280 SECRETS OF ANIMAL LIFE 
transformation (if any) is invisible; all that we do 
observe is the rising of a visible and tangible edifice 
in the transparent medium of the Foraminifer’s 
protoplasm. But the arenaceous forms use visible 
ready-made building materials, and make of them 
an encasement. This is sometimes beautiful, oftener 
perhaps simply quaint; the interest centers in the 
selection of the materials and in the effective 
architecture. First as to selection: a particular 
kind of Foraminifer surrounded by an embarrassing 
wealth of alternatives will have none of them save 
intact sponge-spicules, out of which a transparent 
test is built up. Another kind will use only grains 
of quartz, and a third flakes of mica. What ex- 
periment will show is whether this particulate 
utilization of certain materials is obligatory or facul- 
tative? Is past experience so enregistered within 
the cell that the searching living threads will respond 
to sponge-spicule only? Or would the creature take 
Echinoderm fragments in default of anything 
else? Quite extraordinary is the case of a species 
of Technitella which makes its test of Echinoderm 
plates, and, having no definite oral aperture, 
sends its living threads flowing through the pores 
which these plates possess. How interesting, too, 
is the morsel of fact that one type that makes 
a shell of sponge-spicules will only use them 
intact, while another type will only use them 
broken! It has been suggested that when the 
materials used by a particular type are all of the 
same kind, and therefore of the same specific 
