chambers and the medium in which the organism lives, 
