138 
FORAMINIFERA AND POLYCYSTINA. 
being empty and deserted, each chamber of the Rotalia , or 
any other Eoraminiferous shell, is occupied by a segment of 
sarcode, which is to a great degree independent of the rest, 
and is only connected with those on either side of it by 
delicate threads of the same substance; and the extension of 
the shell is due to the formation of an additional segment of 
sarcode on the outside of the last-formed chamber. Each 
segment has usually the power of putting forth its own 
“ pseudopodia ” through minute apertures in the shell, and 
thus of drawing in its own nourishment through these; but 
even when (as sometimes happens) these food-collecting 
threads are put forth from the last chamber alone, the nutri¬ 
ment there obtained is transmitted to the segments within by 
percolation through the substance of the sarcode, and not 
through any tubular canal.—The accumulation of the shells 
of Eoraminifera in some parts of the existing sea-bottom is 
very remarkable ; and similar accumulations in past ages 
have formed no unimportant part of the crust of the earth— 
a large part of the Chalk-formation having had its origin in 
them, as well as nearly the whole of the ISummulitic limestone 
by which it was succeeded. 
132. Eut animals whose essen- a b 
tial structure seems to be nearly 
the same, may form siliceous in¬ 
stead of calcareous shells; and 
thus are produced those beautiful 
organisms, known under the 
name of Polycystina (fig. 7 9), 
which are occasionally found in 
the existing seas, but whose re¬ 
mains are met with under a far 
greater variety of forms in certain 
of the newer marine deposits. 
There is not in these the same 
tendency to form composite 
structures by the multiplication 
of segments, as in the Eoraminifera; but the complication of 
the individual form is often much greater. Yet, however 
complex the form, the essential composition of these crea¬ 
tures seems to retain the same attribute of simplicity, which 
cannot be conceived capable of further reduction. 
Fig. 79. —Polycystina. 
A, Podocyrtis; B, Rhopalocanium. 
