The only Hydroid Zoophytes or Polyzoans which have
a similar free mode of existence, have either no skeleton at
all, or have hard structures quite unlike the horny sheaths of
the Graptolites.
The second great group of Coelenterate animals (_Actinozoa_)
is represented in the Lower Silurian rocks by numerous Corals.
These, for obvious reasons, are much more abundant in regions
where the Lower Silurian series is largely calcareous (as in
North America) than in districts like Wales, where limestones
are very feebly developed. The Lower Silurian Corals, though
the first of their class, and presenting certain peculiarities,
may be regarded as essentially similar in nature to existing
Corals. These, as is well known, are the calcareous skeletons of
animals--the so-called "Coral-Zoophytes"--closely allied to the
common Sea-anemones in structure and habit. A _simple_ coral (fig.
43) consists of a calcareous cup embedded in the soft tissues of
the flower-like polype, and having at its summit a more or less
deep depression (the "calice") in which the digestive organs
are contained. The space within the coral is divided into
compartments by numerous vertical calcareous plates (the "septa"),
which spring from the inside of the wall of the cup, and of which
some generally reach the centre. _Compound_ corals, again (fig.
44), consist of a greater or less number of structures similar
in structure to the above, but united together in different ways
into a common mass.
Pages:
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182