The Protozoans appear to have flourished extensively in the Lower
Silurian seas, though to a large extent under forms which are
still little understood. We have here for the first time the
appearance of Foraminifera of the ordinary type--one of the most
interesting observations in this collection being that made by
Ehrenberg, who showed that the Lower Silurian sandstones of the
neighbourhood of St Petersburg contained casts in glauconite of
Foraminiferous shells, some of which are referable to the existing
genera _Rotalia_ and _Texularia_. True _Sponges_, belonging to
that section of the group in which the skeleton is calcareous,
are also not unknown, one of the most characteristic genera being
_Astylospongia_ (fig. 37). In this genus are included more or
less globular, often lobed sponges, which are believed not to
have been attached to foreign bodies. In the form here figured
there is a funnel-shaped cavity at the summit; and the entire
mass of the sponge is perforated, as in living examples, by a
system of canals which convey the sea-water to all parts of the
organism. The canals by which the sea-water gains entrance open
on the exterior of the sphere, and those by which it again escapes
from the sponge open into the cup-shaped depression at the summit.
[Illustration: Fig. 37.--_Astylospongia proemorsa_, cut vertically
so as to exhibit the canal-system in the interior.
Pages:
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175